commit 835525a36cfb236d5f88ebea885ba4eb9c6aa062
parent aa58c8dc227553adba1fef5fc3553e1c37a1fb09
Author: Ryan Sepassi <rsepassi@gmail.com>
Date: Fri, 8 May 2026 17:24:24 -0700
C11 std docs
Diffstat:
| A | doc/std/ANNEX-A.txt | | | 603 | +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ |
| A | doc/std/ANNEX-J.txt | | | 618 | +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ |
| A | doc/std/CHAPTER-5.txt | | | 1678 | +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ |
| A | doc/std/CHAPTER-6.txt | | | 9476 | +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ |
4 files changed, 12375 insertions(+), 0 deletions(-)
diff --git a/doc/std/ANNEX-A.txt b/doc/std/ANNEX-A.txt
@@ -0,0 +1,603 @@
+Annex A
+
+ (informative)
+ Language syntax summary
+1 NOTE The notation is described in 6.1.
+
+Contents
+
+A.1 Lexical grammar
+
+Contents
+
+A.1.1 Lexical elements
+
+(6.4) token:
+ keyword
+ identifier
+ constant
+ string-literal
+ punctuator
+(6.4) preprocessing-token:
+ header-name
+ identifier
+ pp-number
+ character-constant
+ string-literal
+ punctuator
+ each non-white-space character that cannot be one of the above
+Contents
+
+A.1.2 Keywords
+
+(6.4.1) keyword: one of
+ auto if unsigned
+ break inline void
+ case int volatile
+ char long while
+ const register _Alignas
+ continue restrict _Alignof
+ default return _Atomic
+ do short _Bool
+ double signed _Complex
+ else sizeof _Generic
+ enum static _Imaginary
+ extern struct _Noreturn
+ float switch _Static_assert
+ for typedef _Thread_local
+ goto union
+Contents
+
+A.1.3 Identifiers
+
+(6.4.2.1) identifier:
+ identifier-nondigit
+ identifier identifier-nondigit
+ identifier digit
+(6.4.2.1) identifier-nondigit:
+ nondigit
+ universal-character-name
+ other implementation-defined characters
+(6.4.2.1) nondigit: one of
+ _ a b c d e f g h i j k l m
+ n o p q r s t u v w x y z
+ A B C D E F G H I J K L M
+ N O P Q R S T U V W X Y Z
+(6.4.2.1) digit: one of
+ 0 1 2 3 4 5 6 7 8 9
+Contents
+
+A.1.4 Universal character names
+
+(6.4.3) universal-character-name:
+ \u hex-quad
+ \U hex-quad hex-quad
+(6.4.3) hex-quad:
+ hexadecimal-digit hexadecimal-digit
+ hexadecimal-digit hexadecimal-digit
+Contents
+
+A.1.5 Constants
+
+(6.4.4) constant:
+ integer-constant
+ floating-constant
+ enumeration-constant
+ character-constant
+(6.4.4.1) integer-constant:
+ decimal-constant integer-suffixopt
+ octal-constant integer-suffixopt
+ hexadecimal-constant integer-suffixopt
+(6.4.4.1) decimal-constant:
+ nonzero-digit
+ decimal-constant digit
+(6.4.4.1) octal-constant:
+ 0
+ octal-constant octal-digit
+(6.4.4.1) hexadecimal-constant:
+ hexadecimal-prefix hexadecimal-digit
+ hexadecimal-constant hexadecimal-digit
+(6.4.4.1) hexadecimal-prefix: one of
+ 0x 0X
+(6.4.4.1) nonzero-digit: one of
+ 1 2 3 4 5 6 7 8 9
+(6.4.4.1) octal-digit: one of
+ 0 1 2 3 4 5 6 7
+(6.4.4.1) hexadecimal-digit: one of
+ 0 1 2 3 4 5 6 7 8 9
+ a b c d e f
+ A B C D E F
+(6.4.4.1) integer-suffix:
+ unsigned-suffix long-suffixopt
+ unsigned-suffix long-long-suffix
+ long-suffix unsigned-suffixopt
+ long-long-suffix unsigned-suffixopt
+(6.4.4.1) unsigned-suffix: one of
+ u U
+(6.4.4.1) long-suffix: one of
+ l L
+(6.4.4.1) long-long-suffix: one of
+ ll LL
+(6.4.4.2) floating-constant:
+ decimal-floating-constant
+ hexadecimal-floating-constant
+(6.4.4.2) decimal-floating-constant:
+ fractional-constant exponent-partopt floating-suffixopt
+ digit-sequence exponent-part floating-suffixopt
+(6.4.4.2) hexadecimal-floating-constant:
+ hexadecimal-prefix hexadecimal-fractional-constant
+ binary-exponent-part floating-suffixopt
+ hexadecimal-prefix hexadecimal-digit-sequence
+ binary-exponent-part floating-suffixopt
+(6.4.4.2) fractional-constant:
+ digit-sequenceopt . digit-sequence
+ digit-sequence .
+(6.4.4.2) exponent-part:
+ e signopt digit-sequence
+ E signopt digit-sequence
+(6.4.4.2) sign: one of
+ + -
+(6.4.4.2) digit-sequence:
+ digit
+ digit-sequence digit
+(6.4.4.2) hexadecimal-fractional-constant:
+ hexadecimal-digit-sequenceopt .
+ hexadecimal-digit-sequence
+ hexadecimal-digit-sequence .
+(6.4.4.2) binary-exponent-part:
+ p signopt digit-sequence
+ P signopt digit-sequence
+(6.4.4.2) hexadecimal-digit-sequence:
+ hexadecimal-digit
+ hexadecimal-digit-sequence hexadecimal-digit
+(6.4.4.2) floating-suffix: one of
+ f l F L
+(6.4.4.3) enumeration-constant:
+ identifier
+(6.4.4.4) character-constant:
+ ' c-char-sequence '
+ L' c-char-sequence '
+ u' c-char-sequence '
+ U' c-char-sequence '
+(6.4.4.4) c-char-sequence:
+ c-char
+ c-char-sequence c-char
+(6.4.4.4) c-char:
+ any member of the source character set except
+ the single-quote ', backslash \, or new-line character
+ escape-sequence
+(6.4.4.4) escape-sequence:
+ simple-escape-sequence
+ octal-escape-sequence
+ hexadecimal-escape-sequence
+ universal-character-name
+(6.4.4.4) simple-escape-sequence: one of
+ \' \" \? \\
+ \a \b \f \n \r \t \v
+(6.4.4.4) octal-escape-sequence:
+ \ octal-digit
+ \ octal-digit octal-digit
+ \ octal-digit octal-digit octal-digit
+(6.4.4.4) hexadecimal-escape-sequence:
+ \x hexadecimal-digit
+ hexadecimal-escape-sequence hexadecimal-digit
+Contents
+
+A.1.6 String literals
+
+(6.4.5) string-literal:
+ encoding-prefixopt " s-char-sequenceopt "
+(6.4.5) encoding-prefix:
+ u8
+ u
+ U
+ L
+(6.4.5) s-char-sequence:
+ s-char
+ s-char-sequence s-char
+(6.4.5) s-char:
+ any member of the source character set except
+ the double-quote ", backslash \, or new-line character
+ escape-sequence
+Contents
+
+A.1.7 Punctuators
+
+(6.4.6) punctuator: one of
+ [ ] ( ) { } . ->
+ ++ -- & * + - ~ !
+ / % << >> < > <= >= == != ^ | && ||
+ ? : ; ...
+ = *= /= %= += -= <<= >>= &= ^= |=
+ , # ##
+ <: :> <% %> %: %:%:
+Contents
+
+A.1.8 Header names
+
+(6.4.7) header-name:
+ < h-char-sequence >
+ " q-char-sequence "
+(6.4.7) h-char-sequence:
+ h-char
+ h-char-sequence h-char
+(6.4.7) h-char:
+ any member of the source character set except
+ the new-line character and >
+(6.4.7) q-char-sequence:
+ q-char
+ q-char-sequence q-char
+(6.4.7) q-char:
+ any member of the source character set except
+ the new-line character and "
+Contents
+
+A.1.9 Preprocessing numbers
+
+(6.4.8) pp-number:
+ digit
+ . digit
+ pp-number digit
+ pp-number identifier-nondigit
+ pp-number e sign
+ pp-number E sign
+ pp-number p sign
+ pp-number P sign
+ pp-number .
+Contents
+
+A.2 Phrase structure grammar
+
+Contents
+
+A.2.1 Expressions
+
+(6.5.1) primary-expression:
+ identifier
+ constant
+ string-literal
+ ( expression )
+ generic-selection
+(6.5.1.1) generic-selection:
+ _Generic ( assignment-expression , generic-assoc-list )
+(6.5.1.1) generic-assoc-list:
+ generic-association
+ generic-assoc-list , generic-association
+(6.5.1.1) generic-association:
+ type-name : assignment-expression
+ default : assignment-expression
+(6.5.2) postfix-expression:
+ primary-expression
+ postfix-expression [ expression ]
+ postfix-expression ( argument-expression-listopt )
+ postfix-expression . identifier
+ postfix-expression -> identifier
+ postfix-expression ++
+ postfix-expression --
+ ( type-name ) { initializer-list }
+ ( type-name ) { initializer-list , }
+(6.5.2) argument-expression-list:
+ assignment-expression
+ argument-expression-list , assignment-expression
+(6.5.3) unary-expression:
+ postfix-expression
+ ++ unary-expression
+ -- unary-expression
+ unary-operator cast-expression
+ sizeof unary-expression
+ sizeof ( type-name )
+ _Alignof ( type-name )
+(6.5.3) unary-operator: one of
+ & * + - ~ !
+(6.5.4) cast-expression:
+ unary-expression
+ ( type-name ) cast-expression
+(6.5.5) multiplicative-expression:
+ cast-expression
+ multiplicative-expression * cast-expression
+ multiplicative-expression / cast-expression
+ multiplicative-expression % cast-expression
+(6.5.6) additive-expression:
+ multiplicative-expression
+ additive-expression + multiplicative-expression
+ additive-expression - multiplicative-expression
+(6.5.7) shift-expression:
+ additive-expression
+ shift-expression << additive-expression
+ shift-expression >> additive-expression
+(6.5.8) relational-expression:
+ shift-expression
+ relational-expression < shift-expression
+ relational-expression > shift-expression
+ relational-expression <= shift-expression
+ relational-expression >= shift-expression
+(6.5.9) equality-expression:
+ relational-expression
+ equality-expression == relational-expression
+ equality-expression != relational-expression
+(6.5.10) AND-expression:
+ equality-expression
+ AND-expression & equality-expression
+(6.5.11) exclusive-OR-expression:
+ AND-expression
+ exclusive-OR-expression ^ AND-expression
+(6.5.12) inclusive-OR-expression:
+ exclusive-OR-expression
+ inclusive-OR-expression | exclusive-OR-expression
+(6.5.13) logical-AND-expression:
+ inclusive-OR-expression
+ logical-AND-expression && inclusive-OR-expression
+(6.5.14) logical-OR-expression:
+ logical-AND-expression
+ logical-OR-expression || logical-AND-expression
+(6.5.15) conditional-expression:
+ logical-OR-expression
+ logical-OR-expression ? expression : conditional-expression
+(6.5.16) assignment-expression:
+ conditional-expression
+ unary-expression assignment-operator assignment-expression
+(6.5.16) assignment-operator: one of
+ = *= /= %= += -= <<= >>= &= ^= |=
+(6.5.17) expression:
+ assignment-expression
+ expression , assignment-expression
+(6.6) constant-expression:
+ conditional-expression
+Contents
+
+A.2.2 Declarations
+
+(6.7) declaration:
+ declaration-specifiers init-declarator-listopt ;
+ static_assert-declaration
+(6.7) declaration-specifiers:
+ storage-class-specifier declaration-specifiersopt
+ type-specifier declaration-specifiersopt
+ type-qualifier declaration-specifiersopt
+ function-specifier declaration-specifiersopt
+ alignment-specifier declaration-specifiersopt
+(6.7) init-declarator-list:
+ init-declarator
+ init-declarator-list , init-declarator
+(6.7) init-declarator:
+ declarator
+ declarator = initializer
+(6.7.1) storage-class-specifier:
+ typedef
+ extern
+ static
+ _Thread_local
+ auto
+ register
+(6.7.2) type-specifier:
+ void
+ char
+ short
+ int
+ long
+ float
+ double
+ signed
+ unsigned
+ _Bool
+ _Complex
+ atomic-type-specifier
+ struct-or-union-specifier
+ enum-specifier
+ typedef-name
+(6.7.2.1) struct-or-union-specifier:
+ struct-or-union identifieropt { struct-declaration-list }
+ struct-or-union identifier
+(6.7.2.1) struct-or-union:
+ struct
+ union
+(6.7.2.1) struct-declaration-list:
+ struct-declaration
+ struct-declaration-list struct-declaration
+(6.7.2.1) struct-declaration:
+ specifier-qualifier-list struct-declarator-listopt ;
+ static_assert-declaration
+(6.7.2.1) specifier-qualifier-list:
+ type-specifier specifier-qualifier-listopt
+ type-qualifier specifier-qualifier-listopt
+(6.7.2.1) struct-declarator-list:
+ struct-declarator
+ struct-declarator-list , struct-declarator
+(6.7.2.1) struct-declarator:
+ declarator
+ declaratoropt : constant-expression
+(6.7.2.2) enum-specifier:
+ enum identifieropt { enumerator-list }
+ enum identifieropt { enumerator-list , }
+ enum identifier
+(6.7.2.2) enumerator-list:
+ enumerator
+ enumerator-list , enumerator
+(6.7.2.2) enumerator:
+ enumeration-constant
+ enumeration-constant = constant-expression
+(6.7.2.4) atomic-type-specifier:
+ _Atomic ( type-name )
+(6.7.3) type-qualifier:
+ const
+ restrict
+ volatile
+ _Atomic
+(6.7.4) function-specifier:
+ inline
+ _Noreturn
+(6.7.5) alignment-specifier:
+ _Alignas ( type-name )
+ _Alignas ( constant-expression )
+(6.7.6) declarator:
+ pointeropt direct-declarator
+(6.7.6) direct-declarator:
+ identifier
+ ( declarator )
+ direct-declarator [ type-qualifier-listopt assignment-expressionopt ]
+ direct-declarator [ static type-qualifier-listopt assignment-expression ]
+ direct-declarator [ type-qualifier-list static assignment-expression ]
+ direct-declarator [ type-qualifier-listopt * ]
+ direct-declarator ( parameter-type-list )
+ direct-declarator ( identifier-listopt )
+(6.7.6) pointer:
+ * type-qualifier-listopt
+ * type-qualifier-listopt pointer
+(6.7.6) type-qualifier-list:
+ type-qualifier
+ type-qualifier-list type-qualifier
+(6.7.6) parameter-type-list:
+ parameter-list
+ parameter-list , ...
+(6.7.6) parameter-list:
+ parameter-declaration
+ parameter-list , parameter-declaration
+(6.7.6) parameter-declaration:
+ declaration-specifiers declarator
+ declaration-specifiers abstract-declaratoropt
+(6.7.6) identifier-list:
+ identifier
+ identifier-list , identifier
+(6.7.7) type-name:
+ specifier-qualifier-list abstract-declaratoropt
+(6.7.7) abstract-declarator:
+ pointer
+ pointeropt direct-abstract-declarator
+(6.7.7) direct-abstract-declarator:
+ ( abstract-declarator )
+ direct-abstract-declaratoropt [ type-qualifier-listopt
+ assignment-expressionopt ]
+ direct-abstract-declaratoropt [ static type-qualifier-listopt
+ assignment-expression ]
+ direct-abstract-declaratoropt [ type-qualifier-list static
+ assignment-expression ]
+ direct-abstract-declaratoropt [ * ]
+ direct-abstract-declaratoropt ( parameter-type-listopt )
+(6.7.8) typedef-name:
+ identifier
+(6.7.9) initializer:
+ assignment-expression
+ { initializer-list }
+ { initializer-list , }
+(6.7.9) initializer-list:
+ designationopt initializer
+ initializer-list , designationopt initializer
+(6.7.9) designation:
+ designator-list =
+(6.7.9) designator-list:
+ designator
+ designator-list designator
+(6.7.9) designator:
+ [ constant-expression ]
+ . identifier
+(6.7.10) static_assert-declaration:
+ _Static_assert ( constant-expression , string-literal ) ;
+Contents
+
+A.2.3 Statements
+
+(6.8) statement:
+ labeled-statement
+ compound-statement
+ expression-statement
+ selection-statement
+ iteration-statement
+ jump-statement
+(6.8.1) labeled-statement:
+ identifier : statement
+ case constant-expression : statement
+ default : statement
+(6.8.2) compound-statement:
+ { block-item-listopt }
+(6.8.2) block-item-list:
+ block-item
+ block-item-list block-item
+(6.8.2) block-item:
+ declaration
+ statement
+(6.8.3) expression-statement:
+ expressionopt ;
+(6.8.4) selection-statement:
+ if ( expression ) statement
+ if ( expression ) statement else statement
+ switch ( expression ) statement
+(6.8.5) iteration-statement:
+ while ( expression ) statement
+ do statement while ( expression ) ;
+ for ( expressionopt ; expressionopt ; expressionopt ) statement
+ for ( declaration expressionopt ; expressionopt ) statement
+(6.8.6) jump-statement:
+ goto identifier ;
+ continue ;
+ break ;
+ return expressionopt ;
+Contents
+
+A.2.4 External definitions
+
+(6.9) translation-unit:
+ external-declaration
+ translation-unit external-declaration
+(6.9) external-declaration:
+ function-definition
+ declaration
+(6.9.1) function-definition:
+ declaration-specifiers declarator declaration-listopt compound-statement
+(6.9.1) declaration-list:
+ declaration
+ declaration-list declaration
+Contents
+
+A.3 Preprocessing directives
+
+(6.10) preprocessing-file:
+ groupopt
+(6.10) group:
+ group-part
+ group group-part
+(6.10) group-part:
+ if-section
+ control-line
+ text-line
+ # non-directive
+(6.10) if-section:
+ if-group elif-groupsopt else-groupopt endif-line
+(6.10) if-group:
+ # if constant-expression new-line groupopt
+ # ifdef identifier new-line groupopt
+ # ifndef identifier new-line groupopt
+(6.10) elif-groups:
+ elif-group
+ elif-groups elif-group
+(6.10) elif-group:
+ # elif constant-expression new-line groupopt
+(6.10) else-group:
+ # else new-line groupopt
+(6.10) endif-line:
+ # endif new-line
+(6.10) control-line:
+ # include pp-tokens new-line
+ # define identifier replacement-list new-line
+ # define identifier lparen identifier-listopt )
+ replacement-list new-line
+ # define identifier lparen ... ) replacement-list new-line
+ # define identifier lparen identifier-list , ... )
+ replacement-list new-line
+ # undef identifier new-line
+ # line pp-tokens new-line
+ # error pp-tokensopt new-line
+ # pragma pp-tokensopt new-line
+ # new-line
+(6.10) text-line:
+ pp-tokensopt new-line
+(6.10) non-directive:
+ pp-tokens new-line
+(6.10) lparen:
+ a ( character not immediately preceded by white-space
+(6.10) replacement-list:
+ pp-tokensopt
+(6.10) pp-tokens:
+ preprocessing-token
+ pp-tokens preprocessing-token
+(6.10) new-line:
+ the new-line character
diff --git a/doc/std/ANNEX-J.txt b/doc/std/ANNEX-J.txt
@@ -0,0 +1,618 @@
+Annex J
+
+ (informative)
+ Portability issues
+1 This annex collects some information about portability that appears in this International Standard.
+
+Contents
+
+J.1 Unspecified behavior
+
+1 The following are unspecified:
+
+The manner and timing of static initialization (5.1.2).
+The termination status returned to the hosted environment if the return type of main is not compatible with int (5.1.2.2.3).
+The values of objects that are neither lock-free atomic objects nor of type volatile sig_atomic_t and the state of the floating-point environment, when the processing of the abstract machine is interrupted by receipt of a signal (5.1.2.3).
+The behavior of the display device if a printing character is written when the active position is at the final position of a line (5.2.2).
+The behavior of the display device if a backspace character is written when the active position is at the initial position of a line (5.2.2).
+The behavior of the display device if a horizontal tab character is written when the active position is at or past the last defined horizontal tabulation position (5.2.2).
+The behavior of the display device if a vertical tab character is written when the active position is at or past the last defined vertical tabulation position (5.2.2).
+How an extended source character that does not correspond to a universal character name counts toward the significant initial characters in an external identifier (5.2.4.1).
+Many aspects of the representations of types (6.2.6).
+The value of padding bytes when storing values in structures or unions (6.2.6.1).
+The values of bytes that correspond to union members other than the one last stored into (6.2.6.1).
+The representation used when storing a value in an object that has more than one object representation for that value (6.2.6.1).
+The values of any padding bits in integer representations (6.2.6.2).
+Whether certain operators can generate negative zeros and whether a negative zero becomes a normal zero when stored in an object (6.2.6.2).
+Whether two string literals result in distinct arrays (6.4.5).
+The order in which subexpressions are evaluated and the order in which side effects take place, except as specified for the function-call (), &&, ||, ? :, and comma operators (6.5).
+The order in which the function designator, arguments, and subexpressions within the arguments are evaluated in a function call (6.5.2.2).
+The order of side effects among compound literal initialization list expressions (6.5.2.5).
+The order in which the operands of an assignment operator are evaluated (6.5.16).
+The alignment of the addressable storage unit allocated to hold a bit-field (6.7.2.1).
+Whether a call to an inline function uses the inline definition or the external definition of the function (6.7.4).
+Whether or not a size expression is evaluated when it is part of the operand of a sizeof operator and changing the value of the size expression would not affect the result of the operator (6.7.6.2).
+The order in which any side effects occur among the initialization list expressions in an initializer (6.7.9).
+The layout of storage for function parameters (6.9.1).
+When a fully expanded macro replacement list contains a function-like macro name as its last preprocessing token and the next preprocessing token from the source file is a (, and the fully expanded replacement of that macro ends with the name of the first macro and the next preprocessing token from the source file is again a (, whether that is considered a nested replacement (6.10.3).
+The order in which # and ## operations are evaluated during macro substitution (6.10.3.2, 6.10.3.3).
+The state of the floating-point status flags when execution passes from a part of the program translated with FENV_ACCESS ''off'' to a part translated with FENV_ACCESS ''on'' (7.6.1).
+The order in which feraiseexcept raises floating-point exceptions, except as stated in F.8.6 (7.6.2.3).
+Whether math_errhandling is a macro or an identifier with external linkage (7.12).
+The results of the frexp functions when the specified value is not a floating-point number (7.12.6.4).
+The numeric result of the ilogb functions when the correct value is outside the range of the return type (7.12.6.5, F.10.3.5).
+The result of rounding when the value is out of range (7.12.9.5, 7.12.9.7, F.10.6.5).
+The value stored by the remquo functions in the object pointed to by quo when y is zero (7.12.10.3).
+Whether a comparison macro argument that is represented in a format wider than its semantic type is converted to the semantic type (7.12.14).
+Whether setjmp is a macro or an identifier with external linkage (7.13).
+Whether va_copy and va_end are macros or identifiers with external linkage (7.16.1).
+The hexadecimal digit before the decimal point when a non-normalized floating-point number is printed with an a or A conversion specifier (7.21.6.1, 7.29.2.1).
+The value of the file position indicator after a successful call to the ungetc function for a text stream, or the ungetwc function for any stream, until all pushed-back characters are read or discarded (7.21.7.10, 7.29.3.10).
+The details of the value stored by the fgetpos function (7.21.9.1).
+The details of the value returned by the ftell function for a text stream (7.21.9.4).
+Whether the strtod, strtof, strtold, wcstod, wcstof, and wcstold functions convert a minus-signed sequence to a negative number directly or by negating the value resulting from converting the corresponding unsigned sequence (7.22.1.3, 7.29.4.1.1).
+The order and contiguity of storage allocated by successive calls to the calloc, malloc, and realloc functions (7.22.3).
+The amount of storage allocated by a successful call to the calloc, malloc, or realloc function when 0 bytes was requested (7.22.3).
+Whether a call to the atexit function that does not happen before the exit function is called will succeed (7.22.4.2).
+Whether a call to the at_quick_exit function that does not happen before the quick_exit function is called will succeed (7.22.4.3).
+Which of two elements that compare as equal is matched by the bsearch function (7.22.5.1).
+The order of two elements that compare as equal in an array sorted by the qsort function (7.22.5.2).
+The encoding of the calendar time returned by the time function (7.27.2.4).
+The characters stored by the strftime or wcsftime function if any of the time values being converted is outside the normal range (7.27.3.5, 7.29.5.1).
+Whether an encoding error occurs if a wchar_t value that does not correspond to a member of the extended character set appears in the format string for a function in 7.29.2 or 7.29.5 and the specified semantics do not require that value to be processed by wcrtomb (7.29.1).
+The conversion state after an encoding error occurs (7.29.6.3.2, 7.29.6.3.3, 7.29.6.4.1, 7.29.6.4.2,
+The resulting value when the ''invalid'' floating-point exception is raised during IEC 60559 floating to integer conversion (F.4).
+Whether conversion of non-integer IEC 60559 floating values to integer raises the ''inexact'' floating-point exception (F.4).
+Whether or when library functions in <math.h> raise the ''inexact'' floating-point exception in an IEC 60559 conformant implementation (F.10).
+Whether or when library functions in <math.h> raise an undeserved ''underflow'' floating-point exception in an IEC 60559 conformant implementation (F.10).
+The exponent value stored by frexp for a NaN or infinity (F.10.3.4).
+The numeric result returned by the lrint, llrint, lround, and llround functions if the rounded value is outside the range of the return type (F.10.6.5, F.10.6.7).
+The sign of one part of the complex result of several math functions for certain special cases in IEC 60559 compatible implementations (G.6.1.1, G.6.2.2, G.6.2.3, G.6.2.4, G.6.2.5, G.6.2.6, G.6.3.1, G.6.4.2).
+Contents
+
+J.2 Undefined behavior
+
+1 The behavior is undefined in the following circumstances:
+
+A ''shall'' or ''shall not'' requirement that appears outside of a constraint is violated (clause 4).
+A nonempty source file does not end in a new-line character which is not immediately preceded by a backslash character or ends in a partial preprocessing token or comment (5.1.1.2).
+Token concatenation produces a character sequence matching the syntax of a universal character name (5.1.1.2).
+A program in a hosted environment does not define a function named main using one of the specified forms (5.1.2.2.1).
+The execution of a program contains a data race (5.1.2.4).
+A character not in the basic source character set is encountered in a source file, except in an identifier, a character constant, a string literal, a header name, a comment, or a preprocessing token that is never converted to a token (5.2.1).
+An identifier, comment, string literal, character constant, or header name contains an invalid multibyte character or does not begin and end in the initial shift state (5.2.1.2).
+The same identifier has both internal and external linkage in the same translation unit (6.2.2).
+An object is referred to outside of its lifetime (6.2.4).
+The value of a pointer to an object whose lifetime has ended is used (6.2.4).
+The value of an object with automatic storage duration is used while it is indeterminate (6.2.4, 6.7.9, 6.8).
+A trap representation is read by an lvalue expression that does not have character type (6.2.6.1).
+A trap representation is produced by a side effect that modifies any part of the object using an lvalue expression that does not have character type (6.2.6.1).
+The operands to certain operators are such that they could produce a negative zero result, but the implementation does not support negative zeros (6.2.6.2).
+Two declarations of the same object or function specify types that are not compatible (6.2.7).
+A program requires the formation of a composite type from a variable length array type whose size is specified by an expression that is not evaluated (6.2.7).
+Conversion to or from an integer type produces a value outside the range that can be represented (6.3.1.4).
+Demotion of one real floating type to another produces a value outside the range that can be represented (6.3.1.5).
+An lvalue does not designate an object when evaluated (6.3.2.1).
+A non-array lvalue with an incomplete type is used in a context that requires the value of the designated object (6.3.2.1).
+An lvalue designating an object of automatic storage duration that could have been declared with the register storage class is used in a context that requires the value of the designated object, but the object is uninitialized. (6.3.2.1).
+An lvalue having array type is converted to a pointer to the initial element of the array, and the array object has register storage class (6.3.2.1).
+An attempt is made to use the value of a void expression, or an implicit or explicit conversion (except to void) is applied to a void expression (6.3.2.2).
+Conversion of a pointer to an integer type produces a value outside the range that can be represented (6.3.2.3).
+Conversion between two pointer types produces a result that is incorrectly aligned (6.3.2.3).
+A pointer is used to call a function whose type is not compatible with the referenced type (6.3.2.3).
+An unmatched ' or " character is encountered on a logical source line during tokenization (6.4).
+A reserved keyword token is used in translation phase 7 or 8 for some purpose other than as a keyword (6.4.1).
+A universal character name in an identifier does not designate a character whose encoding falls into one of the specified ranges (6.4.2.1).
+The initial character of an identifier is a universal character name designating a digit (6.4.2.1).
+Two identifiers differ only in nonsignificant characters (6.4.2.1).
+The identifier __func__ is explicitly declared (6.4.2.2).
+The program attempts to modify a string literal (6.4.5).
+The characters ', \, ", //, or /* occur in the sequence between the < and > delimiters, or the characters ', \, //, or /* occur in the sequence between the " delimiters, in a header name preprocessing token (6.4.7).
+A side effect on a scalar object is unsequenced relative to either a different side effect on the same scalar object or a value computation using the value of the same scalar object (6.5).
+An exceptional condition occurs during the evaluation of an expression (6.5).
+An object has its stored value accessed other than by an lvalue of an allowable type (6.5).
+For a call to a function without a function prototype in scope, the number of arguments does not equal the number of parameters (6.5.2.2).
+For call to a function without a function prototype in scope where the function is defined with a function prototype, either the prototype ends with an ellipsis or the types of the arguments after promotion are not compatible with the types of the parameters (6.5.2.2).
+For a call to a function without a function prototype in scope where the function is not defined with a function prototype, the types of the arguments after promotion are not compatible with those of the parameters after promotion (with certain exceptions) (6.5.2.2).
+A function is defined with a type that is not compatible with the type (of the expression) pointed to by the expression that denotes the called function (6.5.2.2).
+A member of an atomic structure or union is accessed (6.5.2.3).
+The operand of the unary * operator has an invalid value (6.5.3.2).
+A pointer is converted to other than an integer or pointer type (6.5.4).
+The value of the second operand of the / or % operator is zero (6.5.5).
+Addition or subtraction of a pointer into, or just beyond, an array object and an integer type produces a result that does not point into, or just beyond, the same array object (6.5.6).
+Addition or subtraction of a pointer into, or just beyond, an array object and an integer type produces a result that points just beyond the array object and is used as the operand of a unary * operator that is evaluated (6.5.6).
+Pointers that do not point into, or just beyond, the same array object are subtracted (6.5.6).
+An array subscript is out of range, even if an object is apparently accessible with the given subscript (as in the lvalue expression a[1][7] given the declaration int a[4][5]) (6.5.6).
+The result of subtracting two pointers is not representable in an object of type ptrdiff_t (6.5.6).
+An expression is shifted by a negative number or by an amount greater than or equal to the width of the promoted expression (6.5.7).
+An expression having signed promoted type is left-shifted and either the value of the expression is negative or the result of shifting would be not be representable in the promoted type (6.5.7).
+Pointers that do not point to the same aggregate or union (nor just beyond the same array object) are compared using relational operators (6.5.8).
+An object is assigned to an inexactly overlapping object or to an exactly overlapping object with incompatible type (6.5.16.1).
+An expression that is required to be an integer constant expression does not have an integer type; has operands that are not integer constants, enumeration constants, character constants, sizeof expressions whose results are integer constants, _Alignof expressions, or immediately-cast floating constants; or contains casts (outside operands to sizeof and _Alignof operators) other than conversions of arithmetic types to integer types (6.6).
+A constant expression in an initializer is not, or does not evaluate to, one of the following: an arithmetic constant expression, a null pointer constant, an address constant, or an address constant for a complete object type plus or minus an integer constant expression (6.6).
+An arithmetic constant expression does not have arithmetic type; has operands that are not integer constants, floating constants, enumeration constants, character constants, sizeof expressions whose results are integer constants, or _Alignof expressions; or contains casts (outside operands to sizeof or _Alignof operators) other than conversions of arithmetic types to arithmetic types (6.6).
+The value of an object is accessed by an array-subscript [], member-access . or ->, address &, or indirection * operator or a pointer cast in creating an address constant (6.6).
+An identifier for an object is declared with no linkage and the type of the object is incomplete after its declarator, or after its init-declarator if it has an initializer (6.7).
+A function is declared at block scope with an explicit storage-class specifier other than extern (6.7.1).
+A structure or union is defined without any named members (including those specified indirectly via anonymous structures and unions) (6.7.2.1).
+An attempt is made to access, or generate a pointer to just past, a flexible array member of a structure when the referenced object provides no elements for that array (6.7.2.1).
+When the complete type is needed, an incomplete structure or union type is not completed in the same scope by another declaration of the tag that defines the content (6.7.2.3).
+An attempt is made to modify an object defined with a const-qualified type through use of an lvalue with non-const-qualified type (6.7.3).
+An attempt is made to refer to an object defined with a volatile-qualified type through use of an lvalue with non-volatile-qualified type (6.7.3).
+The specification of a function type includes any type qualifiers (6.7.3).
+Two qualified types that are required to be compatible do not have the identically qualified version of a compatible type (6.7.3).
+An object which has been modified is accessed through a restrict-qualified pointer to a const-qualified type, or through a restrict-qualified pointer and another pointer that are not both based on the same object (6.7.3.1).
+A restrict-qualified pointer is assigned a value based on another restricted pointer whose associated block neither began execution before the block associated with this pointer, nor ended before the assignment (6.7.3.1).
+A function with external linkage is declared with an inline function specifier, but is not also defined in the same translation unit (6.7.4).
+A function declared with a _Noreturn function specifier returns to its caller (6.7.4).
+The definition of an object has an alignment specifier and another declaration of that object has a different alignment specifier (6.7.5).
+Declarations of an object in different translation units have different alignment specifiers (6.7.5).
+Two pointer types that are required to be compatible are not identically qualified, or are not pointers to compatible types (6.7.6.1).
+The size expression in an array declaration is not a constant expression and evaluates at program execution time to a nonpositive value (6.7.6.2).
+In a context requiring two array types to be compatible, they do not have compatible element types, or their size specifiers evaluate to unequal values (6.7.6.2).
+A declaration of an array parameter includes the keyword static within the [ and ] and the corresponding argument does not provide access to the first element of an array with at least the specified number of elements (6.7.6.3).
+A storage-class specifier or type qualifier modifies the keyword void as a function parameter type list (6.7.6.3).
+In a context requiring two function types to be compatible, they do not have compatible return types, or their parameters disagree in use of the ellipsis terminator or the number and type of parameters (after default argument promotion, when there is no parameter type list or when one type is specified by a function definition with an identifier list) (6.7.6.3).
+The value of an unnamed member of a structure or union is used (6.7.9).
+The initializer for a scalar is neither a single expression nor a single expression enclosed in braces (6.7.9).
+The initializer for a structure or union object that has automatic storage duration is neither an initializer list nor a single expression that has compatible structure or union type (6.7.9).
+The initializer for an aggregate or union, other than an array initialized by a string literal, is not a brace-enclosed list of initializers for its elements or members (6.7.9).
+An identifier with external linkage is used, but in the program there does not exist exactly one external definition for the identifier, or the identifier is not used and there exist multiple external definitions for the identifier (6.9).
+A function definition includes an identifier list, but the types of the parameters are not declared in a following declaration list (6.9.1).
+An adjusted parameter type in a function definition is not a complete object type (6.9.1).
+A function that accepts a variable number of arguments is defined without a parameter type list that ends with the ellipsis notation (6.9.1).
+The } that terminates a function is reached, and the value of the function call is used by the caller (6.9.1).
+An identifier for an object with internal linkage and an incomplete type is declared with a tentative definition (6.9.2).
+The token defined is generated during the expansion of a #if or #elif preprocessing directive, or the use of the defined unary operator does not match one of the two specified forms prior to macro replacement (6.10.1).
+The #include preprocessing directive that results after expansion does not match one of the two header name forms (6.10.2).
+The character sequence in an #include preprocessing directive does not start with a letter (6.10.2).
+There are sequences of preprocessing tokens within the list of macro arguments that would otherwise act as preprocessing directives (6.10.3).
+The result of the preprocessing operator # is not a valid character string literal (6.10.3.2).
+The result of the preprocessing operator ## is not a valid preprocessing token (6.10.3.3).
+The #line preprocessing directive that results after expansion does not match one of the two well-defined forms, or its digit sequence specifies zero or a number greater than 2147483647 (6.10.4).
+A non-STDC #pragma preprocessing directive that is documented as causing translation failure or some other form of undefined behavior is encountered (6.10.6).
+A #pragma STDC preprocessing directive does not match one of the well-defined forms (6.10.6).
+The name of a predefined macro, or the identifier defined, is the subject of a #define or #undef preprocessing directive (6.10.8).
+An attempt is made to copy an object to an overlapping object by use of a library function, other than as explicitly allowed (e.g., memmove) (clause 7).
+A file with the same name as one of the standard headers, not provided as part of the implementation, is placed in any of the standard places that are searched for included source files (7.1.2).
+A header is included within an external declaration or definition (7.1.2).
+A function, object, type, or macro that is specified as being declared or defined by some standard header is used before any header that declares or defines it is included (7.1.2).
+A standard header is included while a macro is defined with the same name as a keyword (7.1.2).
+The program attempts to declare a library function itself, rather than via a standard header, but the declaration does not have external linkage (7.1.2).
+The program declares or defines a reserved identifier, other than as allowed by 7.1.4 (7.1.3).
+The program removes the definition of a macro whose name begins with an underscore and either an uppercase letter or another underscore (7.1.3).
+An argument to a library function has an invalid value or a type not expected by a function with variable number of arguments (7.1.4).
+The pointer passed to a library function array parameter does not have a value such that all address computations and object accesses are valid (7.1.4).
+The macro definition of assert is suppressed in order to access an actual function (7.2).
+The argument to the assert macro does not have a scalar type (7.2).
+The CX_LIMITED_RANGE, FENV_ACCESS, or FP_CONTRACT pragma is used in any context other than outside all external declarations or preceding all explicit declarations and statements inside a compound statement (7.3.4, 7.6.1, 7.12.2).
+The value of an argument to a character handling function is neither equal to the value of EOF nor representable as an unsigned char (7.4).
+A macro definition of errno is suppressed in order to access an actual object, or the program defines an identifier with the name errno (7.5).
+Part of the program tests floating-point status flags, sets floating-point control modes, or runs under non-default mode settings, but was translated with the state for the FENV_ACCESS pragma ''off'' (7.6.1).
+The exception-mask argument for one of the functions that provide access to the floating-point status flags has a nonzero value not obtained by bitwise OR of the floating-point exception macros (7.6.2).
+The fesetexceptflag function is used to set floating-point status flags that were not specified in the call to the fegetexceptflag function that provided the value of the corresponding fexcept_t object (7.6.2.4).
+The argument to fesetenv or feupdateenv is neither an object set by a call to fegetenv or feholdexcept, nor is it an environment macro (7.6.4.3, 7.6.4.4).
+The value of the result of an integer arithmetic or conversion function cannot be represented (7.8.2.1, 7.8.2.2, 7.8.2.3, 7.8.2.4, 7.22.6.1, 7.22.6.2, 7.22.1).
+The program modifies the string pointed to by the value returned by the setlocale function (7.11.1.1).
+The program modifies the structure pointed to by the value returned by the localeconv function (7.11.2.1).
+A macro definition of math_errhandling is suppressed or the program defines an identifier with the name math_errhandling (7.12).
+An argument to a floating-point classification or comparison macro is not of real floating type (7.12.3, 7.12.14).
+A macro definition of setjmp is suppressed in order to access an actual function, or the program defines an external identifier with the name setjmp (7.13).
+An invocation of the setjmp macro occurs other than in an allowed context (7.13.2.1).
+The longjmp function is invoked to restore a nonexistent environment (7.13.2.1).
+After a longjmp, there is an attempt to access the value of an object of automatic storage duration that does not have volatile-qualified type, local to the function containing the invocation of the corresponding setjmp macro, that was changed between the setjmp invocation and longjmp call (7.13.2.1).
+The program specifies an invalid pointer to a signal handler function (7.14.1.1).
+A signal handler returns when the signal corresponded to a computational exception (7.14.1.1).
+A signal handler called in response to SIGFPE, SIGILL, SIGSEGV, or any other implementation-defined value corresponding to a computational exception returns (7.14.1.1).
+A signal occurs as the result of calling the abort or raise function, and the signal handler calls the raise function (7.14.1.1).
+A signal occurs other than as the result of calling the abort or raise function, and the signal handler refers to an object with static or thread storage duration that is not a lock-free atomic object other than by assigning a value to an object declared as volatile sig_atomic_t, or calls any function in the standard library other than the abort function, the _Exit function, the quick_exit function, or the signal function (for the same signal number) (7.14.1.1).
+The value of errno is referred to after a signal occurred other than as the result of calling the abort or raise function and the corresponding signal handler obtained a SIG_ERR return from a call to the signal function (7.14.1.1).
+A signal is generated by an asynchronous signal handler (7.14.1.1).
+The signal function is used in a multi-threaded program (7.14.1.1).
+A function with a variable number of arguments attempts to access its varying arguments other than through a properly declared and initialized va_list object, or before the va_start macro is invoked (7.16, 7.16.1.1, 7.16.1.4).
+The macro va_arg is invoked using the parameter ap that was passed to a function that invoked the macro va_arg with the same parameter (7.16).
+A macro definition of va_start, va_arg, va_copy, or va_end is suppressed in order to access an actual function, or the program defines an external identifier with the name va_copy or va_end (7.16.1).
+The va_start or va_copy macro is invoked without a corresponding invocation of the va_end macro in the same function, or vice versa (7.16.1, 7.16.1.2, 7.16.1.3, 7.16.1.4).
+The type parameter to the va_arg macro is not such that a pointer to an object of that type can be obtained simply by postfixing a * (7.16.1.1).
+The va_arg macro is invoked when there is no actual next argument, or with a specified type that is not compatible with the promoted type of the actual next argument, with certain exceptions (7.16.1.1).
+The va_copy or va_start macro is called to initialize a va_list that was previously initialized by either macro without an intervening invocation of the va_end macro for the same va_list (7.16.1.2, 7.16.1.4).
+The parameter parmN of a va_start macro is declared with the register storage class, with a function or array type, or with a type that is not compatible with the type that results after application of the default argument promotions (7.16.1.4).
+The member designator parameter of an offsetof macro is an invalid right operand of the . operator for the type parameter, or designates a bit-field (7.19).
+The argument in an instance of one of the integer-constant macros is not a decimal, octal, or hexadecimal constant, or it has a value that exceeds the limits for the corresponding type (7.20.4).
+A byte input/output function is applied to a wide-oriented stream, or a wide character input/output function is applied to a byte-oriented stream (7.21.2).
+Use is made of any portion of a file beyond the most recent wide character written to a wide-oriented stream (7.21.2).
+The value of a pointer to a FILE object is used after the associated file is closed (7.21.3).
+The stream for the fflush function points to an input stream or to an update stream in which the most recent operation was input (7.21.5.2).
+The string pointed to by the mode argument in a call to the fopen function does not exactly match one of the specified character sequences (7.21.5.3).
+An output operation on an update stream is followed by an input operation without an intervening call to the fflush function or a file positioning function, or an input operation on an update stream is followed by an output operation with an intervening call to a file positioning function (7.21.5.3).
+An attempt is made to use the contents of the array that was supplied in a call to the setvbuf function (7.21.5.6).
+There are insufficient arguments for the format in a call to one of the formatted input/output functions, or an argument does not have an appropriate type (7.21.6.1, 7.21.6.2, 7.29.2.1, 7.29.2.2).
+The format in a call to one of the formatted input/output functions or to the strftime or wcsftime function is not a valid multibyte character sequence that begins and ends in its initial shift state (7.21.6.1, 7.21.6.2, 7.27.3.5, 7.29.2.1, 7.29.2.2, 7.29.5.1).
+In a call to one of the formatted output functions, a precision appears with a conversion specifier other than those described (7.21.6.1, 7.29.2.1).
+A conversion specification for a formatted output function uses an asterisk to denote an argument-supplied field width or precision, but the corresponding argument is not provided (7.21.6.1, 7.29.2.1).
+A conversion specification for a formatted output function uses a # or 0 flag with a conversion specifier other than those described (7.21.6.1, 7.29.2.1).
+A conversion specification for one of the formatted input/output functions uses a length modifier with a conversion specifier other than those described (7.21.6.1, 7.21.6.2, 7.29.2.1, 7.29.2.2).
+An s conversion specifier is encountered by one of the formatted output functions, and the argument is missing the null terminator (unless a precision is specified that does not require null termination) (7.21.6.1, 7.29.2.1).
+An n conversion specification for one of the formatted input/output functions includes any flags, an assignment-suppressing character, a field width, or a precision (7.21.6.1, 7.21.6.2, 7.29.2.1, 7.29.2.2).
+A % conversion specifier is encountered by one of the formatted input/output functions, but the complete conversion specification is not exactly %% (7.21.6.1, 7.21.6.2, 7.29.2.1, 7.29.2.2).
+An invalid conversion specification is found in the format for one of the formatted input/output functions, or the strftime or wcsftime function (7.21.6.1, 7.21.6.2, 7.27.3.5, 7.29.2.1, 7.29.2.2, 7.29.5.1).
+The number of characters or wide characters transmitted by a formatted output function (or written to an array, or that would have been written to an array) is greater than INT_MAX (7.21.6.1, 7.29.2.1).
+The number of input items assigned by a formatted input function is greater than INT_MAX (7.21.6.2, 7.29.2.2).
+The result of a conversion by one of the formatted input functions cannot be represented in the corresponding object, or the receiving object does not have an appropriate type (7.21.6.2, 7.29.2.2).
+A c, s, or [ conversion specifier is encountered by one of the formatted input functions, and the array pointed to by the corresponding argument is not large enough to accept the input sequence (and a null terminator if the conversion specifier is s or [) (7.21.6.2, 7.29.2.2).
+A c, s, or [ conversion specifier with an l qualifier is encountered by one of the formatted input functions, but the input is not a valid multibyte character sequence that begins in the initial shift state (7.21.6.2, 7.29.2.2).
+The input item for a %p conversion by one of the formatted input functions is not a value converted earlier during the same program execution (7.21.6.2, 7.29.2.2).
+The vfprintf, vfscanf, vprintf, vscanf, vsnprintf, vsprintf, vsscanf, vfwprintf, vfwscanf, vswprintf, vswscanf, vwprintf, or vwscanf function is called with an improperly initialized va_list argument, or the argument is used (other than in an invocation of va_end) after the function returns (7.21.6.8, 7.21.6.9, 7.21.6.10, 7.21.6.11, 7.21.6.12, 7.21.6.13, 7.21.6.14, 7.29.2.5, 7.29.2.6, 7.29.2.7, 7.29.2.8, 7.29.2.9, 7.29.2.10).
+The contents of the array supplied in a call to the fgets or fgetws function are used after a read error occurred (7.21.7.2, 7.29.3.2).
+The file position indicator for a binary stream is used after a call to the ungetc function where its value was zero before the call (7.21.7.10).
+The file position indicator for a stream is used after an error occurred during a call to the fread or fwrite function (7.21.8.1, 7.21.8.2).
+A partial element read by a call to the fread function is used (7.21.8.1).
+The fseek function is called for a text stream with a nonzero offset and either the offset was not returned by a previous successful call to the ftell function on a stream associated with the same file or whence is not SEEK_SET (7.21.9.2).
+The fsetpos function is called to set a position that was not returned by a previous successful call to the fgetpos function on a stream associated with the same file (7.21.9.3).
+A non-null pointer returned by a call to the calloc, malloc, or realloc function with a zero requested size is used to access an object (7.22.3).
+The value of a pointer that refers to space deallocated by a call to the free or realloc function is used (7.22.3).
+The alignment requested of the aligned_alloc function is not valid or not supported by the implementation, or the size requested is not an integral multiple of the alignment (7.22.3.1).
+The pointer argument to the free or realloc function does not match a pointer earlier returned by a memory management function, or the space has been deallocated by a call to free or realloc (7.22.3.3, 7.22.3.5).
+The value of the object allocated by the malloc function is used (7.22.3.4).
+The value of any bytes in a new object allocated by the realloc function beyond the size of the old object are used (7.22.3.5).
+The program calls the exit or quick_exit function more than once, or calls both functions (7.22.4.4, 7.22.4.7).
+During the call to a function registered with the atexit or at_quick_exit function, a call is made to the longjmp function that would terminate the call to the registered function (7.22.4.4, 7.22.4.7).
+The string set up by the getenv or strerror function is modified by the program (7.22.4.6, 7.24.6.2).
+A signal is raised while the quick_exit function is executing (7.22.4.7).
+A command is executed through the system function in a way that is documented as causing termination or some other form of undefined behavior (7.22.4.8).
+A searching or sorting utility function is called with an invalid pointer argument, even if the number of elements is zero (7.22.5).
+The comparison function called by a searching or sorting utility function alters the contents of the array being searched or sorted, or returns ordering values inconsistently (7.22.5).
+The array being searched by the bsearch function does not have its elements in proper order (7.22.5.1).
+The current conversion state is used by a multibyte/wide character conversion function after changing the LC_CTYPE category (7.22.7).
+A string or wide string utility function is instructed to access an array beyond the end of an object (7.24.1, 7.29.4).
+A string or wide string utility function is called with an invalid pointer argument, even if the length is zero (7.24.1, 7.29.4).
+The contents of the destination array are used after a call to the strxfrm, strftime, wcsxfrm, or wcsftime function in which the specified length was too small to hold the entire null-terminated result (7.24.4.5, 7.27.3.5, 7.29.4.4.4, 7.29.5.1).
+The first argument in the very first call to the strtok or wcstok is a null pointer (7.24.5.8, 7.29.4.5.7).
+The type of an argument to a type-generic macro is not compatible with the type of the corresponding parameter of the selected function (7.25).
+A complex argument is supplied for a generic parameter of a type-generic macro that has no corresponding complex function (7.25).
+At least one member of the broken-down time passed to asctime contains a value outside its normal range, or the calculated year exceeds four digits or is less than the year 1000 (7.27.3.1).
+The argument corresponding to an s specifier without an l qualifier in a call to the fwprintf function does not point to a valid multibyte character sequence that begins in the initial shift state (7.29.2.11).
+In a call to the wcstok function, the object pointed to by ptr does not have the value stored by the previous call for the same wide string (7.29.4.5.7).
+An mbstate_t object is used inappropriately (7.29.6).
+The value of an argument of type wint_t to a wide character classification or case mapping function is neither equal to the value of WEOF nor representable as a wchar_t (7.30.1).
+The iswctype function is called using a different LC_CTYPE category from the one in effect for the call to the wctype function that returned the description (7.30.2.2.1).
+The towctrans function is called using a different LC_CTYPE category from the one in effect for the call to the wctrans function that returned the description (7.30.3.2.1).
+Contents
+
+J.3 Implementation-defined behavior
+
+1 A conforming implementation is required to document its choice of behavior in each of the areas listed in this subclause. The following are implementation-defined:
+
+Contents
+
+J.3.1 Translation
+
+1
+
+How a diagnostic is identified (3.10, 5.1.1.3).
+Whether each nonempty sequence of white-space characters other than new-line is retained or replaced by one space character in translation phase 3 (5.1.1.2).
+Contents
+
+J.3.2 Environment
+
+1
+
+The mapping between physical source file multibyte characters and the source character set in translation phase 1 (5.1.1.2).
+The name and type of the function called at program startup in a freestanding environment (5.1.2.1).
+The effect of program termination in a freestanding environment (5.1.2.1).
+An alternative manner in which the main function may be defined (5.1.2.2.1).
+The values given to the strings pointed to by the argv argument to main (5.1.2.2.1).
+What constitutes an interactive device (5.1.2.3).
+Whether a program can have more than one thread of execution in a freestanding environment (5.1.2.4).
+The set of signals, their semantics, and their default handling (7.14).
+Signal values other than SIGFPE, SIGILL, and SIGSEGV that correspond to a computational exception (7.14.1.1).
+Signals for which the equivalent of signal(sig, SIG_IGN); is executed at program startup (7.14.1.1).
+The set of environment names and the method for altering the environment list used by the getenv function (7.22.4.6).
+The manner of execution of the string by the system function (7.22.4.8).
+Contents
+
+J.3.3 Identifiers
+
+1
+
+Which additional multibyte characters may appear in identifiers and their correspondence to universal character names (6.4.2).
+The number of significant initial characters in an identifier (5.2.4.1, 6.4.2).
+Contents
+
+J.3.4 Characters
+
+1
+
+The number of bits in a byte (3.6).
+The values of the members of the execution character set (5.2.1).
+The unique value of the member of the execution character set produced for each of the standard alphabetic escape sequences (5.2.2).
+The value of a char object into which has been stored any character other than a member of the basic execution character set (6.2.5).
+Which of signed char or unsigned char has the same range, representation, and behavior as ''plain'' char (6.2.5, 6.3.1.1).
+The mapping of members of the source character set (in character constants and string literals) to members of the execution character set (6.4.4.4, 5.1.1.2).
+The value of an integer character constant containing more than one character or containing a character or escape sequence that does not map to a single-byte execution character (6.4.4.4).
+The value of a wide character constant containing more than one multibyte character or a single multibyte character that maps to multiple members of the extended execution character set, or containing a multibyte character or escape sequence not represented in the extended execution character set (6.4.4.4).
+The current locale used to convert a wide character constant consisting of a single multibyte character that maps to a member of the extended execution character set into a corresponding wide character code (6.4.4.4).
+Whether differently-prefixed wide string literal tokens can be concatenated and, if so, the treatment of the resulting multibyte character sequence (6.4.5).
+The current locale used to convert a wide string literal into corresponding wide character codes (6.4.5).
+The value of a string literal containing a multibyte character or escape sequence not represented in the execution character set (6.4.5).
+The encoding of any of wchar_t, char16_t, and char32_t where the corresponding standard encoding macro (__STDC_ISO_10646__, __STDC_UTF_16__, or __STDC_UTF_32__) is not defined (6.10.8.2).
+Contents
+
+J.3.5 Integers
+
+1
+
+Any extended integer types that exist in the implementation (6.2.5).
+Whether signed integer types are represented using sign and magnitude, two's complement, or ones' complement, and whether the extraordinary value is a trap representation or an ordinary value (6.2.6.2).
+The rank of any extended integer type relative to another extended integer type with the same precision (6.3.1.1).
+The result of, or the signal raised by, converting an integer to a signed integer type when the value cannot be represented in an object of that type (6.3.1.3).
+The results of some bitwise operations on signed integers (6.5).
+Contents
+
+J.3.6 Floating point
+
+1
+
+The accuracy of the floating-point operations and of the library functions in <math.h> and <complex.h> that return floating-point results (5.2.4.2.2).
+The accuracy of the conversions between floating-point internal representations and string representations performed by the library functions in <stdio.h>, <stdlib.h>, and <wchar.h> (5.2.4.2.2).
+The rounding behaviors characterized by non-standard values of FLT_ROUNDS (5.2.4.2.2).
+The evaluation methods characterized by non-standard negative values of FLT_EVAL_METHOD (5.2.4.2.2).
+The direction of rounding when an integer is converted to a floating-point number that cannot exactly represent the original value (6.3.1.4).
+The direction of rounding when a floating-point number is converted to a narrower floating-point number (6.3.1.5).
+How the nearest representable value or the larger or smaller representable value immediately adjacent to the nearest representable value is chosen for certain floating constants (6.4.4.2).
+Whether and how floating expressions are contracted when not disallowed by the FP_CONTRACT pragma (6.5).
+The default state for the FENV_ACCESS pragma (7.6.1).
+Additional floating-point exceptions, rounding modes, environments, and classifications, and their macro names (7.6, 7.12).
+The default state for the FP_CONTRACT pragma (7.12.2).
+Contents
+
+J.3.7 Arrays and pointers
+
+1
+
+The result of converting a pointer to an integer or vice versa (6.3.2.3).
+The size of the result of subtracting two pointers to elements of the same array (6.5.6).
+Contents
+
+J.3.8 Hints
+
+1
+
+The extent to which suggestions made by using the register storage-class specifier are effective (6.7.1).
+The extent to which suggestions made by using the inline function specifier are effective (6.7.4).
+Contents
+
+J.3.9 Structures, unions, enumerations, and bit-fields
+
+1
+
+Whether a ''plain'' int bit-field is treated as a signed int bit-field or as an unsigned int bit-field (6.7.2, 6.7.2.1).
+Allowable bit-field types other than _Bool, signed int, and unsigned int (6.7.2.1).
+Whether atomic types are permitted for bit-fields (6.7.2.1).
+Whether a bit-field can straddle a storage-unit boundary (6.7.2.1).
+The order of allocation of bit-fields within a unit (6.7.2.1).
+The alignment of non-bit-field members of structures (6.7.2.1). This should present no problem unless binary data written by one implementation is read by another.
+The integer type compatible with each enumerated type (6.7.2.2).
+Contents
+
+J.3.10 Qualifiers
+
+1
+
+What constitutes an access to an object that has volatile-qualified type (6.7.3).
+Contents
+
+J.3.11 Preprocessing directives
+
+1
+
+The locations within #pragma directives where header name preprocessing tokens are recognized (6.4, 6.4.7).
+How sequences in both forms of header names are mapped to headers or external source file names (6.4.7).
+Whether the value of a character constant in a constant expression that controls conditional inclusion matches the value of the same character constant in the execution character set (6.10.1).
+Whether the value of a single-character character constant in a constant expression that controls conditional inclusion may have a negative value (6.10.1).
+The places that are searched for an included < > delimited header, and how the places are specified or the header is identified (6.10.2).
+How the named source file is searched for in an included " " delimited header (6.10.2).
+The method by which preprocessing tokens (possibly resulting from macro expansion) in a #include directive are combined into a header name (6.10.2).
+The nesting limit for #include processing (6.10.2).
+Whether the # operator inserts a \ character before the \ character that begins a universal character name in a character constant or string literal (6.10.3.2).
+The behavior on each recognized non-STDC #pragma directive (6.10.6).
+The definitions for __DATE__ and __TIME__ when respectively, the date and time of translation are not available (6.10.8.1).
+Contents
+
+J.3.12 Library functions
+
+1
+
+Any library facilities available to a freestanding program, other than the minimal set required by clause 4 (5.1.2.1).
+The format of the diagnostic printed by the assert macro (7.2.1.1).
+The representation of the floating-point status flags stored by the fegetexceptflag function (7.6.2.2).
+Whether the feraiseexcept function raises the ''inexact'' floating-point exception in addition to the ''overflow'' or ''underflow'' floating-point exception (7.6.2.3).
+Strings other than "C" and "" that may be passed as the second argument to the setlocale function (7.11.1.1).
+The types defined for float_t and double_t when the value of the FLT_EVAL_METHOD macro is less than 0 (7.12).
+Domain errors for the mathematics functions, other than those required by this International Standard (7.12.1).
+The values returned by the mathematics functions on domain errors or pole errors (7.12.1).
+The values returned by the mathematics functions on underflow range errors, whether errno is set to the value of the macro ERANGE when the integer expression math_errhandling & MATH_ERRNO is nonzero, and whether the ''underflow'' floating-point exception is raised when the integer expression math_errhandling & MATH_ERREXCEPT is nonzero. (7.12.1).
+Whether a domain error occurs or zero is returned when an fmod function has a second argument of zero (7.12.10.1).
+Whether a domain error occurs or zero is returned when a remainder function has a second argument of zero (7.12.10.2).
+The base-2 logarithm of the modulus used by the remquo functions in reducing the quotient (7.12.10.3).
+Whether a domain error occurs or zero is returned when a remquo function has a second argument of zero (7.12.10.3).
+Whether the equivalent of signal(sig, SIG_DFL); is executed prior to the call of a signal handler, and, if not, the blocking of signals that is performed (7.14.1.1).
+The null pointer constant to which the macro NULL expands (7.19).
+Whether the last line of a text stream requires a terminating new-line character (7.21.2).
+Whether space characters that are written out to a text stream immediately before a new-line character appear when read in (7.21.2).
+The number of null characters that may be appended to data written to a binary stream (7.21.2).
+Whether the file position indicator of an append-mode stream is initially positioned at the beginning or end of the file (7.21.3).
+Whether a write on a text stream causes the associated file to be truncated beyond that point (7.21.3).
+The characteristics of file buffering (7.21.3).
+Whether a zero-length file actually exists (7.21.3).
+The rules for composing valid file names (7.21.3).
+Whether the same file can be simultaneously open multiple times (7.21.3).
+The nature and choice of encodings used for multibyte characters in files (7.21.3).
+The effect of the remove function on an open file (7.21.4.1).
+The effect if a file with the new name exists prior to a call to the rename function (7.21.4.2).
+Whether an open temporary file is removed upon abnormal program termination (7.21.4.3).
+Which changes of mode are permitted (if any), and under what circumstances (7.21.5.4).
+The style used to print an infinity or NaN, and the meaning of any n-char or n-wchar sequence printed for a NaN (7.21.6.1, 7.29.2.1).
+The output for %p conversion in the fprintf or fwprintf function (7.21.6.1, 7.29.2.1).
+The interpretation of a - character that is neither the first nor the last character, nor the second where a ^ character is the first, in the scanlist for %[ conversion in the fscanf or fwscanf function (7.21.6.2, 7.29.2.1).
+The set of sequences matched by a %p conversion and the interpretation of the corresponding input item in the fscanf or fwscanf function (7.21.6.2, 7.29.2.2).
+The value to which the macro errno is set by the fgetpos, fsetpos, or ftell functions on failure (7.21.9.1, 7.21.9.3, 7.21.9.4).
+The meaning of any n-char or n-wchar sequence in a string representing a NaN that is converted by the strtod, strtof, strtold, wcstod, wcstof, or wcstold function (7.22.1.3, 7.29.4.1.1).
+Whether or not the strtod, strtof, strtold, wcstod, wcstof, or wcstold function sets errno to ERANGE when underflow occurs (7.22.1.3, 7.29.4.1.1).
+Whether the calloc, malloc, and realloc functions return a null pointer or a pointer to an allocated object when the size requested is zero (7.22.3).
+Whether open streams with unwritten buffered data are flushed, open streams are closed, or temporary files are removed when the abort or _Exit function is called (7.22.4.1, 7.22.4.5).
+The termination status returned to the host environment by the abort, exit, _Exit, or quick_exit function (7.22.4.1, 7.22.4.4, 7.22.4.5, 7.22.4.7).
+The value returned by the system function when its argument is not a null pointer (7.22.4.8).
+The range and precision of times representable in clock_t and time_t (7.27). *
+The local time zone and Daylight Saving Time (7.27.1).
+The era for the clock function (7.27.2.1).
+The TIME_UTC epoch (7.27.2.5).
+The replacement string for the %Z specifier to the strftime, and wcsftime functions in the "C" locale (7.27.3.5, 7.29.5.1).
+Whether the functions in <math.h> honor the rounding direction mode in an IEC 60559 conformant implementation, unless explicitly specified otherwise (F.10).
+Contents
+
+J.3.13 Architecture
+
+1
+
+The values or expressions assigned to the macros specified in the headers <float.h>, <limits.h>, and <stdint.h> (5.2.4.2, 7.20.2, 7.20.3).
+The result of attempting to indirectly access an object with automatic or thread storage duration from a thread other than the one with which it is associated (6.2.4).
+The number, order, and encoding of bytes in any object (when not explicitly specified in this International Standard) (6.2.6.1).
+Whether any extended alignments are supported and the contexts in which they are supported (6.2.8).
+Valid alignment values other than those returned by an _Alignof expression for fundamental types, if any (6.2.8).
+The value of the result of the sizeof and _Alignof operators (6.5.3.4).
+Contents
+
+J.4 Locale-specific behavior
+
+1 The following characteristics of a hosted environment are locale-specific and are required to be documented by the implementation:
+
+Additional members of the source and execution character sets beyond the basic character set (5.2.1).
+The presence, meaning, and representation of additional multibyte characters in the execution character set beyond the basic character set (5.2.1.2).
+The shift states used for the encoding of multibyte characters (5.2.1.2).
+The direction of writing of successive printing characters (5.2.2).
+The decimal-point character (7.1.1).
+The set of printing characters (7.4, 7.30.2).
+The set of control characters (7.4, 7.30.2).
+The sets of characters tested for by the isalpha, isblank, islower, ispunct, isspace, isupper, iswalpha, iswblank, iswlower, iswpunct, iswspace, or iswupper functions (7.4.1.2, 7.4.1.3, 7.4.1.7, 7.4.1.9, 7.4.1.10, 7.4.1.11, 7.30.2.1.2, 7.30.2.1.3, 7.30.2.1.7, 7.30.2.1.9, 7.30.2.1.10, 7.30.2.1.11).
+The native environment (7.11.1.1).
+Additional subject sequences accepted by the numeric conversion functions (7.22.1, 7.29.4.1).
+The collation sequence of the execution character set (7.24.4.3, 7.29.4.4.2).
+The contents of the error message strings set up by the strerror function (7.24.6.2).
+The formats for time and date (7.27.3.5, 7.29.5.1).
+Character mappings that are supported by the towctrans function (7.30.1).
+Character classifications that are supported by the iswctype function (7.30.1).
+Contents
+
+J.5 Common extensions
+
+1 The following extensions are widely used in many systems, but are not portable to all implementations. The inclusion of any extension that may cause a strictly conforming program to become invalid renders an implementation nonconforming. Examples of such extensions are new keywords, extra library functions declared in standard headers, or predefined macros with names that do not begin with an underscore.
+
+Contents
+
+J.5.1 Environment arguments
+
+1 In a hosted environment, the main function receives a third argument, char *envp[], that points to a null-terminated array of pointers to char, each of which points to a string that provides information about the environment for this execution of the program (5.1.2.2.1).
+
+Contents
+
+J.5.2 Specialized identifiers
+
+1 Characters other than the underscore _, letters, and digits, that are not part of the basic source character set (such as the dollar sign $, or characters in national character sets) may appear in an identifier (6.4.2).
+
+Contents
+
+J.5.3 Lengths and cases of identifiers
+
+1 All characters in identifiers (with or without external linkage) are significant (6.4.2).
+
+Contents
+
+J.5.4 Scopes of identifiers
+
+1 A function identifier, or the identifier of an object the declaration of which contains the keyword extern, has file scope (6.2.1).
+
+Contents
+
+J.5.5 Writable string literals
+
+1 String literals are modifiable (in which case, identical string literals should denote distinct objects) (6.4.5).
+
+Contents
+
+J.5.6 Other arithmetic types
+
+1 Additional arithmetic types, such as __int128 or double double, and their appropriate conversions are defined (6.2.5, 6.3.1). Additional floating types may have more range or precision than long double, may be used for evaluating expressions of other floating types, and may be used to define float_t or double_t. Additional floating types may also have less range or precision than float.
+
+Contents
+
+J.5.7 Function pointer casts
+
+1 A pointer to an object or to void may be cast to a pointer to a function, allowing data to be invoked as a function (6.5.4).
+
+2 A pointer to a function may be cast to a pointer to an object or to void, allowing a function to be inspected or modified (for example, by a debugger) (6.5.4).
+
+Contents
+
+J.5.8 Extended bit-field types
+
+1 A bit-field may be declared with a type other than _Bool, unsigned int, or signed int, with an appropriate maximum width (6.7.2.1).
+
+Contents
+
+J.5.9 The fortran keyword
+
+1 The fortran function specifier may be used in a function declaration to indicate that calls suitable for FORTRAN should be generated, or that a different representation for the external name is to be generated (6.7.4).
+
+Contents
+
+J.5.10 The asm keyword
+
+1 The asm keyword may be used to insert assembly language directly into the translator output (6.8). The most common implementation is via a statement of the form:
+
+ asm ( character-string-literal );
+Contents
+
+J.5.11 Multiple external definitions
+
+1 There may be more than one external definition for the identifier of an object, with or without the explicit use of the keyword extern; if the definitions disagree, or more than one is initialized, the behavior is undefined (6.9.2).
+
+Contents
+
+J.5.12 Predefined macro names
+
+1 Macro names that do not begin with an underscore, describing the translation and execution environments, are defined by the implementation before translation begins (6.10.8).
+
+Contents
+
+J.5.13 Floating-point status flags
+
+1 If any floating-point status flags are set on normal termination after all calls to functions registered by the atexit function have been made (see 7.22.4.4), the implementation writes some diagnostics indicating the fact to the stderr stream, if it is still open,
+
+Contents
+
+J.5.14 Extra arguments for signal handlers
+
+1 Handlers for specific signals are called with extra arguments in addition to the signal number (7.14.1.1).
+
+Contents
+
+J.5.15 Additional stream types and file-opening modes
+
+1 Additional mappings from files to streams are supported (7.21.2).
+
+2 Additional file-opening modes may be specified by characters appended to the mode argument of the fopen function (7.21.5.3).
+
+Contents
+
+J.5.16 Defined file position indicator
+
+1 The file position indicator is decremented by each successful call to the ungetc or ungetwc function for a text stream, except if its value was zero before a call (7.21.7.10, 7.29.3.10).
+
+Contents
+
+J.5.17 Math error reporting
+
+1 Functions declared in <complex.h> and <math.h> raise SIGFPE to report errors instead of, or in addition to, setting errno or raising floating-point exceptions (7.3, 7.12).
+
+
diff --git a/doc/std/CHAPTER-5.txt b/doc/std/CHAPTER-5.txt
@@ -0,0 +1,1678 @@
+5. Environment
+
+
+1
+ An implementation translates C source files and executes C programs in two data-
+ processing-system environments, which will be called the translation environment and
+ the execution environment in this International Standard. Their characteristics define and
+ constrain the results of executing conforming C programs constructed according to the
+ syntactic and semantic rules for conforming implementations.
+
+ Forward references: In this clause, only a few of many possible forward references
+ have been noted.
+
+
+Contents
+
+5.1 Conceptual models
+
+
+Contents
+
+5.1.1 Translation environment
+
+
+Contents
+
+5.1.1.1 Program structure
+
+
+1
+ A C program need not all be translated at the same time. The text of the program is kept
+ in units called source files, (or preprocessing files) in this International Standard. A
+ source file together with all the headers and source files included via the preprocessing
+ directive #include is known as a preprocessing translation unit. After preprocessing, a
+ preprocessing translation unit is called a translation unit. Previously translated translation
+ units may be preserved individually or in libraries. The separate translation units of a
+ program communicate by (for example) calls to functions whose identifiers have external
+ linkage, manipulation of objects whose identifiers have external linkage, or manipulation
+ of data files. Translation units may be separately translated and then later linked to
+ produce an executable program.
+
+ Forward references: linkages of identifiers (6.2.2), external definitions (6.9),
+ preprocessing directives (6.10).
+
+
+Contents
+
+5.1.1.2 Translation phases
+
+
+1
+ The precedence among the syntax rules of translation is specified by the following
+ phases.6)
+
+
+- Physical source file multibyte characters are mapped, in an implementation-
+ defined manner, to the source character set (introducing new-line characters for
+ end-of-line indicators) if necessary. Trigraph sequences are replaced by
+ corresponding single-character internal representations.
+
+
+
+
+
+- Each instance of a backslash character (\) immediately followed by a new-line
+ character is deleted, splicing physical source lines to form logical source lines.
+ Only the last backslash on any physical source line shall be eligible for being part
+ of such a splice. A source file that is not empty shall end in a new-line character,
+ which shall not be immediately preceded by a backslash character before any such
+ splicing takes place.
+
+- The source file is decomposed into preprocessing tokens7) and sequences of
+ white-space characters (including comments). A source file shall not end in a
+ partial preprocessing token or in a partial comment. Each comment is replaced by
+ one space character. New-line characters are retained. Whether each nonempty
+ sequence of white-space characters other than new-line is retained or replaced by
+ one space character is implementation-defined.
+
+- Preprocessing directives are executed, macro invocations are expanded, and
+ _Pragma unary operator expressions are executed. If a character sequence that
+ matches the syntax of a universal character name is produced by token
+ concatenation (6.10.3.3), the behavior is undefined. A #include preprocessing
+ directive causes the named header or source file to be processed from phase 1
+ through phase 4, recursively. All preprocessing directives are then deleted.
+
+- Each source character set member and escape sequence in character constants and
+ string literals is converted to the corresponding member of the execution character
+ set; if there is no corresponding member, it is converted to an implementation-
+ defined member other than the null (wide) character.8)
+
+- Adjacent string literal tokens are concatenated.
+
+- White-space characters separating tokens are no longer significant. Each
+ preprocessing token is converted into a token. The resulting tokens are
+ syntactically and semantically analyzed and translated as a translation unit.
+
+- All external object and function references are resolved. Library components are
+ linked to satisfy external references to functions and objects not defined in the
+ current translation. All such translator output is collected into a program image
+ which contains information needed for execution in its execution environment.
+
+
+ Forward references: universal character names (6.4.3), lexical elements (6.4),
+ preprocessing directives (6.10), trigraph sequences (5.2.1.1), external definitions (6.9).
+
+
+
+
+
+Footnotes
+
+6) Implementations shall behave as if these separate phases occur, even though many are typically folded
+ together in practice. Source files, translation units, and translated translation units need not
+ necessarily be stored as files, nor need there be any one-to-one correspondence between these entities
+ and any external representation. The description is conceptual only, and does not specify any
+ particular implementation.
+
+
+7) As described in 6.4, the process of dividing a source file's characters into preprocessing tokens is
+ context-dependent. For example, see the handling of < within a #include preprocessing directive.
+
+
+8) An implementation need not convert all non-corresponding source characters to the same execution
+ character.
+
+
+Contents
+
+5.1.1.3 Diagnostics
+
+
+1
+ A conforming implementation shall produce at least one diagnostic message (identified in
+ an implementation-defined manner) if a preprocessing translation unit or translation unit
+ contains a violation of any syntax rule or constraint, even if the behavior is also explicitly
+ specified as undefined or implementation-defined. Diagnostic messages need not be
+ produced in other circumstances.9)
+
+2
+ EXAMPLE An implementation shall issue a diagnostic for the translation unit:
+
+
+ char i;
+ int i;
+
+
+ because in those cases where wording in this International Standard describes the behavior for a construct
+ as being both a constraint error and resulting in undefined behavior, the constraint error shall be diagnosed.
+
+
+
+Footnotes
+
+9) The intent is that an implementation should identify the nature of, and where possible localize, each
+ violation. Of course, an implementation is free to produce any number of diagnostics as long as a
+ valid program is still correctly translated. It may also successfully translate an invalid program.
+
+
+Contents
+
+5.1.2 Execution environments
+
+
+1
+ Two execution environments are defined: freestanding and hosted. In both cases,
+ program startup occurs when a designated C function is called by the execution
+ environment. All objects with static storage duration shall be initialized (set to their
+ initial values) before program startup. The manner and timing of such initialization are
+ otherwise unspecified. Program termination returns control to the execution
+ environment.
+
+ Forward references: storage durations of objects (6.2.4), initialization (6.7.9).
+
+
+Contents
+
+5.1.2.1 Freestanding environment
+
+
+1
+ In a freestanding environment (in which C program execution may take place without any
+ benefit of an operating system), the name and type of the function called at program
+ startup are implementation-defined. Any library facilities available to a freestanding
+ program, other than the minimal set required by clause 4, are implementation-defined.
+
+2
+ The effect of program termination in a freestanding environment is implementation-
+ defined.
+
+
+Contents
+
+5.1.2.2 Hosted environment
+
+
+1
+ A hosted environment need not be provided, but shall conform to the following
+ specifications if present.
+
+
+
+
+
+
+Contents
+
+5.1.2.2.1 Program startup
+
+
+1
+ The function called at program startup is named main. The implementation declares no
+ prototype for this function. It shall be defined with a return type of int and with no
+ parameters:
+
+
+ int main(void) { /* ... */ }
+
+
+ or with two parameters (referred to here as argc and argv, though any names may be
+ used, as they are local to the function in which they are declared):
+
+
+ int main(int argc, char *argv[]) { /* ... */ }
+
+
+ or equivalent;10) or in some other implementation-defined manner.
+
+2
+ If they are declared, the parameters to the main function shall obey the following
+ constraints:
+
+
+- The value of argc shall be nonnegative.
+
+- argv[argc] shall be a null pointer.
+
+- If the value of argc is greater than zero, the array members argv[0] through
+ argv[argc-1] inclusive shall contain pointers to strings, which are given
+ implementation-defined values by the host environment prior to program startup. The
+ intent is to supply to the program information determined prior to program startup
+ from elsewhere in the hosted environment. If the host environment is not capable of
+ supplying strings with letters in both uppercase and lowercase, the implementation
+ shall ensure that the strings are received in lowercase.
+
+- If the value of argc is greater than zero, the string pointed to by argv[0]
+ represents the program name; argv[0][0] shall be the null character if the
+ program name is not available from the host environment. If the value of argc is
+ greater than one, the strings pointed to by argv[1] through argv[argc-1]
+ represent the program parameters.
+
+- The parameters argc and argv and the strings pointed to by the argv array shall
+ be modifiable by the program, and retain their last-stored values between program
+ startup and program termination.
+
+
+Footnotes
+
+10) Thus, int can be replaced by a typedef name defined as int, or the type of argv can be written as
+ char ** argv, and so on.
+
+
+Contents
+
+5.1.2.2.2 Program execution
+
+
+1
+ In a hosted environment, a program may use all the functions, macros, type definitions,
+ and objects described in the library clause (clause 7).
+
+
+
+
+
+
+Contents
+
+5.1.2.2.3 Program termination
+
+
+1
+ If the return type of the main function is a type compatible with int, a return from the
+ initial call to the main function is equivalent to calling the exit function with the value
+ returned by the main function as its argument;11) reaching the } that terminates the
+ main function returns a value of 0. If the return type is not compatible with int, the
+ termination status returned to the host environment is unspecified.
+
+ Forward references: definition of terms (7.1.1), the exit function (7.22.4.4).
+
+
+Footnotes
+
+11) In accordance with 6.2.4, the lifetimes of objects with automatic storage duration declared in main
+ will have ended in the former case, even where they would not have in the latter.
+
+
+Contents
+
+5.1.2.3 Program execution
+
+
+1
+ The semantic descriptions in this International Standard describe the behavior of an
+ abstract machine in which issues of optimization are irrelevant.
+
+2
+ Accessing a volatile object, modifying an object, modifying a file, or calling a function
+ that does any of those operations are all side effects,12) which are changes in the state of
+ the execution environment. Evaluation of an expression in general includes both value
+ computations and initiation of side effects. Value computation for an lvalue expression
+ includes determining the identity of the designated object.
+
+3
+ Sequenced before is an asymmetric, transitive, pair-wise relation between evaluations
+ executed by a single thread, which induces a partial order among those evaluations.
+ Given any two evaluations A and B, if A is sequenced before B, then the execution of A
+ shall precede the execution of B. (Conversely, if A is sequenced before B, then B is
+ sequenced after A.) If A is not sequenced before or after B, then A and B are
+ unsequenced. Evaluations A and B are indeterminately sequenced when A is sequenced
+ either before or after B, but it is unspecified which.13) The presence of a sequence point
+ between the evaluation of expressions A and B implies that every value computation and
+ side effect associated with A is sequenced before every value computation and side effect
+ associated with B. (A summary of the sequence points is given in annex C.)
+
+4
+ In the abstract machine, all expressions are evaluated as specified by the semantics. An
+ actual implementation need not evaluate part of an expression if it can deduce that its
+ value is not used and that no needed side effects are produced (including any caused by
+
+
+ calling a function or accessing a volatile object).
+
+5
+ When the processing of the abstract machine is interrupted by receipt of a signal, the
+ values of objects that are neither lock-free atomic objects nor of type volatile
+ sig_atomic_t are unspecified, as is the state of the floating-point environment. The
+ value of any object modified by the handler that is neither a lock-free atomic object nor of
+ type volatile sig_atomic_t becomes indeterminate when the handler exits, as
+ does the state of the floating-point environment if it is modified by the handler and not
+ restored to its original state.
+
+6
+ The least requirements on a conforming implementation are:
+
+
+- Accesses to volatile objects are evaluated strictly according to the rules of the abstract
+ machine.
+
+- At program termination, all data written into files shall be identical to the result that
+ execution of the program according to the abstract semantics would have produced.
+
+- The input and output dynamics of interactive devices shall take place as specified in
+ 7.21.3. The intent of these requirements is that unbuffered or line-buffered output
+ appear as soon as possible, to ensure that prompting messages actually appear prior to
+ a program waiting for input.
+
+ This is the observable behavior of the program.
+
+7
+ What constitutes an interactive device is implementation-defined.
+
+8
+ More stringent correspondences between abstract and actual semantics may be defined by
+ each implementation.
+
+9
+ EXAMPLE 1 An implementation might define a one-to-one correspondence between abstract and actual
+ semantics: at every sequence point, the values of the actual objects would agree with those specified by the
+ abstract semantics. The keyword volatile would then be redundant.
+
+10
+ Alternatively, an implementation might perform various optimizations within each translation unit, such
+ that the actual semantics would agree with the abstract semantics only when making function calls across
+ translation unit boundaries. In such an implementation, at the time of each function entry and function
+ return where the calling function and the called function are in different translation units, the values of all
+ externally linked objects and of all objects accessible via pointers therein would agree with the abstract
+ semantics. Furthermore, at the time of each such function entry the values of the parameters of the called
+ function and of all objects accessible via pointers therein would agree with the abstract semantics. In this
+ type of implementation, objects referred to by interrupt service routines activated by the signal function
+ would require explicit specification of volatile storage, as well as other implementation-defined
+ restrictions.
+
+
+11
+ EXAMPLE 2 In executing the fragment
+
+
+ char c1, c2;
+ /* ... */
+ c1 = c1 + c2;
+
+
+ the ''integer promotions'' require that the abstract machine promote the value of each variable to int size
+ and then add the two ints and truncate the sum. Provided the addition of two chars can be done without
+
+ overflow, or with overflow wrapping silently to produce the correct result, the actual execution need only
+ produce the same result, possibly omitting the promotions.
+
+
+12
+ EXAMPLE 3 Similarly, in the fragment
+
+
+ float f1, f2;
+ double d;
+ /* ... */
+ f1 = f2 * d;
+
+
+ the multiplication may be executed using single-precision arithmetic if the implementation can ascertain
+ that the result would be the same as if it were executed using double-precision arithmetic (for example, if d
+ were replaced by the constant 2.0, which has type double).
+
+
+13
+ EXAMPLE 4 Implementations employing wide registers have to take care to honor appropriate
+ semantics. Values are independent of whether they are represented in a register or in memory. For
+ example, an implicit spilling of a register is not permitted to alter the value. Also, an explicit store and load
+ is required to round to the precision of the storage type. In particular, casts and assignments are required to
+ perform their specified conversion. For the fragment
+
+
+ double d1, d2;
+ float f;
+ d1 = f = expression;
+ d2 = (float) expression;
+
+
+ the values assigned to d1 and d2 are required to have been converted to float.
+
+
+14
+ EXAMPLE 5 Rearrangement for floating-point expressions is often restricted because of limitations in
+ precision as well as range. The implementation cannot generally apply the mathematical associative rules
+ for addition or multiplication, nor the distributive rule, because of roundoff error, even in the absence of
+ overflow and underflow. Likewise, implementations cannot generally replace decimal constants in order to
+ rearrange expressions. In the following fragment, rearrangements suggested by mathematical rules for real
+ numbers are often not valid (see F.9).
+
+
+ double x, y, z;
+ /* ... */
+ x = (x * y) * z; // not equivalent to x *= y * z;
+ z = (x - y) + y ; // not equivalent to z = x;
+ z = x + x * y; // not equivalent to z = x * (1.0 + y);
+ y = x / 5.0; // not equivalent to y = x * 0.2;
+
+
+
+
+15
+ EXAMPLE 6 To illustrate the grouping behavior of expressions, in the following fragment
+
+
+ int a, b;
+ /* ... */
+ a = a + 32760 + b + 5;
+
+
+ the expression statement behaves exactly the same as
+
+
+ a = (((a + 32760) + b) + 5);
+
+
+ due to the associativity and precedence of these operators. Thus, the result of the sum (a + 32760) is
+ next added to b, and that result is then added to 5 which results in the value assigned to a. On a machine in
+ which overflows produce an explicit trap and in which the range of values representable by an int is
+ [-32768, +32767], the implementation cannot rewrite this expression as
+
+
+ a = ((a + b) + 32765);
+
+
+ since if the values for a and b were, respectively, -32754 and -15, the sum a + b would produce a trap
+
+ while the original expression would not; nor can the expression be rewritten either as
+
+
+ a = ((a + 32765) + b);
+
+
+ or
+
+
+ a = (a + (b + 32765));
+
+
+ since the values for a and b might have been, respectively, 4 and -8 or -17 and 12. However, on a machine
+ in which overflow silently generates some value and where positive and negative overflows cancel, the
+ above expression statement can be rewritten by the implementation in any of the above ways because the
+ same result will occur.
+
+
+16
+ EXAMPLE 7 The grouping of an expression does not completely determine its evaluation. In the
+ following fragment
+
+
+ #include <stdio.h>
+ int sum;
+ char *p;
+ /* ... */
+ sum = sum * 10 - '0' + (*p++ = getchar());
+
+
+ the expression statement is grouped as if it were written as
+
+
+ sum = (((sum * 10) - '0') + ((*(p++)) = (getchar())));
+
+
+ but the actual increment of p can occur at any time between the previous sequence point and the next
+ sequence point (the ;), and the call to getchar can occur at any point prior to the need of its returned
+ value.
+
+
+ Forward references: expressions (6.5), type qualifiers (6.7.3), statements (6.8), floating-
+ point environment <fenv.h> (7.6), the signal function (7.14), files (7.21.3).
+
+
+Footnotes
+
+12) The IEC 60559 standard for binary floating-point arithmetic requires certain user-accessible status
+ flags and control modes. Floating-point operations implicitly set the status flags; modes affect result
+ values of floating-point operations. Implementations that support such floating-point state are
+ required to regard changes to it as side effects -- see annex F for details. The floating-point
+ environment library <fenv.h> provides a programming facility for indicating when these side
+ effects matter, freeing the implementations in other cases.
+
+
+13) The executions of unsequenced evaluations can interleave. Indeterminately sequenced evaluations
+ cannot interleave, but can be executed in any order.
+
+
+Contents
+
+5.1.2.4 Multi-threaded executions and data races
+
+
+1
+ Under a hosted implementation, a program can have more than one thread of execution
+ (or thread) running concurrently. The execution of each thread proceeds as defined by
+ the remainder of this standard. The execution of the entire program consists of an
+ execution of all of its threads.14) Under a freestanding implementation, it is
+ implementation-defined whether a program can have more than one thread of execution.
+
+2
+ The value of an object visible to a thread T at a particular point is the initial value of the
+ object, a value stored in the object by T , or a value stored in the object by another thread,
+ according to the rules below.
+
+3
+ NOTE 1 In some cases, there may instead be undefined behavior. Much of this section is motivated by
+ the desire to support atomic operations with explicit and detailed visibility constraints. However, it also
+ implicitly supports a simpler view for more restricted programs.
+
+
+4
+ Two expression evaluations conflict if one of them modifies a memory location and the
+ other one reads or modifies the same memory location.
+
+
+
+
+5
+ The library defines a number of atomic operations (7.17) and operations on mutexes
+ (7.26.4) that are specially identified as synchronization operations. These operations play
+ a special role in making assignments in one thread visible to another. A synchronization
+ operation on one or more memory locations is either an acquire operation, a release
+ operation, both an acquire and release operation, or a consume operation. A
+ synchronization operation without an associated memory location is a fence and can be
+ either an acquire fence, a release fence, or both an acquire and release fence. In addition,
+ there are relaxed atomic operations, which are not synchronization operations, and
+ atomic read-modify-write operations, which have special characteristics.
+
+6
+ NOTE 2 For example, a call that acquires a mutex will perform an acquire operation on the locations
+ composing the mutex. Correspondingly, a call that releases the same mutex will perform a release
+ operation on those same locations. Informally, performing a release operation on A forces prior side effects
+ on other memory locations to become visible to other threads that later perform an acquire or consume
+ operation on A. We do not include relaxed atomic operations as synchronization operations although, like
+ synchronization operations, they cannot contribute to data races.
+
+
+7
+ All modifications to a particular atomic object M occur in some particular total order,
+ called the modification order of M. If A and B are modifications of an atomic object M,
+ and A happens before B, then A shall precede B in the modification order of M, which is
+ defined below.
+
+8
+ NOTE 3 This states that the modification orders must respect the ''happens before'' relation.
+
+
+9
+ NOTE 4 There is a separate order for each atomic object. There is no requirement that these can be
+ combined into a single total order for all objects. In general this will be impossible since different threads
+ may observe modifications to different variables in inconsistent orders.
+
+
+10
+ A release sequence headed by a release operation A on an atomic object M is a maximal
+ contiguous sub-sequence of side effects in the modification order of M, where the first
+ operation is A and every subsequent operation either is performed by the same thread that
+ performed the release or is an atomic read-modify-write operation.
+
+11
+ Certain library calls synchronize with other library calls performed by another thread. In
+ particular, an atomic operation A that performs a release operation on an object M
+ synchronizes with an atomic operation B that performs an acquire operation on M and
+ reads a value written by any side effect in the release sequence headed by A.
+
+12
+ NOTE 5 Except in the specified cases, reading a later value does not necessarily ensure visibility as
+ described below. Such a requirement would sometimes interfere with efficient implementation.
+
+
+13
+ NOTE 6 The specifications of the synchronization operations define when one reads the value written by
+ another. For atomic variables, the definition is clear. All operations on a given mutex occur in a single total
+ order. Each mutex acquisition ''reads the value written'' by the last mutex release.
+
+
+14
+ An evaluation A carries a dependency 15) to an evaluation B if:
+
+
+
+
+- the value of A is used as an operand of B, unless:
+
+
+- B is an invocation of the kill_dependency macro,
+
+
+- A is the left operand of a && or || operator,
+
+
+- A is the left operand of a ? : operator, or
+
+
+- A is the left operand of a , operator;
+
+ or
+
+- A writes a scalar object or bit-field M, B reads from M the value written by A, and A
+ is sequenced before B, or
+
+- for some evaluation X, A carries a dependency to X and X carries a dependency to B.
+
+
+15
+ An evaluation A is dependency-ordered before16) an evaluation B if:
+
+
+- A performs a release operation on an atomic object M, and, in another thread, B
+ performs a consume operation on M and reads a value written by any side effect in
+ the release sequence headed by A, or
+
+- for some evaluation X, A is dependency-ordered before X and X carries a
+ dependency to B.
+
+
+16
+ An evaluation A inter-thread happens before an evaluation B if A synchronizes with B, A
+ is dependency-ordered before B, or, for some evaluation X:
+
+
+- A synchronizes with X and X is sequenced before B,
+
+- A is sequenced before X and X inter-thread happens before B, or
+
+- A inter-thread happens before X and X inter-thread happens before B.
+
+
+17
+ NOTE 7 The ''inter-thread happens before'' relation describes arbitrary concatenations of ''sequenced
+ before'', ''synchronizes with'', and ''dependency-ordered before'' relationships, with two exceptions. The
+ first exception is that a concatenation is not permitted to end with ''dependency-ordered before'' followed
+ by ''sequenced before''. The reason for this limitation is that a consume operation participating in a
+ ''dependency-ordered before'' relationship provides ordering only with respect to operations to which this
+ consume operation actually carries a dependency. The reason that this limitation applies only to the end of
+ such a concatenation is that any subsequent release operation will provide the required ordering for a prior
+ consume operation. The second exception is that a concatenation is not permitted to consist entirely of
+ ''sequenced before''. The reasons for this limitation are (1) to permit ''inter-thread happens before'' to be
+ transitively closed and (2) the ''happens before'' relation, defined below, provides for relationships
+ consisting entirely of ''sequenced before''.
+
+
+18
+ An evaluation A happens before an evaluation B if A is sequenced before B or A inter-
+ thread happens before B.
+
+
+
+
+
+19
+ A visible side effect A on an object M with respect to a value computation B of M
+ satisfies the conditions:
+
+
+- A happens before B, and
+
+- there is no other side effect X to M such that A happens before X and X happens
+ before B.
+
+ The value of a non-atomic scalar object M, as determined by evaluation B, shall be the
+ value stored by the visible side effect A.
+
+20
+ NOTE 8 If there is ambiguity about which side effect to a non-atomic object is visible, then there is a data
+ race and the behavior is undefined.
+
+
+21
+ NOTE 9 This states that operations on ordinary variables are not visibly reordered. This is not actually
+ detectable without data races, but it is necessary to ensure that data races, as defined here, and with suitable
+ restrictions on the use of atomics, correspond to data races in a simple interleaved (sequentially consistent)
+ execution.
+
+
+22
+ The visible sequence of side effects on an atomic object M, with respect to a value
+ computation B of M, is a maximal contiguous sub-sequence of side effects in the
+ modification order of M, where the first side effect is visible with respect to B, and for
+ every subsequent side effect, it is not the case that B happens before it. The value of an
+ atomic object M, as determined by evaluation B, shall be the value stored by some
+ operation in the visible sequence of M with respect to B. Furthermore, if a value
+ computation A of an atomic object M happens before a value computation B of M, and
+ the value computed by A corresponds to the value stored by side effect X, then the value
+ computed by B shall either equal the value computed by A, or be the value stored by side
+ effect Y , where Y follows X in the modification order of M.
+
+23
+ NOTE 10 This effectively disallows compiler reordering of atomic operations to a single object, even if
+ both operations are ''relaxed'' loads. By doing so, we effectively make the ''cache coherence'' guarantee
+ provided by most hardware available to C atomic operations.
+
+
+24
+ NOTE 11 The visible sequence depends on the ''happens before'' relation, which in turn depends on the
+ values observed by loads of atomics, which we are restricting here. The intended reading is that there must
+ exist an association of atomic loads with modifications they observe that, together with suitably chosen
+ modification orders and the ''happens before'' relation derived as described above, satisfy the resulting
+ constraints as imposed here.
+
+
+25
+ The execution of a program contains a data race if it contains two conflicting actions in
+ different threads, at least one of which is not atomic, and neither happens before the
+ other. Any such data race results in undefined behavior.
+
+26
+ NOTE 12 It can be shown that programs that correctly use simple mutexes and
+ memory_order_seq_cst operations to prevent all data races, and use no other synchronization
+ operations, behave as though the operations executed by their constituent threads were simply interleaved,
+ with each value computation of an object being the last value stored in that interleaving. This is normally
+ referred to as ''sequential consistency''. However, this applies only to data-race-free programs, and data-
+ race-free programs cannot observe most program transformations that do not change single-threaded
+ program semantics. In fact, most single-threaded program transformations continue to be allowed, since
+ any program that behaves differently as a result must contain undefined behavior.
+
+
+27
+ NOTE 13 Compiler transformations that introduce assignments to a potentially shared memory location
+ that would not be modified by the abstract machine are generally precluded by this standard, since such an
+ assignment might overwrite another assignment by a different thread in cases in which an abstract machine
+ execution would not have encountered a data race. This includes implementations of data member
+ assignment that overwrite adjacent members in separate memory locations. We also generally preclude
+ reordering of atomic loads in cases in which the atomics in question may alias, since this may violate the
+ "visible sequence" rules.
+
+
+28
+ NOTE 14 Transformations that introduce a speculative read of a potentially shared memory location may
+ not preserve the semantics of the program as defined in this standard, since they potentially introduce a data
+ race. However, they are typically valid in the context of an optimizing compiler that targets a specific
+ machine with well-defined semantics for data races. They would be invalid for a hypothetical machine that
+ is not tolerant of races or provides hardware race detection.
+
+
+Footnotes
+
+14) The execution can usually be viewed as an interleaving of all of the threads. However, some kinds of
+ atomic operations, for example, allow executions inconsistent with a simple interleaving as described
+ below.
+
+
+15) The ''carries a dependency'' relation is a subset of the ''sequenced before'' relation, and is similarly
+ strictly intra-thread.
+
+
+16) The ''dependency-ordered before'' relation is analogous to the ''synchronizes with'' relation, but uses
+ release/consume in place of release/acquire.
+
+
+Contents
+
+5.2 Environmental considerations
+
+
+Contents
+
+5.2.1 Character sets
+
+
+1
+ Two sets of characters and their associated collating sequences shall be defined: the set in
+ which source files are written (the source character set), and the set interpreted in the
+ execution environment (the execution character set). Each set is further divided into a
+ basic character set, whose contents are given by this subclause, and a set of zero or more
+ locale-specific members (which are not members of the basic character set) called
+ extended characters. The combined set is also called the extended character set. The
+ values of the members of the execution character set are implementation-defined.
+
+2
+ In a character constant or string literal, members of the execution character set shall be
+ represented by corresponding members of the source character set or by escape
+ sequences consisting of the backslash \ followed by one or more characters. A byte with
+ all bits set to 0, called the null character, shall exist in the basic execution character set; it
+ is used to terminate a character string.
+
+3
+ Both the basic source and basic execution character sets shall have the following
+ members: the 26 uppercase letters of the Latin alphabet
+
+
+ A B C D E F G H I J K L M
+ N O P Q R S T U V W X Y Z
+
+
+ the 26 lowercase letters of the Latin alphabet
+
+
+ a b c d e f g h i j k l m
+ n o p q r s t u v w x y z
+
+
+ the 10 decimal digits
+
+
+ 0 1 2 3 4 5 6 7 8 9
+
+
+ the following 29 graphic characters
+
+
+ ! " # % & ' ( ) * + , - . / :
+ ; < = > ? [ \ ] ^ _ { | } ~
+
+
+ the space character, and control characters representing horizontal tab, vertical tab, and
+ form feed. The representation of each member of the source and execution basic
+ character sets shall fit in a byte. In both the source and execution basic character sets, the
+ value of each character after 0 in the above list of decimal digits shall be one greater than
+ the value of the previous. In source files, there shall be some way of indicating the end of
+ each line of text; this International Standard treats such an end-of-line indicator as if it
+ were a single new-line character. In the basic execution character set, there shall be
+ control characters representing alert, backspace, carriage return, and new line. If any
+ other characters are encountered in a source file (except in an identifier, a character
+ constant, a string literal, a header name, a comment, or a preprocessing token that is never
+
+ converted to a token), the behavior is undefined.
+
+4
+ A letter is an uppercase letter or a lowercase letter as defined above; in this International
+ Standard the term does not include other characters that are letters in other alphabets.
+
+5
+ The universal character name construct provides a way to name other characters.
+
+ Forward references: universal character names (6.4.3), character constants (6.4.4.4),
+ preprocessing directives (6.10), string literals (6.4.5), comments (6.4.9), string (7.1.1).
+
+
+Contents
+
+5.2.1.1 Trigraph sequences
+
+
+1
+ Before any other processing takes place, each occurrence of one of the following
+ sequences of three characters (called trigraph sequences17)) is replaced with the
+ corresponding single character.
+
+
+ ??= # ??) ] ??! |
+ ??( [ ??' ^ ??> }
+ ??/ \ ??< { ??- ~
+
+
+ No other trigraph sequences exist. Each ? that does not begin one of the trigraphs listed
+ above is not changed.
+
+2
+ EXAMPLE 1
+
+
+ ??=define arraycheck(a, b) a??(b??) ??!??! b??(a??)
+
+
+ becomes
+
+
+ #define arraycheck(a, b) a[b] || b[a]
+
+
+
+
+3
+ EXAMPLE 2 The following source line
+
+
+ printf("Eh???/n");
+
+
+ becomes (after replacement of the trigraph sequence ??/)
+
+
+ printf("Eh?\n");
+
+
+
+
+
+Footnotes
+
+17) The trigraph sequences enable the input of characters that are not defined in the Invariant Code Set as
+ described in ISO/IEC 646, which is a subset of the seven-bit US ASCII code set.
+
+
+Contents
+
+5.2.1.2 Multibyte characters
+
+
+1
+ The source character set may contain multibyte characters, used to represent members of
+ the extended character set. The execution character set may also contain multibyte
+ characters, which need not have the same encoding as for the source character set. For
+ both character sets, the following shall hold:
+
+
+- The basic character set shall be present and each character shall be encoded as a
+ single byte.
+
+- The presence, meaning, and representation of any additional members is locale-
+ specific.
+
+
+
+- A multibyte character set may have a state-dependent encoding, wherein each
+ sequence of multibyte characters begins in an initial shift state and enters other
+ locale-specific shift states when specific multibyte characters are encountered in the
+ sequence. While in the initial shift state, all single-byte characters retain their usual
+ interpretation and do not alter the shift state. The interpretation for subsequent bytes
+ in the sequence is a function of the current shift state.
+
+- A byte with all bits zero shall be interpreted as a null character independent of shift
+ state. Such a byte shall not occur as part of any other multibyte character.
+
+
+2
+ For source files, the following shall hold:
+
+
+- An identifier, comment, string literal, character constant, or header name shall begin
+ and end in the initial shift state.
+
+- An identifier, comment, string literal, character constant, or header name shall consist
+ of a sequence of valid multibyte characters.
+
+
+Contents
+
+5.2.2 Character display semantics
+
+
+1
+ The active position is that location on a display device where the next character output by
+ the fputc function would appear. The intent of writing a printing character (as defined
+ by the isprint function) to a display device is to display a graphic representation of
+ that character at the active position and then advance the active position to the next
+ position on the current line. The direction of writing is locale-specific. If the active
+ position is at the final position of a line (if there is one), the behavior of the display device
+ is unspecified.
+
+2
+ Alphabetic escape sequences representing nongraphic characters in the execution
+ character set are intended to produce actions on display devices as follows:
+
+ \a (alert) Produces an audible or visible alert without changing the active position.
+ \b (backspace) Moves the active position to the previous position on the current line. If
+ the active position is at the initial position of a line, the behavior of the display
+ device is unspecified.
+ \f (form feed) Moves the active position to the initial position at the start of the next
+ logical page.
+ \n (new line) Moves the active position to the initial position of the next line.
+ \r (carriage return) Moves the active position to the initial position of the current line.
+ \t (horizontal tab) Moves the active position to the next horizontal tabulation position
+ on the current line. If the active position is at or past the last defined horizontal
+ tabulation position, the behavior of the display device is unspecified.
+ \v (vertical tab) Moves the active position to the initial position of the next vertical
+
+ tabulation position. If the active position is at or past the last defined vertical
+ tabulation position, the behavior of the display device is unspecified.
+
+
+3
+ Each of these escape sequences shall produce a unique implementation-defined value
+ which can be stored in a single char object. The external representations in a text file
+ need not be identical to the internal representations, and are outside the scope of this
+ International Standard.
+
+ Forward references: the isprint function (7.4.1.8), the fputc function (7.21.7.3).
+
+
+Contents
+
+5.2.3 Signals and interrupts
+
+
+1
+ Functions shall be implemented such that they may be interrupted at any time by a signal,
+ or may be called by a signal handler, or both, with no alteration to earlier, but still active,
+ invocations' control flow (after the interruption), function return values, or objects with
+ automatic storage duration. All such objects shall be maintained outside the function
+ image (the instructions that compose the executable representation of a function) on a
+ per-invocation basis.
+
+
+Contents
+
+5.2.4 Environmental limits
+
+
+1
+ Both the translation and execution environments constrain the implementation of
+ language translators and libraries. The following summarizes the language-related
+ environmental limits on a conforming implementation; the library-related limits are
+ discussed in clause 7.
+
+
+Contents
+
+5.2.4.1 Translation limits
+
+
+1
+ The implementation shall be able to translate and execute at least one program that
+ contains at least one instance of every one of the following limits:18)
+
+
+- 127 nesting levels of blocks
+
+- 63 nesting levels of conditional inclusion
+
+- 12 pointer, array, and function declarators (in any combinations) modifying an
+ arithmetic, structure, union, or void type in a declaration
+
+- 63 nesting levels of parenthesized declarators within a full declarator
+
+- 63 nesting levels of parenthesized expressions within a full expression
+
+- 63 significant initial characters in an internal identifier or a macro name (each
+ universal character name or extended source character is considered a single
+ character)
+
+- 31 significant initial characters in an external identifier (each universal character name
+ specifying a short identifier of 0000FFFF or less is considered 6 characters, each
+
+ universal character name specifying a short identifier of 00010000 or more is
+ considered 10 characters, and each extended source character is considered the same
+ number of characters as the corresponding universal character name, if any)19)
+
+- 4095 external identifiers in one translation unit
+
+- 511 identifiers with block scope declared in one block
+
+- 4095 macro identifiers simultaneously defined in one preprocessing translation unit
+
+- 127 parameters in one function definition
+
+- 127 arguments in one function call
+
+- 127 parameters in one macro definition
+
+- 127 arguments in one macro invocation
+
+- 4095 characters in a logical source line
+
+- 4095 characters in a string literal (after concatenation)
+
+- 65535 bytes in an object (in a hosted environment only)
+
+- 15 nesting levels for #included files
+
+- 1023 case labels for a switch statement (excluding those for any nested switch
+ statements)
+
+- 1023 members in a single structure or union
+
+- 1023 enumeration constants in a single enumeration
+
+- 63 levels of nested structure or union definitions in a single struct-declaration-list
+
+
+Footnotes
+
+18) Implementations should avoid imposing fixed translation limits whenever possible.
+
+
+19) See ''future language directions'' (6.11.3).
+
+
+Contents
+
+5.2.4.2 Numerical limits
+
+
+1
+ An implementation is required to document all the limits specified in this subclause,
+ which are specified in the headers <limits.h> and <float.h>. Additional limits are
+ specified in <stdint.h>.
+
+ Forward references: integer types <stdint.h> (7.20).
+
+
+Contents
+
+5.2.4.2.1 Sizes of integer types <limits.h>
+
+
+1
+ The values given below shall be replaced by constant expressions suitable for use in #if
+ preprocessing directives. Moreover, except for CHAR_BIT and MB_LEN_MAX, the
+ following shall be replaced by expressions that have the same type as would an
+ expression that is an object of the corresponding type converted according to the integer
+ promotions. Their implementation-defined values shall be equal or greater in magnitude
+
+
+
+ (absolute value) to those shown, with the same sign.
+
+
+- number of bits for smallest object that is not a bit-field (byte)
+
+
+ CHAR_BIT 8
+
+
+- minimum value for an object of type signed char
+
+
+ SCHAR_MIN -127 // -(27 - 1)
+
+
+- maximum value for an object of type signed char
+
+
+ SCHAR_MAX +127 // 27 - 1
+
+
+- maximum value for an object of type unsigned char
+
+
+ UCHAR_MAX 255 // 28 - 1
+
+
+- minimum value for an object of type char
+
+
+ CHAR_MIN see below
+
+
+- maximum value for an object of type char
+
+
+ CHAR_MAX see below
+
+
+- maximum number of bytes in a multibyte character, for any supported locale
+
+
+ MB_LEN_MAX 1
+
+
+- minimum value for an object of type short int
+
+
+ SHRT_MIN -32767 // -(215 - 1)
+
+
+- maximum value for an object of type short int
+
+
+ SHRT_MAX +32767 // 215 - 1
+
+
+- maximum value for an object of type unsigned short int
+
+
+ USHRT_MAX 65535 // 216 - 1
+
+
+- minimum value for an object of type int
+
+
+ INT_MIN -32767 // -(215 - 1)
+
+
+- maximum value for an object of type int
+
+
+ INT_MAX +32767 // 215 - 1
+
+
+- maximum value for an object of type unsigned int
+
+
+ UINT_MAX 65535 // 216 - 1
+
+
+- minimum value for an object of type long int
+
+
+ LONG_MIN -2147483647 // -(231 - 1)
+
+
+- maximum value for an object of type long int
+
+
+ LONG_MAX +2147483647 // 231 - 1
+
+
+- maximum value for an object of type unsigned long int
+
+
+ ULONG_MAX 4294967295 // 232 - 1
+
+
+- minimum value for an object of type long long int
+
+
+ LLONG_MIN -9223372036854775807 // -(263 - 1)
+
+
+- maximum value for an object of type long long int
+
+
+ LLONG_MAX +9223372036854775807 // 263 - 1
+
+
+- maximum value for an object of type unsigned long long int
+
+
+ ULLONG_MAX 18446744073709551615 // 264 - 1
+
+
+2
+ If the value of an object of type char is treated as a signed integer when used in an
+ expression, the value of CHAR_MIN shall be the same as that of SCHAR_MIN and the
+ value of CHAR_MAX shall be the same as that of SCHAR_MAX. Otherwise, the value of
+ CHAR_MIN shall be 0 and the value of CHAR_MAX shall be the same as that of
+ UCHAR_MAX.20) The value UCHAR_MAX shall equal 2CHAR_BIT - 1.
+
+ Forward references: representations of types (6.2.6), conditional inclusion (6.10.1).
+
+
+Footnotes
+
+20) See 6.2.5.
+
+
+Contents
+
+5.2.4.2.2 Characteristics of floating types <float.h>
+
+
+1
+ The characteristics of floating types are defined in terms of a model that describes a
+ representation of floating-point numbers and values that provide information about an
+ implementation's floating-point arithmetic.21) The following parameters are used to
+ define the model for each floating-point type:
+
+
+ s sign ((+-)1)
+ b base or radix of exponent representation (an integer > 1)
+ e exponent (an integer between a minimum emin and a maximum emax )
+ p precision (the number of base-b digits in the significand)
+ fk nonnegative integers less than b (the significand digits)
+
+
+2
+ A floating-point number (x) is defined by the following model:
+
+
+ p
+ x = s be (Sum) fk b-k , emin <= e <= emax
+ k=1
+
+
+
+
+3
+ In addition to normalized floating-point numbers ( f1 > 0 if x != 0), floating types may be
+ able to contain other kinds of floating-point numbers, such as subnormal floating-point
+ numbers (x != 0, e = emin , f1 = 0) and unnormalized floating-point numbers (x != 0,
+ e > emin , f1 = 0), and values that are not floating-point numbers, such as infinities and
+ NaNs. A NaN is an encoding signifying Not-a-Number. A quiet NaN propagates
+ through almost every arithmetic operation without raising a floating-point exception; a
+ signaling NaN generally raises a floating-point exception when occurring as an
+
+
+
+ arithmetic operand.22)
+
+4
+ An implementation may give zero and values that are not floating-point numbers (such as
+ infinities and NaNs) a sign or may leave them unsigned. Wherever such values are
+ unsigned, any requirement in this International Standard to retrieve the sign shall produce
+ an unspecified sign, and any requirement to set the sign shall be ignored.
+
+5
+ The minimum range of representable values for a floating type is the most negative finite
+ floating-point number representable in that type through the most positive finite floating-
+ point number representable in that type. In addition, if negative infinity is representable
+ in a type, the range of that type is extended to all negative real numbers; likewise, if
+ positive infinity is representable in a type, the range of that type is extended to all positive
+ real numbers.
+
+6
+ The accuracy of the floating-point operations (+, -, *, /) and of the library functions in
+ <math.h> and <complex.h> that return floating-point results is implementation-
+ defined, as is the accuracy of the conversion between floating-point internal
+ representations and string representations performed by the library functions in
+ <stdio.h>, <stdlib.h>, and <wchar.h>. The implementation may state that the
+ accuracy is unknown.
+
+7
+ All integer values in the <float.h> header, except FLT_ROUNDS, shall be constant
+ expressions suitable for use in #if preprocessing directives; all floating values shall be
+ constant expressions. All except DECIMAL_DIG, FLT_EVAL_METHOD, FLT_RADIX,
+ and FLT_ROUNDS have separate names for all three floating-point types. The floating-point
+ model representation is provided for all values except FLT_EVAL_METHOD and
+ FLT_ROUNDS.
+
+8
+ The rounding mode for floating-point addition is characterized by the implementation-
+ defined value of FLT_ROUNDS:23)
+
+
+ -1 indeterminable
+ 0 toward zero
+ 1 to nearest
+ 2 toward positive infinity
+ 3 toward negative infinity
+
+
+ All other values for FLT_ROUNDS characterize implementation-defined rounding
+ behavior.
+
+
+
+
+9
+ Except for assignment and cast (which remove all extra range and precision), the values
+ yielded by operators with floating operands and values subject to the usual arithmetic
+ conversions and of floating constants are evaluated to a format whose range and precision
+ may be greater than required by the type. The use of evaluation formats is characterized
+ by the implementation-defined value of FLT_EVAL_METHOD:24)
+
+
+ -1 indeterminable;
+ 0 evaluate all operations and constants just to the range and precision of the
+ type;
+ 1 evaluate operations and constants of type float and double to the
+ range and precision of the double type, evaluate long double
+ operations and constants to the range and precision of the long double
+ type;
+ 2 evaluate all operations and constants to the range and precision of the
+ long double type.
+
+
+ All other negative values for FLT_EVAL_METHOD characterize implementation-defined
+ behavior.
+
+10
+ The presence or absence of subnormal numbers is characterized by the implementation-
+ defined values of FLT_HAS_SUBNORM, DBL_HAS_SUBNORM, and
+ LDBL_HAS_SUBNORM:
+
+
+ -1 indeterminable25)
+ 0 absent26) (type does not support subnormal numbers)
+ 1 present (type does support subnormal numbers)
+
+
+11
+ The values given in the following list shall be replaced by constant expressions with
+ implementation-defined values that are greater or equal in magnitude (absolute value) to
+ those shown, with the same sign:
+
+
+- radix of exponent representation, b
+
+
+ FLT_RADIX 2
+
+
+- number of base-FLT_RADIX digits in the floating-point significand, p
+
+
+ FLT_MANT_DIG
+ DBL_MANT_DIG
+ LDBL_MANT_DIG
+
+
+- number of decimal digits, n, such that any floating-point number with p radix b digits
+ can be rounded to a floating-point number with n decimal digits and back again
+ without change to the value,
+
+
+ { p log10 b if b is a power of 10
+ {
+ { [^1 + p log10 b^] otherwise
+
+
+ FLT_DECIMAL_DIG 6
+ DBL_DECIMAL_DIG 10
+ LDBL_DECIMAL_DIG 10
+
+
+- number of decimal digits, n, such that any floating-point number in the widest
+ supported floating type with pmax radix b digits can be rounded to a floating-point
+ number with n decimal digits and back again without change to the value,
+
+
+ { pmax log10 b if b is a power of 10
+ {
+ { [^1 + pmax log10 b^] otherwise
+
+
+ DECIMAL_DIG 10
+
+
+- number of decimal digits, q, such that any floating-point number with q decimal digits
+ can be rounded into a floating-point number with p radix b digits and back again
+ without change to the q decimal digits,
+
+
+ { p log10 b if b is a power of 10
+ {
+ { [_( p - 1) log10 b_] otherwise
+
+ FLT_DIG 6
+ DBL_DIG 10
+ LDBL_DIG 10
+
+
+- minimum negative integer such that FLT_RADIX raised to one less than that power is
+ a normalized floating-point number, emin
+
+
+ FLT_MIN_EXP
+ DBL_MIN_EXP
+ LDBL_MIN_EXP
+
+
+- minimum negative integer such that 10 raised to that power is in the range of
+ normalized floating-point numbers, [^log10 bemin-1^]
+
+
+ FLT_MIN_10_EXP -37
+ DBL_MIN_10_EXP -37
+ LDBL_MIN_10_EXP -37
+
+
+- maximum integer such that FLT_RADIX raised to one less than that power is a
+ representable finite floating-point number, emax
+
+
+ FLT_MAX_EXP
+ DBL_MAX_EXP
+ LDBL_MAX_EXP
+
+
+- maximum integer such that 10 raised to that power is in the range of representable
+ finite floating-point numbers, [_log10 ((1 - b-p)bemax)_]
+
+
+ FLT_MAX_10_EXP +37
+ DBL_MAX_10_EXP +37
+ LDBL_MAX_10_EXP +37
+
+
+12
+ The values given in the following list shall be replaced by constant expressions with
+ implementation-defined values that are greater than or equal to those shown:
+
+
+- maximum representable finite floating-point number, (1 - b-p)bemax
+
+
+ FLT_MAX 1E+37
+ DBL_MAX 1E+37
+ LDBL_MAX 1E+37
+
+
+13
+ The values given in the following list shall be replaced by constant expressions with
+ implementation-defined (positive) values that are less than or equal to those shown:
+
+
+- the difference between 1 and the least value greater than 1 that is representable in the
+ given floating point type, b1-p
+
+
+ FLT_EPSILON 1E-5
+ DBL_EPSILON 1E-9
+ LDBL_EPSILON 1E-9
+
+
+- minimum normalized positive floating-point number, bemin-1
+
+
+ FLT_MIN 1E-37
+ DBL_MIN 1E-37
+ LDBL_MIN 1E-37
+
+
+- minimum positive floating-point number27)
+ FLT_TRUE_MIN 1E-37
+ DBL_TRUE_MIN 1E-37
+ LDBL_TRUE_MIN 1E-37
+
+
+Recommended practice
+
+14
+ Conversion from (at least) double to decimal with DECIMAL_DIG digits and back
+ should be the identity function.
+
+15
+ EXAMPLE 1 The following describes an artificial floating-point representation that meets the minimum
+ requirements of this International Standard, and the appropriate values in a <float.h> header for type
+ float:
+
+
+ 6
+ x = s 16e (Sum) fk 16-k , -31 <= e <= +32
+ k=1
+
+
+ FLT_RADIX 16
+ FLT_MANT_DIG 6
+ FLT_EPSILON 9.53674316E-07F
+ FLT_DECIMAL_DIG 9
+ FLT_DIG 6
+ FLT_MIN_EXP -31
+ FLT_MIN 2.93873588E-39F
+ FLT_MIN_10_EXP -38
+ FLT_MAX_EXP +32
+ FLT_MAX 3.40282347E+38F
+ FLT_MAX_10_EXP +38
+
+
+
+
+16
+ EXAMPLE 2 The following describes floating-point representations that also meet the requirements for
+ single-precision and double-precision numbers in IEC 60559,28) and the appropriate values in a
+ <float.h> header for types float and double:
+
+
+ 24
+ xf = s 2e (Sum) fk 2-k , -125 <= e <= +128
+ k=1
+
+
+ 53
+ xd = s 2e (Sum) fk 2-k , -1021 <= e <= +1024
+ k=1
+
+
+ FLT_RADIX 2
+ DECIMAL_DIG 17
+ FLT_MANT_DIG 24
+ FLT_EPSILON 1.19209290E-07F // decimal constant
+ FLT_EPSILON 0X1P-23F // hex constant
+ FLT_DECIMAL_DIG 9
+
+
+
+
+
+
+ FLT_DIG 6
+ FLT_MIN_EXP -125
+ FLT_MIN 1.17549435E-38F // decimal constant
+ FLT_MIN 0X1P-126F // hex constant
+ FLT_TRUE_MIN 1.40129846E-45F // decimal constant
+ FLT_TRUE_MIN 0X1P-149F // hex constant
+ FLT_HAS_SUBNORM 1
+ FLT_MIN_10_EXP -37
+ FLT_MAX_EXP +128
+ FLT_MAX 3.40282347E+38F // decimal constant
+ FLT_MAX 0X1.fffffeP127F // hex constant
+ FLT_MAX_10_EXP +38
+ DBL_MANT_DIG 53
+ DBL_EPSILON 2.2204460492503131E-16 // decimal constant
+ DBL_EPSILON 0X1P-52 // hex constant
+ DBL_DECIMAL_DIG 17
+ DBL_DIG 15
+ DBL_MIN_EXP -1021
+ DBL_MIN 2.2250738585072014E-308 // decimal constant
+ DBL_MIN 0X1P-1022 // hex constant
+ DBL_TRUE_MIN 4.9406564584124654E-324 // decimal constant
+ DBL_TRUE_MIN 0X1P-1074 // hex constant
+ DBL_HAS_SUBNORM 1
+ DBL_MIN_10_EXP -307
+ DBL_MAX_EXP +1024
+ DBL_MAX 1.7976931348623157E+308 // decimal constant
+ DBL_MAX 0X1.fffffffffffffP1023 // hex constant
+ DBL_MAX_10_EXP +308
+
+
+ If a type wider than double were supported, then DECIMAL_DIG would be greater than 17. For
+ example, if the widest type were to use the minimal-width IEC 60559 double-extended format (64 bits of
+ precision), then DECIMAL_DIG would be 21.
+
+
+ Forward references: conditional inclusion (6.10.1), complex arithmetic
+ <complex.h> (7.3), extended multibyte and wide character utilities <wchar.h>
+ (7.29), floating-point environment <fenv.h> (7.6), general utilities <stdlib.h>
+ (7.22), input/output <stdio.h> (7.21), mathematics <math.h> (7.12).
+
+
+Footnotes
+
+21) The floating-point model is intended to clarify the description of each floating-point characteristic and
+ does not require the floating-point arithmetic of the implementation to be identical.
+
+
+22) IEC 60559:1989 specifies quiet and signaling NaNs. For implementations that do not support
+ IEC 60559:1989, the terms quiet NaN and signaling NaN are intended to apply to encodings with
+ similar behavior.
+
+
+23) Evaluation of FLT_ROUNDS correctly reflects any execution-time change of rounding mode through
+ the function fesetround in <fenv.h>.
+
+
+24) The evaluation method determines evaluation formats of expressions involving all floating types, not
+ just real types. For example, if FLT_EVAL_METHOD is 1, then the product of two float
+ _Complex operands is represented in the double _Complex format, and its parts are evaluated to
+ double.
+
+
+25) Characterization as indeterminable is intended if floating-point operations do not consistently interpret
+ subnormal representations as zero, nor as nonzero.
+
+
+26) Characterization as absent is intended if no floating-point operations produce subnormal results from
+ non-subnormal inputs, even if the type format includes representations of subnormal numbers.
+
+
+27) If the presence or absence of subnormal numbers is indeterminable, then the value is intended to be a
+ positive number no greater than the minimum normalized positive number for the type.
+
+
+28) The floating-point model in that standard sums powers of b from zero, so the values of the exponent
+ limits are one less than shown here.
+
+
+Contents
diff --git a/doc/std/CHAPTER-6.txt b/doc/std/CHAPTER-6.txt
@@ -0,0 +1,9476 @@
+6. Language
+
+
+Contents
+
+6.1 Notation
+
+
+1
+ In the syntax notation used in this clause, syntactic categories (nonterminals) are
+ indicated by italic type, and literal words and character set members (terminals) by bold
+ type. A colon (:) following a nonterminal introduces its definition. Alternative
+ definitions are listed on separate lines, except when prefaced by the words ''one of''. An
+ optional symbol is indicated by the subscript ''opt'', so that
+
+
+ { expressionopt }
+
+
+ indicates an optional expression enclosed in braces.
+
+2
+ When syntactic categories are referred to in the main text, they are not italicized and
+ words are separated by spaces instead of hyphens.
+
+3
+ A summary of the language syntax is given in annex A.
+
+
+Contents
+
+6.2 Concepts
+
+
+Contents
+
+6.2.1 Scopes of identifiers
+
+
+1
+ An identifier can denote an object; a function; a tag or a member of a structure, union, or
+ enumeration; a typedef name; a label name; a macro name; or a macro parameter. The
+ same identifier can denote different entities at different points in the program. A member
+ of an enumeration is called an enumeration constant. Macro names and macro
+ parameters are not considered further here, because prior to the semantic phase of
+ program translation any occurrences of macro names in the source file are replaced by the
+ preprocessing token sequences that constitute their macro definitions.
+
+2
+ For each different entity that an identifier designates, the identifier is visible (i.e., can be
+ used) only within a region of program text called its scope. Different entities designated
+ by the same identifier either have different scopes, or are in different name spaces. There
+ are four kinds of scopes: function, file, block, and function prototype. (A function
+ prototype is a declaration of a function that declares the types of its parameters.)
+
+3
+ A label name is the only kind of identifier that has function scope. It can be used (in a
+ goto statement) anywhere in the function in which it appears, and is declared implicitly
+ by its syntactic appearance (followed by a : and a statement).
+
+4
+ Every other identifier has scope determined by the placement of its declaration (in a
+ declarator or type specifier). If the declarator or type specifier that declares the identifier
+ appears outside of any block or list of parameters, the identifier has file scope, which
+ terminates at the end of the translation unit. If the declarator or type specifier that
+ declares the identifier appears inside a block or within the list of parameter declarations in
+ a function definition, the identifier has block scope, which terminates at the end of the
+ associated block. If the declarator or type specifier that declares the identifier appears
+
+ within the list of parameter declarations in a function prototype (not part of a function
+ definition), the identifier has function prototype scope, which terminates at the end of the
+ function declarator. If an identifier designates two different entities in the same name
+ space, the scopes might overlap. If so, the scope of one entity (the inner scope) will end
+ strictly before the scope of the other entity (the outer scope). Within the inner scope, the
+ identifier designates the entity declared in the inner scope; the entity declared in the outer
+ scope is hidden (and not visible) within the inner scope.
+
+5
+ Unless explicitly stated otherwise, where this International Standard uses the term
+ ''identifier'' to refer to some entity (as opposed to the syntactic construct), it refers to the
+ entity in the relevant name space whose declaration is visible at the point the identifier
+ occurs.
+
+6
+ Two identifiers have the same scope if and only if their scopes terminate at the same
+ point.
+
+7
+ Structure, union, and enumeration tags have scope that begins just after the appearance of
+ the tag in a type specifier that declares the tag. Each enumeration constant has scope that
+ begins just after the appearance of its defining enumerator in an enumerator list. Any
+ other identifier has scope that begins just after the completion of its declarator.
+
+8
+ As a special case, a type name (which is not a declaration of an identifier) is considered to
+ have a scope that begins just after the place within the type name where the omitted
+ identifier would appear were it not omitted.
+
+ Forward references: declarations (6.7), function calls (6.5.2.2), function definitions
+ (6.9.1), identifiers (6.4.2), macro replacement (6.10.3), name spaces of identifiers (6.2.3),
+ source file inclusion (6.10.2), statements (6.8).
+
+
+Contents
+
+6.2.2 Linkages of identifiers
+
+
+1
+ An identifier declared in different scopes or in the same scope more than once can be
+ made to refer to the same object or function by a process called linkage.29) There are
+ three kinds of linkage: external, internal, and none.
+
+2
+ In the set of translation units and libraries that constitutes an entire program, each
+ declaration of a particular identifier with external linkage denotes the same object or
+ function. Within one translation unit, each declaration of an identifier with internal
+ linkage denotes the same object or function. Each declaration of an identifier with no
+ linkage denotes a unique entity.
+
+3
+ If the declaration of a file scope identifier for an object or a function contains the storage-
+ class specifier static, the identifier has internal linkage.30)
+
+
+
+
+
+4
+ For an identifier declared with the storage-class specifier extern in a scope in which a
+ prior declaration of that identifier is visible,31) if the prior declaration specifies internal or
+ external linkage, the linkage of the identifier at the later declaration is the same as the
+ linkage specified at the prior declaration. If no prior declaration is visible, or if the prior
+ declaration specifies no linkage, then the identifier has external linkage.
+
+5
+ If the declaration of an identifier for a function has no storage-class specifier, its linkage
+ is determined exactly as if it were declared with the storage-class specifier extern. If
+ the declaration of an identifier for an object has file scope and no storage-class specifier,
+ its linkage is external.
+
+6
+ The following identifiers have no linkage: an identifier declared to be anything other than
+ an object or a function; an identifier declared to be a function parameter; a block scope
+ identifier for an object declared without the storage-class specifier extern.
+
+7
+ If, within a translation unit, the same identifier appears with both internal and external
+ linkage, the behavior is undefined.
+
+ Forward references: declarations (6.7), expressions (6.5), external definitions (6.9),
+ statements (6.8).
+
+
+Footnotes
+
+29) There is no linkage between different identifiers.
+
+
+30) A function declaration can contain the storage-class specifier static only if it is at file scope; see
+ 6.7.1.
+
+
+31) As specified in 6.2.1, the later declaration might hide the prior declaration.
+
+
+Contents
+
+6.2.3 Name spaces of identifiers
+
+
+1
+ If more than one declaration of a particular identifier is visible at any point in a
+ translation unit, the syntactic context disambiguates uses that refer to different entities.
+ Thus, there are separate name spaces for various categories of identifiers, as follows:
+
+
+- label names (disambiguated by the syntax of the label declaration and use);
+
+- the tags of structures, unions, and enumerations (disambiguated by following any32)
+ of the keywords struct, union, or enum);
+
+- the members of structures or unions; each structure or union has a separate name
+ space for its members (disambiguated by the type of the expression used to access the
+ member via the . or -> operator);
+
+- all other identifiers, called ordinary identifiers (declared in ordinary declarators or as
+ enumeration constants).
+
+
+ Forward references: enumeration specifiers (6.7.2.2), labeled statements (6.8.1),
+ structure and union specifiers (6.7.2.1), structure and union members (6.5.2.3), tags
+ (6.7.2.3), the goto statement (6.8.6.1).
+
+
+
+Footnotes
+
+32) There is only one name space for tags even though three are possible.
+
+
+Contents
+
+6.2.4 Storage durations of objects
+
+
+1
+ An object has a storage duration that determines its lifetime. There are four storage
+ durations: static, thread, automatic, and allocated. Allocated storage is described in
+ 7.22.3.
+
+2
+ The lifetime of an object is the portion of program execution during which storage is
+ guaranteed to be reserved for it. An object exists, has a constant address,33) and retains
+ its last-stored value throughout its lifetime.34) If an object is referred to outside of its
+ lifetime, the behavior is undefined. The value of a pointer becomes indeterminate when
+ the object it points to (or just past) reaches the end of its lifetime.
+
+3
+ An object whose identifier is declared without the storage-class specifier
+ _Thread_local, and either with external or internal linkage or with the storage-class
+ specifier static, has static storage duration. Its lifetime is the entire execution of the
+ program and its stored value is initialized only once, prior to program startup.
+
+4
+ An object whose identifier is declared with the storage-class specifier _Thread_local
+ has thread storage duration. Its lifetime is the entire execution of the thread for which it
+ is created, and its stored value is initialized when the thread is started. There is a distinct
+ object per thread, and use of the declared name in an expression refers to the object
+ associated with the thread evaluating the expression. The result of attempting to
+ indirectly access an object with thread storage duration from a thread other than the one
+ with which the object is associated is implementation-defined.
+
+5
+ An object whose identifier is declared with no linkage and without the storage-class
+ specifier static has automatic storage duration, as do some compound literals. The
+ result of attempting to indirectly access an object with automatic storage duration from a
+ thread other than the one with which the object is associated is implementation-defined.
+
+6
+ For such an object that does not have a variable length array type, its lifetime extends
+ from entry into the block with which it is associated until execution of that block ends in
+ any way. (Entering an enclosed block or calling a function suspends, but does not end,
+ execution of the current block.) If the block is entered recursively, a new instance of the
+ object is created each time. The initial value of the object is indeterminate. If an
+ initialization is specified for the object, it is performed each time the declaration or
+ compound literal is reached in the execution of the block; otherwise, the value becomes
+ indeterminate each time the declaration is reached.
+
+
+
+
+
+7
+ For such an object that does have a variable length array type, its lifetime extends from
+ the declaration of the object until execution of the program leaves the scope of the
+ declaration.35) If the scope is entered recursively, a new instance of the object is created
+ each time. The initial value of the object is indeterminate.
+
+8
+ A non-lvalue expression with structure or union type, where the structure or union
+ contains a member with array type (including, recursively, members of all contained
+ structures and unions) refers to an object with automatic storage duration and temporary
+ lifetime.36) Its lifetime begins when the expression is evaluated and its initial value is the
+ value of the expression. Its lifetime ends when the evaluation of the containing full
+ expression or full declarator ends. Any attempt to modify an object with temporary
+ lifetime results in undefined behavior.
+
+ Forward references: array declarators (6.7.6.2), compound literals (6.5.2.5), declarators
+ (6.7.6), function calls (6.5.2.2), initialization (6.7.9), statements (6.8).
+
+
+Footnotes
+
+33) The term ''constant address'' means that two pointers to the object constructed at possibly different
+ times will compare equal. The address may be different during two different executions of the same
+ program.
+
+
+34) In the case of a volatile object, the last store need not be explicit in the program.
+
+
+35) Leaving the innermost block containing the declaration, or jumping to a point in that block or an
+ embedded block prior to the declaration, leaves the scope of the declaration.
+
+
+36) The address of such an object is taken implicitly when an array member is accessed.
+
+
+Contents
+
+6.2.5 Types
+
+
+1
+ The meaning of a value stored in an object or returned by a function is determined by the
+ type of the expression used to access it. (An identifier declared to be an object is the
+ simplest such expression; the type is specified in the declaration of the identifier.) Types
+ are partitioned into object types (types that describe objects) and function types (types
+ that describe functions). At various points within a translation unit an object type may be
+ incomplete (lacking sufficient information to determine the size of objects of that type) or
+ complete (having sufficient information).37)
+
+2
+ An object declared as type _Bool is large enough to store the values 0 and 1.
+
+3
+ An object declared as type char is large enough to store any member of the basic
+ execution character set. If a member of the basic execution character set is stored in a
+ char object, its value is guaranteed to be nonnegative. If any other character is stored in
+ a char object, the resulting value is implementation-defined but shall be within the range
+ of values that can be represented in that type.
+
+4
+ There are five standard signed integer types, designated as signed char, short
+ int, int, long int, and long long int. (These and other types may be
+ designated in several additional ways, as described in 6.7.2.) There may also be
+ implementation-defined extended signed integer types.38) The standard and extended
+ signed integer types are collectively called signed integer types.39)
+
+
+
+5
+ An object declared as type signed char occupies the same amount of storage as a
+ ''plain'' char object. A ''plain'' int object has the natural size suggested by the
+ architecture of the execution environment (large enough to contain any value in the range
+ INT_MIN to INT_MAX as defined in the header <limits.h>).
+
+6
+ For each of the signed integer types, there is a corresponding (but different) unsigned
+ integer type (designated with the keyword unsigned) that uses the same amount of
+ storage (including sign information) and has the same alignment requirements. The type
+ _Bool and the unsigned integer types that correspond to the standard signed integer
+ types are the standard unsigned integer types. The unsigned integer types that
+ correspond to the extended signed integer types are the extended unsigned integer types.
+ The standard and extended unsigned integer types are collectively called unsigned integer
+ types.40)
+
+7
+ The standard signed integer types and standard unsigned integer types are collectively
+ called the standard integer types, the extended signed integer types and extended
+ unsigned integer types are collectively called the extended integer types.
+
+8
+ For any two integer types with the same signedness and different integer conversion rank
+ (see 6.3.1.1), the range of values of the type with smaller integer conversion rank is a
+ subrange of the values of the other type.
+
+9
+ The range of nonnegative values of a signed integer type is a subrange of the
+ corresponding unsigned integer type, and the representation of the same value in each
+ type is the same.41) A computation involving unsigned operands can never overflow,
+ because a result that cannot be represented by the resulting unsigned integer type is
+ reduced modulo the number that is one greater than the largest value that can be
+ represented by the resulting type.
+
+10
+ There are three real floating types, designated as float, double, and long
+ double.42) The set of values of the type float is a subset of the set of values of the
+ type double; the set of values of the type double is a subset of the set of values of the
+ type long double.
+
+
+
+
+11
+ There are three complex types, designated as float _Complex, double
+ _Complex, and long double _Complex.43) (Complex types are a conditional
+ feature that implementations need not support; see 6.10.8.3.) The real floating and
+ complex types are collectively called the floating types.
+
+12
+ For each floating type there is a corresponding real type, which is always a real floating
+ type. For real floating types, it is the same type. For complex types, it is the type given
+ by deleting the keyword _Complex from the type name.
+
+13
+ Each complex type has the same representation and alignment requirements as an array
+ type containing exactly two elements of the corresponding real type; the first element is
+ equal to the real part, and the second element to the imaginary part, of the complex
+ number.
+
+14
+ The type char, the signed and unsigned integer types, and the floating types are
+ collectively called the basic types. The basic types are complete object types. Even if the
+ implementation defines two or more basic types to have the same representation, they are
+ nevertheless different types.44)
+
+15
+ The three types char, signed char, and unsigned char are collectively called
+ the character types. The implementation shall define char to have the same range,
+ representation, and behavior as either signed char or unsigned char.45)
+
+16
+ An enumeration comprises a set of named integer constant values. Each distinct
+ enumeration constitutes a different enumerated type.
+
+17
+ The type char, the signed and unsigned integer types, and the enumerated types are
+ collectively called integer types. The integer and real floating types are collectively called
+ real types.
+
+18
+ Integer and floating types are collectively called arithmetic types. Each arithmetic type
+ belongs to one type domain: the real type domain comprises the real types, the complex
+ type domain comprises the complex types.
+
+19
+ The void type comprises an empty set of values; it is an incomplete object type that
+ cannot be completed.
+
+
+
+
+
+20
+ Any number of derived types can be constructed from the object and function types, as
+ follows:
+
+
+- An array type describes a contiguously allocated nonempty set of objects with a
+ particular member object type, called the element type. The element type shall be
+ complete whenever the array type is specified. Array types are characterized by their
+ element type and by the number of elements in the array. An array type is said to be
+ derived from its element type, and if its element type is T , the array type is sometimes
+ called ''array of T ''. The construction of an array type from an element type is called
+ ''array type derivation''.
+
+- A structure type describes a sequentially allocated nonempty set of member objects
+ (and, in certain circumstances, an incomplete array), each of which has an optionally
+ specified name and possibly distinct type.
+
+- A union type describes an overlapping nonempty set of member objects, each of
+ which has an optionally specified name and possibly distinct type.
+
+- A function type describes a function with specified return type. A function type is
+ characterized by its return type and the number and types of its parameters. A
+ function type is said to be derived from its return type, and if its return type is T , the
+ function type is sometimes called ''function returning T ''. The construction of a
+ function type from a return type is called ''function type derivation''.
+
+- A pointer type may be derived from a function type or an object type, called the
+ referenced type. A pointer type describes an object whose value provides a reference
+ to an entity of the referenced type. A pointer type derived from the referenced type T
+ is sometimes called ''pointer to T ''. The construction of a pointer type from a
+ referenced type is called ''pointer type derivation''. A pointer type is a complete
+ object type.
+
+- An atomic type describes the type designated by the construct _Atomic ( type-
+ name ). (Atomic types are a conditional feature that implementations need not
+ support; see 6.10.8.3.)
+
+ These methods of constructing derived types can be applied recursively.
+
+21
+ Arithmetic types and pointer types are collectively called scalar types. Array and
+ structure types are collectively called aggregate types.46)
+
+22
+ An array type of unknown size is an incomplete type. It is completed, for an identifier of
+ that type, by specifying the size in a later declaration (with internal or external linkage).
+ A structure or union type of unknown content (as described in 6.7.2.3) is an incomplete
+
+
+
+ type. It is completed, for all declarations of that type, by declaring the same structure or
+ union tag with its defining content later in the same scope.
+
+23
+ A type has known constant size if the type is not incomplete and is not a variable length
+ array type.
+
+24
+ Array, function, and pointer types are collectively called derived declarator types. A
+ declarator type derivation from a type T is the construction of a derived declarator type
+ from T by the application of an array-type, a function-type, or a pointer-type derivation to
+ T.
+
+25
+ A type is characterized by its type category, which is either the outermost derivation of a
+ derived type (as noted above in the construction of derived types), or the type itself if the
+ type consists of no derived types.
+
+26
+ Any type so far mentioned is an unqualified type. Each unqualified type has several
+ qualified versions of its type,47) corresponding to the combinations of one, two, or all
+ three of the const, volatile, and restrict qualifiers. The qualified or unqualified
+ versions of a type are distinct types that belong to the same type category and have the
+ same representation and alignment requirements.48) A derived type is not qualified by the
+ qualifiers (if any) of the type from which it is derived.
+
+27
+ Further, there is the _Atomic qualifier. The presence of the _Atomic qualifier
+ designates an atomic type. The size, representation, and alignment of an atomic type
+ need not be the same as those of the corresponding unqualified type. Therefore, this
+ Standard explicitly uses the phrase ''atomic, qualified or unqualified type'' whenever the
+ atomic version of a type is permitted along with the other qualified versions of a type.
+ The phrase ''qualified or unqualified type'', without specific mention of atomic, does not
+ include the atomic types.
+
+28
+ A pointer to void shall have the same representation and alignment requirements as a
+ pointer to a character type.48) Similarly, pointers to qualified or unqualified versions of
+ compatible types shall have the same representation and alignment requirements. All
+ pointers to structure types shall have the same representation and alignment requirements
+ as each other. All pointers to union types shall have the same representation and
+ alignment requirements as each other. Pointers to other types need not have the same
+ representation or alignment requirements.
+
+29
+ EXAMPLE 1 The type designated as ''float *'' has type ''pointer to float''. Its type category is
+ pointer, not a floating type. The const-qualified version of this type is designated as ''float * const''
+ whereas the type designated as ''const float *'' is not a qualified type -- its type is ''pointer to const-
+
+
+
+ qualified float'' and is a pointer to a qualified type.
+
+
+30
+ EXAMPLE 2 The type designated as ''struct tag (*[5])(float)'' has type ''array of pointer to
+ function returning struct tag''. The array has length five and the function has a single parameter of type
+ float. Its type category is array.
+
+
+ Forward references: compatible type and composite type (6.2.7), declarations (6.7).
+
+
+Footnotes
+
+37) A type may be incomplete or complete throughout an entire translation unit, or it may change states at
+ different points within a translation unit.
+
+
+38) Implementation-defined keywords shall have the form of an identifier reserved for any use as
+ described in 7.1.3.
+
+
+39) Therefore, any statement in this Standard about signed integer types also applies to the extended
+ signed integer types.
+
+
+40) Therefore, any statement in this Standard about unsigned integer types also applies to the extended
+ unsigned integer types.
+
+
+41) The same representation and alignment requirements are meant to imply interchangeability as
+ arguments to functions, return values from functions, and members of unions.
+
+
+42) See ''future language directions'' (6.11.1).
+
+
+43) A specification for imaginary types is in annex G.
+
+
+44) An implementation may define new keywords that provide alternative ways to designate a basic (or
+ any other) type; this does not violate the requirement that all basic types be different.
+ Implementation-defined keywords shall have the form of an identifier reserved for any use as
+ described in 7.1.3.
+
+
+45) CHAR_MIN, defined in <limits.h>, will have one of the values 0 or SCHAR_MIN, and this can be
+ used to distinguish the two options. Irrespective of the choice made, char is a separate type from the
+ other two and is not compatible with either.
+
+
+46) Note that aggregate type does not include union type because an object with union type can only
+ contain one member at a time.
+
+
+47) See 6.7.3 regarding qualified array and function types.
+
+
+48) The same representation and alignment requirements are meant to imply interchangeability as
+ arguments to functions, return values from functions, and members of unions.
+
+
+Contents
+
+6.2.6 Representations of types
+
+
+Contents
+
+6.2.6.1 General
+
+
+1
+ The representations of all types are unspecified except as stated in this subclause.
+
+2
+ Except for bit-fields, objects are composed of contiguous sequences of one or more bytes,
+ the number, order, and encoding of which are either explicitly specified or
+ implementation-defined.
+
+3
+ Values stored in unsigned bit-fields and objects of type unsigned char shall be
+ represented using a pure binary notation.49)
+
+4
+ Values stored in non-bit-field objects of any other object type consist of n x CHAR_BIT
+ bits, where n is the size of an object of that type, in bytes. The value may be copied into
+ an object of type unsigned char [n] (e.g., by memcpy); the resulting set of bytes is
+ called the object representation of the value. Values stored in bit-fields consist of m bits,
+ where m is the size specified for the bit-field. The object representation is the set of m
+ bits the bit-field comprises in the addressable storage unit holding it. Two values (other
+ than NaNs) with the same object representation compare equal, but values that compare
+ equal may have different object representations.
+
+5
+ Certain object representations need not represent a value of the object type. If the stored
+ value of an object has such a representation and is read by an lvalue expression that does
+ not have character type, the behavior is undefined. If such a representation is produced
+ by a side effect that modifies all or any part of the object by an lvalue expression that
+ does not have character type, the behavior is undefined.50) Such a representation is called
+ a trap representation.
+
+6
+ When a value is stored in an object of structure or union type, including in a member
+ object, the bytes of the object representation that correspond to any padding bytes take
+ unspecified values.51) The value of a structure or union object is never a trap
+
+
+
+ representation, even though the value of a member of the structure or union object may be
+ a trap representation.
+
+7
+ When a value is stored in a member of an object of union type, the bytes of the object
+ representation that do not correspond to that member but do correspond to other members
+ take unspecified values.
+
+8
+ Where an operator is applied to a value that has more than one object representation,
+ which object representation is used shall not affect the value of the result.52) Where a
+ value is stored in an object using a type that has more than one object representation for
+ that value, it is unspecified which representation is used, but a trap representation shall
+ not be generated.
+
+9
+ Loads and stores of objects with atomic types are done with
+ memory_order_seq_cst semantics.
+
+ Forward references: declarations (6.7), expressions (6.5), lvalues, arrays, and function
+ designators (6.3.2.1), order and consistency (7.17.3).
+
+
+Footnotes
+
+49) A positional representation for integers that uses the binary digits 0 and 1, in which the values
+ represented by successive bits are additive, begin with 1, and are multiplied by successive integral
+ powers of 2, except perhaps the bit with the highest position. (Adapted from the American National
+ Dictionary for Information Processing Systems.) A byte contains CHAR_BIT bits, and the values of
+ type unsigned char range from 0 to 2CHAR_BIT - 1.
+
+
+50) Thus, an automatic variable can be initialized to a trap representation without causing undefined
+ behavior, but the value of the variable cannot be used until a proper value is stored in it.
+
+
+51) Thus, for example, structure assignment need not copy any padding bits.
+
+
+52) It is possible for objects x and y with the same effective type T to have the same value when they are
+ accessed as objects of type T, but to have different values in other contexts. In particular, if == is
+ defined for type T, then x == y does not imply that memcmp(&x, &y, sizeof (T)) == 0.
+ Furthermore, x == y does not necessarily imply that x and y have the same value; other operations
+ on values of type T may distinguish between them.
+
+
+Contents
+
+6.2.6.2 Integer types
+
+
+1
+ For unsigned integer types other than unsigned char, the bits of the object
+ representation shall be divided into two groups: value bits and padding bits (there need
+ not be any of the latter). If there are N value bits, each bit shall represent a different
+ power of 2 between 1 and 2N - 1, so that objects of that type shall be capable of
+ representing values from 0 to 2N - 1 using a pure binary representation; this shall be
+ known as the value representation. The values of any padding bits are unspecified.53)
+
+2
+ For signed integer types, the bits of the object representation shall be divided into three
+ groups: value bits, padding bits, and the sign bit. There need not be any padding bits;
+ signed char shall not have any padding bits. There shall be exactly one sign bit.
+ Each bit that is a value bit shall have the same value as the same bit in the object
+ representation of the corresponding unsigned type (if there are M value bits in the signed
+ type and N in the unsigned type, then M <= N ). If the sign bit is zero, it shall not affect
+
+
+ the resulting value. If the sign bit is one, the value shall be modified in one of the
+ following ways:
+
+
+- the corresponding value with sign bit 0 is negated (sign and magnitude);
+
+- the sign bit has the value -(2M) (two's complement);
+
+- the sign bit has the value -(2M- 1) (ones' complement).
+
+ Which of these applies is implementation-defined, as is whether the value with sign bit 1
+ and all value bits zero (for the first two), or with sign bit and all value bits 1 (for ones'
+ complement), is a trap representation or a normal value. In the case of sign and
+ magnitude and ones' complement, if this representation is a normal value it is called a
+ negative zero.
+
+3
+ If the implementation supports negative zeros, they shall be generated only by:
+
+
+- the &, |, ^, ~, <<, and >> operators with operands that produce such a value;
+
+- the +, -, *, /, and % operators where one operand is a negative zero and the result is
+ zero;
+
+- compound assignment operators based on the above cases.
+
+ It is unspecified whether these cases actually generate a negative zero or a normal zero,
+ and whether a negative zero becomes a normal zero when stored in an object.
+
+4
+ If the implementation does not support negative zeros, the behavior of the &, |, ^, ~, <<,
+ and >> operators with operands that would produce such a value is undefined.
+
+5
+ The values of any padding bits are unspecified.54) A valid (non-trap) object representation
+ of a signed integer type where the sign bit is zero is a valid object representation of the
+ corresponding unsigned type, and shall represent the same value. For any integer type,
+ the object representation where all the bits are zero shall be a representation of the value
+ zero in that type.
+
+6
+ The precision of an integer type is the number of bits it uses to represent values,
+ excluding any sign and padding bits. The width of an integer type is the same but
+ including any sign bit; thus for unsigned integer types the two values are the same, while
+ for signed integer types the width is one greater than the precision.
+
+
+
+
+
+
+Footnotes
+
+53) Some combinations of padding bits might generate trap representations, for example, if one padding
+ bit is a parity bit. Regardless, no arithmetic operation on valid values can generate a trap
+ representation other than as part of an exceptional condition such as an overflow, and this cannot occur
+ with unsigned types. All other combinations of padding bits are alternative object representations of
+ the value specified by the value bits.
+
+
+54) Some combinations of padding bits might generate trap representations, for example, if one padding
+ bit is a parity bit. Regardless, no arithmetic operation on valid values can generate a trap
+ representation other than as part of an exceptional condition such as an overflow. All other
+ combinations of padding bits are alternative object representations of the value specified by the value
+ bits.
+
+
+Contents
+
+6.2.7 Compatible type and composite type
+
+
+1
+ Two types have compatible type if their types are the same. Additional rules for
+ determining whether two types are compatible are described in 6.7.2 for type specifiers,
+ in 6.7.3 for type qualifiers, and in 6.7.6 for declarators.55) Moreover, two structure,
+ union, or enumerated types declared in separate translation units are compatible if their
+ tags and members satisfy the following requirements: If one is declared with a tag, the
+ other shall be declared with the same tag. If both are completed anywhere within their
+ respective translation units, then the following additional requirements apply: there shall
+ be a one-to-one correspondence between their members such that each pair of
+ corresponding members are declared with compatible types; if one member of the pair is
+ declared with an alignment specifier, the other is declared with an equivalent alignment
+ specifier; and if one member of the pair is declared with a name, the other is declared
+ with the same name. For two structures, corresponding members shall be declared in the
+ same order. For two structures or unions, corresponding bit-fields shall have the same
+ widths. For two enumerations, corresponding members shall have the same values.
+
+2
+ All declarations that refer to the same object or function shall have compatible type;
+ otherwise, the behavior is undefined.
+
+3
+ A composite type can be constructed from two types that are compatible; it is a type that
+ is compatible with both of the two types and satisfies the following conditions:
+
+
+- If both types are array types, the following rules are applied:
+
+
+- If one type is an array of known constant size, the composite type is an array of
+ that size.
+
+- Otherwise, if one type is a variable length array whose size is specified by an
+ expression that is not evaluated, the behavior is undefined.
+
+- Otherwise, if one type is a variable length array whose size is specified, the
+ composite type is a variable length array of that size.
+
+- Otherwise, if one type is a variable length array of unspecified size, the composite
+ type is a variable length array of unspecified size.
+
+- Otherwise, both types are arrays of unknown size and the composite type is an
+ array of unknown size.
+
+ The element type of the composite type is the composite type of the two element
+ types.
+
+- If only one type is a function type with a parameter type list (a function prototype),
+ the composite type is a function prototype with the parameter type list.
+
+
+
+
+- If both types are function types with parameter type lists, the type of each parameter
+ in the composite parameter type list is the composite type of the corresponding
+ parameters.
+
+ These rules apply recursively to the types from which the two types are derived.
+
+4
+ For an identifier with internal or external linkage declared in a scope in which a prior
+ declaration of that identifier is visible,56) if the prior declaration specifies internal or
+ external linkage, the type of the identifier at the later declaration becomes the composite
+ type.
+
+ Forward references: array declarators (6.7.6.2).
+
+5
+ EXAMPLE Given the following two file scope declarations:
+
+
+ int f(int (*)(), double (*)[3]);
+ int f(int (*)(char *), double (*)[]);
+
+
+ The resulting composite type for the function is:
+
+
+ int f(int (*)(char *), double (*)[3]);
+
+
+
+
+
+Footnotes
+
+55) Two types need not be identical to be compatible.
+
+
+56) As specified in 6.2.1, the later declaration might hide the prior declaration.
+
+
+Contents
+
+6.2.8 Alignment of objects
+
+
+1
+ Complete object types have alignment requirements which place restrictions on the
+ addresses at which objects of that type may be allocated. An alignment is an
+ implementation-defined integer value representing the number of bytes between
+ successive addresses at which a given object can be allocated. An object type imposes an
+ alignment requirement on every object of that type: stricter alignment can be requested
+ using the _Alignas keyword.
+
+2
+ A fundamental alignment is represented by an alignment less than or equal to the greatest
+ alignment supported by the implementation in all contexts, which is equal to
+ _Alignof (max_align_t).
+
+3
+ An extended alignment is represented by an alignment greater than
+ _Alignof (max_align_t). It is implementation-defined whether any extended
+ alignments are supported and the contexts in which they are supported. A type having an
+ extended alignment requirement is an over-aligned type.57)
+
+4
+ Alignments are represented as values of the type size_t. Valid alignments include only
+ those values returned by an _Alignof expression for fundamental types, plus an
+ additional implementation-defined set of values, which may be empty. Every valid
+ alignment value shall be a nonnegative integral power of two.
+
+
+
+
+5
+ Alignments have an order from weaker to stronger or stricter alignments. Stricter
+ alignments have larger alignment values. An address that satisfies an alignment
+ requirement also satisfies any weaker valid alignment requirement.
+
+6
+ The alignment requirement of a complete type can be queried using an _Alignof
+ expression. The types char, signed char, and unsigned char shall have the
+ weakest alignment requirement.
+
+7
+ Comparing alignments is meaningful and provides the obvious results:
+
+
+- Two alignments are equal when their numeric values are equal.
+
+- Two alignments are different when their numeric values are not equal.
+
+- When an alignment is larger than another it represents a stricter alignment.
+
+
+Footnotes
+
+57) Every over-aligned type is, or contains, a structure or union type with a member to which an extended
+ alignment has been applied.
+
+
+Contents
+
+6.3 Conversions
+
+
+1
+ Several operators convert operand values from one type to another automatically. This
+ subclause specifies the result required from such an implicit conversion, as well as those
+ that result from a cast operation (an explicit conversion). The list in 6.3.1.8 summarizes
+ the conversions performed by most ordinary operators; it is supplemented as required by
+ the discussion of each operator in 6.5.
+
+2
+ Conversion of an operand value to a compatible type causes no change to the value or the
+ representation.
+
+ Forward references: cast operators (6.5.4).
+
+
+Contents
+
+6.3.1 Arithmetic operands
+
+
+Contents
+
+6.3.1.1 Boolean, characters, and integers
+
+
+1
+ Every integer type has an integer conversion rank defined as follows:
+
+
+- No two signed integer types shall have the same rank, even if they have the same
+ representation.
+
+- The rank of a signed integer type shall be greater than the rank of any signed integer
+ type with less precision.
+
+- The rank of long long int shall be greater than the rank of long int, which
+ shall be greater than the rank of int, which shall be greater than the rank of short
+ int, which shall be greater than the rank of signed char.
+
+- The rank of any unsigned integer type shall equal the rank of the corresponding
+ signed integer type, if any.
+
+- The rank of any standard integer type shall be greater than the rank of any extended
+ integer type with the same width.
+
+- The rank of char shall equal the rank of signed char and unsigned char.
+
+- The rank of _Bool shall be less than the rank of all other standard integer types.
+
+- The rank of any enumerated type shall equal the rank of the compatible integer type
+ (see 6.7.2.2).
+
+- The rank of any extended signed integer type relative to another extended signed
+ integer type with the same precision is implementation-defined, but still subject to the
+ other rules for determining the integer conversion rank.
+
+- For all integer types T1, T2, and T3, if T1 has greater rank than T2 and T2 has
+ greater rank than T3, then T1 has greater rank than T3.
+
+
+2
+ The following may be used in an expression wherever an int or unsigned int may
+ be used:
+
+
+- An object or expression with an integer type (other than int or unsigned int)
+ whose integer conversion rank is less than or equal to the rank of int and
+ unsigned int.
+
+- A bit-field of type _Bool, int, signed int, or unsigned int.
+
+ If an int can represent all values of the original type (as restricted by the width, for a
+ bit-field), the value is converted to an int; otherwise, it is converted to an unsigned
+ int. These are called the integer promotions.58) All other types are unchanged by the
+ integer promotions.
+
+3
+ The integer promotions preserve value including sign. As discussed earlier, whether a
+ ''plain'' char is treated as signed is implementation-defined.
+
+ Forward references: enumeration specifiers (6.7.2.2), structure and union specifiers
+ (6.7.2.1).
+
+
+Footnotes
+
+58) The integer promotions are applied only: as part of the usual arithmetic conversions, to certain
+ argument expressions, to the operands of the unary +, -, and ~ operators, and to both operands of the
+ shift operators, as specified by their respective subclauses.
+
+
+Contents
+
+6.3.1.2 Boolean type
+
+
+1
+ When any scalar value is converted to _Bool, the result is 0 if the value compares equal
+ to 0; otherwise, the result is 1.59)
+
+
+Footnotes
+
+59) NaNs do not compare equal to 0 and thus convert to 1.
+
+
+Contents
+
+6.3.1.3 Signed and unsigned integers
+
+
+1
+ When a value with integer type is converted to another integer type other than _Bool, if
+ the value can be represented by the new type, it is unchanged.
+
+2
+ Otherwise, if the new type is unsigned, the value is converted by repeatedly adding or
+ subtracting one more than the maximum value that can be represented in the new type
+ until the value is in the range of the new type.60)
+
+3
+ Otherwise, the new type is signed and the value cannot be represented in it; either the
+ result is implementation-defined or an implementation-defined signal is raised.
+
+
+Footnotes
+
+60) The rules describe arithmetic on the mathematical value, not the value of a given type of expression.
+
+
+Contents
+
+6.3.1.4 Real floating and integer
+
+
+1
+ When a finite value of real floating type is converted to an integer type other than _Bool,
+ the fractional part is discarded (i.e., the value is truncated toward zero). If the value of
+ the integral part cannot be represented by the integer type, the behavior is undefined.61)
+
+
+
+
+2
+ When a value of integer type is converted to a real floating type, if the value being
+ converted can be represented exactly in the new type, it is unchanged. If the value being
+ converted is in the range of values that can be represented but cannot be represented
+ exactly, the result is either the nearest higher or nearest lower representable value, chosen
+ in an implementation-defined manner. If the value being converted is outside the range of
+ values that can be represented, the behavior is undefined. Results of some implicit
+ conversions may be represented in greater range and precision than that required by the
+ new type (see 6.3.1.8 and 6.8.6.4).
+
+
+Footnotes
+
+61) The remaindering operation performed when a value of integer type is converted to unsigned type
+ need not be performed when a value of real floating type is converted to unsigned type. Thus, the
+ range of portable real floating values is (-1, Utype_MAX+1).
+
+
+Contents
+
+6.3.1.5 Real floating types
+
+
+1
+ When a value of real floating type is converted to a real floating type, if the value being
+ converted can be represented exactly in the new type, it is unchanged. If the value being
+ converted is in the range of values that can be represented but cannot be represented
+ exactly, the result is either the nearest higher or nearest lower representable value, chosen
+ in an implementation-defined manner. If the value being converted is outside the range of
+ values that can be represented, the behavior is undefined. Results of some implicit
+ conversions may be represented in greater range and precision than that required by the
+ new type (see 6.3.1.8 and 6.8.6.4).
+
+
+Contents
+
+6.3.1.6 Complex types
+
+
+1
+ When a value of complex type is converted to another complex type, both the real and
+ imaginary parts follow the conversion rules for the corresponding real types.
+
+
+Contents
+
+6.3.1.7 Real and complex
+
+
+1
+ When a value of real type is converted to a complex type, the real part of the complex
+ result value is determined by the rules of conversion to the corresponding real type and
+ the imaginary part of the complex result value is a positive zero or an unsigned zero.
+
+2
+ When a value of complex type is converted to a real type, the imaginary part of the
+ complex value is discarded and the value of the real part is converted according to the
+ conversion rules for the corresponding real type.
+
+
+Contents
+
+6.3.1.8 Usual arithmetic conversions
+
+
+1
+ Many operators that expect operands of arithmetic type cause conversions and yield result
+ types in a similar way. The purpose is to determine a common real type for the operands
+ and result. For the specified operands, each operand is converted, without change of type
+ domain, to a type whose corresponding real type is the common real type. Unless
+ explicitly stated otherwise, the common real type is also the corresponding real type of
+ the result, whose type domain is the type domain of the operands if they are the same,
+ and complex otherwise. This pattern is called the usual arithmetic conversions:
+
+
+- First, if the corresponding real type of either operand is long double, the other
+ operand is converted, without change of type domain, to a type whose
+ corresponding real type is long double.
+
+- Otherwise, if the corresponding real type of either operand is double, the other
+ operand is converted, without change of type domain, to a type whose
+ corresponding real type is double.
+
+- Otherwise, if the corresponding real type of either operand is float, the other
+ operand is converted, without change of type domain, to a type whose
+ corresponding real type is float.62)
+
+- Otherwise, the integer promotions are performed on both operands. Then the
+ following rules are applied to the promoted operands:
+
+
+- If both operands have the same type, then no further conversion is needed.
+
+- Otherwise, if both operands have signed integer types or both have unsigned
+ integer types, the operand with the type of lesser integer conversion rank is
+ converted to the type of the operand with greater rank.
+
+- Otherwise, if the operand that has unsigned integer type has rank greater or
+ equal to the rank of the type of the other operand, then the operand with
+ signed integer type is converted to the type of the operand with unsigned
+ integer type.
+
+- Otherwise, if the type of the operand with signed integer type can represent
+ all of the values of the type of the operand with unsigned integer type, then
+ the operand with unsigned integer type is converted to the type of the
+ operand with signed integer type.
+
+- Otherwise, both operands are converted to the unsigned integer type
+ corresponding to the type of the operand with signed integer type.
+
+
+2
+ The values of floating operands and of the results of floating expressions may be
+ represented in greater range and precision than that required by the type; the types are not
+ changed thereby.63)
+
+
+
+
+
+
+Footnotes
+
+62) For example, addition of a double _Complex and a float entails just the conversion of the
+ float operand to double (and yields a double _Complex result).
+
+
+63) The cast and assignment operators are still required to remove extra range and precision.
+
+
+Contents
+
+6.3.2 Other operands
+
+
+Contents
+
+6.3.2.1 Lvalues, arrays, and function designators
+
+
+1
+ An lvalue is an expression (with an object type other than void) that potentially
+ designates an object;64) if an lvalue does not designate an object when it is evaluated, the
+ behavior is undefined. When an object is said to have a particular type, the type is
+ specified by the lvalue used to designate the object. A modifiable lvalue is an lvalue that
+ does not have array type, does not have an incomplete type, does not have a const-
+ qualified type, and if it is a structure or union, does not have any member (including,
+ recursively, any member or element of all contained aggregates or unions) with a const-
+ qualified type.
+
+2
+ Except when it is the operand of the sizeof operator, the _Alignof operator, the
+ unary & operator, the ++ operator, the -- operator, or the left operand of the . operator
+ or an assignment operator, an lvalue that does not have array type is converted to the
+ value stored in the designated object (and is no longer an lvalue); this is called lvalue
+ conversion. If the lvalue has qualified type, the value has the unqualified version of the
+ type of the lvalue; additionally, if the lvalue has atomic type, the value has the non-atomic
+ version of the type of the lvalue; otherwise, the value has the type of the lvalue. If the
+ lvalue has an incomplete type and does not have array type, the behavior is undefined. If
+ the lvalue designates an object of automatic storage duration that could have been
+ declared with the register storage class (never had its address taken), and that object
+ is uninitialized (not declared with an initializer and no assignment to it has been
+ performed prior to use), the behavior is undefined.
+
+3
+ Except when it is the operand of the sizeof operator, the _Alignof operator, or the
+ unary & operator, or is a string literal used to initialize an array, an expression that has
+ type ''array of type'' is converted to an expression with type ''pointer to type'' that points
+ to the initial element of the array object and is not an lvalue. If the array object has
+ register storage class, the behavior is undefined.
+
+4
+ A function designator is an expression that has function type. Except when it is the
+ operand of the sizeof operator, the _Alignof operator,65) or the unary & operator, a
+ function designator with type ''function returning type'' is converted to an expression that
+
+
+
+ has type ''pointer to function returning type''.
+
+ Forward references: address and indirection operators (6.5.3.2), assignment operators
+ (6.5.16), common definitions <stddef.h> (7.19), initialization (6.7.9), postfix
+ increment and decrement operators (6.5.2.4), prefix increment and decrement operators
+ (6.5.3.1), the sizeof and _Alignof operators (6.5.3.4), structure and union members
+ (6.5.2.3).
+
+
+Footnotes
+
+64) The name ''lvalue'' comes originally from the assignment expression E1 = E2, in which the left
+ operand E1 is required to be a (modifiable) lvalue. It is perhaps better considered as representing an
+ object ''locator value''. What is sometimes called ''rvalue'' is in this International Standard described
+ as the ''value of an expression''.
+ An obvious example of an lvalue is an identifier of an object. As a further example, if E is a unary
+ expression that is a pointer to an object, *E is an lvalue that designates the object to which E points.
+
+
+65) Because this conversion does not occur, the operand of the sizeof or _Alignof operator remains
+ a function designator and violates the constraints in 6.5.3.4.
+
+
+Contents
+
+6.3.2.2 void
+
+
+1
+ The (nonexistent) value of a void expression (an expression that has type void) shall not
+ be used in any way, and implicit or explicit conversions (except to void) shall not be
+ applied to such an expression. If an expression of any other type is evaluated as a void
+ expression, its value or designator is discarded. (A void expression is evaluated for its
+ side effects.)
+
+
+Contents
+
+6.3.2.3 Pointers
+
+
+1
+ A pointer to void may be converted to or from a pointer to any object type. A pointer to
+ any object type may be converted to a pointer to void and back again; the result shall
+ compare equal to the original pointer.
+
+2
+ For any qualifier q, a pointer to a non-q-qualified type may be converted to a pointer to
+ the q-qualified version of the type; the values stored in the original and converted pointers
+ shall compare equal.
+
+3
+ An integer constant expression with the value 0, or such an expression cast to type
+ void *, is called a null pointer constant.66) If a null pointer constant is converted to a
+ pointer type, the resulting pointer, called a null pointer, is guaranteed to compare unequal
+ to a pointer to any object or function.
+
+4
+ Conversion of a null pointer to another pointer type yields a null pointer of that type.
+ Any two null pointers shall compare equal.
+
+5
+ An integer may be converted to any pointer type. Except as previously specified, the
+ result is implementation-defined, might not be correctly aligned, might not point to an
+ entity of the referenced type, and might be a trap representation.67)
+
+6
+ Any pointer type may be converted to an integer type. Except as previously specified, the
+ result is implementation-defined. If the result cannot be represented in the integer type,
+ the behavior is undefined. The result need not be in the range of values of any integer
+ type.
+
+
+
+
+7
+ A pointer to an object type may be converted to a pointer to a different object type. If the
+ resulting pointer is not correctly aligned68) for the referenced type, the behavior is
+ undefined. Otherwise, when converted back again, the result shall compare equal to the
+ original pointer. When a pointer to an object is converted to a pointer to a character type,
+ the result points to the lowest addressed byte of the object. Successive increments of the
+ result, up to the size of the object, yield pointers to the remaining bytes of the object.
+
+8
+ A pointer to a function of one type may be converted to a pointer to a function of another
+ type and back again; the result shall compare equal to the original pointer. If a converted
+ pointer is used to call a function whose type is not compatible with the referenced type,
+ the behavior is undefined.
+
+ Forward references: cast operators (6.5.4), equality operators (6.5.9), integer types
+ capable of holding object pointers (7.20.1.4), simple assignment (6.5.16.1).
+
+
+
+
+
+
+Footnotes
+
+66) The macro NULL is defined in <stddef.h> (and other headers) as a null pointer constant; see 7.19.
+
+
+67) The mapping functions for converting a pointer to an integer or an integer to a pointer are intended to
+ be consistent with the addressing structure of the execution environment.
+
+
+68) In general, the concept ''correctly aligned'' is transitive: if a pointer to type A is correctly aligned for a
+ pointer to type B, which in turn is correctly aligned for a pointer to type C, then a pointer to type A is
+ correctly aligned for a pointer to type C.
+
+
+Contents
+
+6.4 Lexical elements
+
+
+Syntax
+
+1
+
+
+ token:
+ keyword
+ identifier
+ constant
+ string-literal
+ punctuator
+ preprocessing-token:
+ header-name
+ identifier
+ pp-number
+ character-constant
+ string-literal
+ punctuator
+ each non-white-space character that cannot be one of the above
+
+
+Constraints
+
+2
+ Each preprocessing token that is converted to a token shall have the lexical form of a
+ keyword, an identifier, a constant, a string literal, or a punctuator.
+
+Semantics
+
+3
+ A token is the minimal lexical element of the language in translation phases 7 and 8. The
+ categories of tokens are: keywords, identifiers, constants, string literals, and punctuators.
+ A preprocessing token is the minimal lexical element of the language in translation
+ phases 3 through 6. The categories of preprocessing tokens are: header names,
+ identifiers, preprocessing numbers, character constants, string literals, punctuators, and
+ single non-white-space characters that do not lexically match the other preprocessing
+ token categories.69) If a ' or a " character matches the last category, the behavior is
+ undefined. Preprocessing tokens can be separated by white space; this consists of
+ comments (described later), or white-space characters (space, horizontal tab, new-line,
+ vertical tab, and form-feed), or both. As described in 6.10, in certain circumstances
+ during translation phase 4, white space (or the absence thereof) serves as more than
+ preprocessing token separation. White space may appear within a preprocessing token
+ only as part of a header name or between the quotation characters in a character constant
+ or string literal.
+
+
+
+
+
+4
+ If the input stream has been parsed into preprocessing tokens up to a given character, the
+ next preprocessing token is the longest sequence of characters that could constitute a
+ preprocessing token. There is one exception to this rule: header name preprocessing
+ tokens are recognized only within #include preprocessing directives and in
+ implementation-defined locations within #pragma directives. In such contexts, a
+ sequence of characters that could be either a header name or a string literal is recognized
+ as the former.
+
+5
+ EXAMPLE 1 The program fragment 1Ex is parsed as a preprocessing number token (one that is not a
+ valid floating or integer constant token), even though a parse as the pair of preprocessing tokens 1 and Ex
+ might produce a valid expression (for example, if Ex were a macro defined as +1). Similarly, the program
+ fragment 1E1 is parsed as a preprocessing number (one that is a valid floating constant token), whether or
+ not E is a macro name.
+
+
+6
+ EXAMPLE 2 The program fragment x+++++y is parsed as x ++ ++ + y, which violates a constraint on
+ increment operators, even though the parse x ++ + ++ y might yield a correct expression.
+
+
+ Forward references: character constants (6.4.4.4), comments (6.4.9), expressions (6.5),
+ floating constants (6.4.4.2), header names (6.4.7), macro replacement (6.10.3), postfix
+ increment and decrement operators (6.5.2.4), prefix increment and decrement operators
+ (6.5.3.1), preprocessing directives (6.10), preprocessing numbers (6.4.8), string literals
+ (6.4.5).
+
+
+Footnotes
+
+69) An additional category, placemarkers, is used internally in translation phase 4 (see 6.10.3.3); it cannot
+ occur in source files.
+
+
+Contents
+
+6.4.1 Keywords
+
+
+Syntax
+
+1
+
+
+ keyword: one of
+ auto if unsigned
+ break inline void
+ case int volatile
+ char long while
+ const register _Alignas
+ continue restrict _Alignof
+ default return _Atomic
+ do short _Bool
+ double signed _Complex
+ else sizeof _Generic
+ enum static _Imaginary
+ extern struct _Noreturn
+ float switch _Static_assert
+ for typedef _Thread_local
+ goto union
+
+
+Semantics
+
+2
+ The above tokens (case sensitive) are reserved (in translation phases 7 and 8) for use as
+ keywords, and shall not be used otherwise. The keyword _Imaginary is reserved for
+
+ specifying imaginary types.70)
+
+
+Footnotes
+
+70) One possible specification for imaginary types appears in annex G.
+
+
+Contents
+
+6.4.2 Identifiers
+
+
+Contents
+
+6.4.2.1 General
+
+
+Syntax
+
+1
+
+
+ identifier:
+ identifier-nondigit
+ identifier identifier-nondigit
+ identifier digit
+ identifier-nondigit:
+ nondigit
+ universal-character-name
+ other implementation-defined characters
+ nondigit: one of
+ _ a b c d e f g h i j k l m
+ n o p q r s t u v w x y z
+ A B C D E F G H I J K L M
+ N O P Q R S T U V W X Y Z
+ digit: one of
+ 0 1 2 3 4 5 6 7 8 9
+
+
+Semantics
+
+2
+ An identifier is a sequence of nondigit characters (including the underscore _, the
+ lowercase and uppercase Latin letters, and other characters) and digits, which designates
+ one or more entities as described in 6.2.1. Lowercase and uppercase letters are distinct.
+ There is no specific limit on the maximum length of an identifier.
+
+3
+ Each universal character name in an identifier shall designate a character whose encoding
+ in ISO/IEC 10646 falls into one of the ranges specified in D.1.71) The initial character
+ shall not be a universal character name designating a character whose encoding falls into
+ one of the ranges specified in D.2. An implementation may allow multibyte characters
+ that are not part of the basic source character set to appear in identifiers; which characters
+ and their correspondence to universal character names is implementation-defined.
+
+
+
+
+
+4
+ When preprocessing tokens are converted to tokens during translation phase 7, if a
+ preprocessing token could be converted to either a keyword or an identifier, it is converted
+ to a keyword.
+
+Implementation limits
+
+5
+ As discussed in 5.2.4.1, an implementation may limit the number of significant initial
+ characters in an identifier; the limit for an external name (an identifier that has external
+ linkage) may be more restrictive than that for an internal name (a macro name or an
+ identifier that does not have external linkage). The number of significant characters in an
+ identifier is implementation-defined.
+
+6
+ Any identifiers that differ in a significant character are different identifiers. If two
+ identifiers differ only in nonsignificant characters, the behavior is undefined.
+
+ Forward references: universal character names (6.4.3), macro replacement (6.10.3).
+
+
+Footnotes
+
+71) On systems in which linkers cannot accept extended characters, an encoding of the universal character
+ name may be used in forming valid external identifiers. For example, some otherwise unused
+ character or sequence of characters may be used to encode the \u in a universal character name.
+ Extended characters may produce a long external identifier.
+
+
+Contents
+
+6.4.2.2 Predefined identifiers
+
+
+Semantics
+
+1
+ The identifier __func__ shall be implicitly declared by the translator as if,
+ immediately following the opening brace of each function definition, the declaration
+
+
+ static const char __func__[] = "function-name";
+
+
+ appeared, where function-name is the name of the lexically-enclosing function.72)
+
+2
+ This name is encoded as if the implicit declaration had been written in the source
+ character set and then translated into the execution character set as indicated in translation
+ phase 5.
+
+3
+ EXAMPLE Consider the code fragment:
+
+
+ #include <stdio.h>
+ void myfunc(void)
+ {
+ printf("%s\n", __func__);
+ /* ... */
+ }
+
+
+ Each time the function is called, it will print to the standard output stream:
+
+
+ myfunc
+
+
+
+
+ Forward references: function definitions (6.9.1).
+
+
+
+
+
+
+Footnotes
+
+72) Since the name __func__ is reserved for any use by the implementation (7.1.3), if any other
+ identifier is explicitly declared using the name __func__, the behavior is undefined.
+
+
+Contents
+
+6.4.3 Universal character names
+
+
+Syntax
+
+1
+
+
+ universal-character-name:
+ \u hex-quad
+ \U hex-quad hex-quad
+ hex-quad:
+ hexadecimal-digit hexadecimal-digit
+ hexadecimal-digit hexadecimal-digit
+
+
+Constraints
+
+2
+ A universal character name shall not specify a character whose short identifier is less than
+ 00A0 other than 0024 ($), 0040 (@), or 0060 ('), nor one in the range D800 through
+ DFFF inclusive.73)
+
+Description
+
+3
+ Universal character names may be used in identifiers, character constants, and string
+ literals to designate characters that are not in the basic character set.
+
+Semantics
+
+4
+ The universal character name \Unnnnnnnn designates the character whose eight-digit
+ short identifier (as specified by ISO/IEC 10646) is nnnnnnnn.74) Similarly, the universal
+ character name \unnnn designates the character whose four-digit short identifier is nnnn
+ (and whose eight-digit short identifier is 0000nnnn).
+
+
+
+
+
+
+Footnotes
+
+73) The disallowed characters are the characters in the basic character set and the code positions reserved
+ by ISO/IEC 10646 for control characters, the character DELETE, and the S-zone (reserved for use by
+ UTF-16).
+
+
+
+74) Short identifiers for characters were first specified in ISO/IEC 10646-1/AMD9:1997.
+
+
+Contents
+
+6.4.4 Constants
+
+
+Syntax
+
+1
+
+
+ constant:
+ integer-constant
+ floating-constant
+ enumeration-constant
+ character-constant
+
+
+Constraints
+
+2
+ Each constant shall have a type and the value of a constant shall be in the range of
+ representable values for its type.
+
+Semantics
+
+3
+ Each constant has a type, determined by its form and value, as detailed later.
+
+
+Contents
+
+6.4.4.1 Integer constants
+
+
+Syntax
+
+1
+
+
+ integer-constant:
+ decimal-constant integer-suffixopt
+ octal-constant integer-suffixopt
+ hexadecimal-constant integer-suffixopt
+ decimal-constant:
+ nonzero-digit
+ decimal-constant digit
+ octal-constant:
+ 0
+ octal-constant octal-digit
+ hexadecimal-constant:
+ hexadecimal-prefix hexadecimal-digit
+ hexadecimal-constant hexadecimal-digit
+ hexadecimal-prefix: one of
+ 0x 0X
+ nonzero-digit: one of
+ 1 2 3 4 5 6 7 8 9
+ octal-digit: one of
+ 0 1 2 3 4 5 6 7
+ hexadecimal-digit: one of
+ 0 1 2 3 4 5 6 7 8 9
+ a b c d e f
+ A B C D E F
+ integer-suffix:
+ unsigned-suffix long-suffixopt
+ unsigned-suffix long-long-suffix
+ long-suffix unsigned-suffixopt
+ long-long-suffix unsigned-suffixopt
+ unsigned-suffix: one of
+ u U
+ long-suffix: one of
+ l L
+ long-long-suffix: one of
+ ll LL
+
+
+Description
+
+2
+ An integer constant begins with a digit, but has no period or exponent part. It may have a
+ prefix that specifies its base and a suffix that specifies its type.
+
+3
+ A decimal constant begins with a nonzero digit and consists of a sequence of decimal
+ digits. An octal constant consists of the prefix 0 optionally followed by a sequence of the
+ digits 0 through 7 only. A hexadecimal constant consists of the prefix 0x or 0X followed
+ by a sequence of the decimal digits and the letters a (or A) through f (or F) with values
+ 10 through 15 respectively.
+
+Semantics
+
+4
+ The value of a decimal constant is computed base 10; that of an octal constant, base 8;
+ that of a hexadecimal constant, base 16. The lexically first digit is the most significant.
+
+5
+ The type of an integer constant is the first of the corresponding list in which its value can
+ be represented.
+
+
+ Suffix Decimal Constant Octal or Hexadecimal Constant
+
+ none
+
+
+int
+long int
+long long int
+
+
+
+
+int
+unsigned int
+long int
+unsigned long int
+long long int
+unsigned long long int
+
+
+ u or U
+
+
+unsigned int
+unsigned long int
+unsigned long long int
+
+
+
+
+unsigned int
+unsigned long int
+unsigned long long int
+
+
+ l or L
+
+
+long int
+long long int
+
+
+
+
+long int
+unsigned long int
+long long int
+unsigned long long int
+
+
+ Both u or U and l or L
+
+
+unsigned long int
+unsigned long long int
+
+
+
+
+unsigned long int
+unsigned long long int
+
+
+ ll or LL
+
+
+long long int
+
+
+
+
+long long int
+unsigned long long int
+
+
+ Both u or U and ll or LL
+
+
+unsigned long long int
+
+
+
+
+unsigned long long int
+
+
+6
+ If an integer constant cannot be represented by any type in its list, it may have an
+ extended integer type, if the extended integer type can represent its value. If all of the
+ types in the list for the constant are signed, the extended integer type shall be signed. If
+ all of the types in the list for the constant are unsigned, the extended integer type shall be
+ unsigned. If the list contains both signed and unsigned types, the extended integer type
+ may be signed or unsigned. If an integer constant cannot be represented by any type in
+ its list and has no extended integer type, then the integer constant has no type.
+
+
+Contents
+
+6.4.4.2 Floating constants
+
+
+Syntax
+
+1
+
+
+ floating-constant:
+ decimal-floating-constant
+ hexadecimal-floating-constant
+ decimal-floating-constant:
+ fractional-constant exponent-partopt floating-suffixopt
+ digit-sequence exponent-part floating-suffixopt
+ hexadecimal-floating-constant:
+ hexadecimal-prefix hexadecimal-fractional-constant
+ binary-exponent-part floating-suffixopt
+ hexadecimal-prefix hexadecimal-digit-sequence
+ binary-exponent-part floating-suffixopt
+ fractional-constant:
+ digit-sequenceopt . digit-sequence
+ digit-sequence .
+ exponent-part:
+ e signopt digit-sequence
+ E signopt digit-sequence
+ sign: one of
+ + -
+ digit-sequence:
+ digit
+ digit-sequence digit
+ hexadecimal-fractional-constant:
+ hexadecimal-digit-sequenceopt .
+ hexadecimal-digit-sequence
+ hexadecimal-digit-sequence .
+ binary-exponent-part:
+ p signopt digit-sequence
+ P signopt digit-sequence
+ hexadecimal-digit-sequence:
+ hexadecimal-digit
+ hexadecimal-digit-sequence hexadecimal-digit
+ floating-suffix: one of
+ f l F L
+
+
+Description
+
+2
+ A floating constant has a significand part that may be followed by an exponent part and a
+ suffix that specifies its type. The components of the significand part may include a digit
+ sequence representing the whole-number part, followed by a period (.), followed by a
+ digit sequence representing the fraction part. The components of the exponent part are an
+ e, E, p, or P followed by an exponent consisting of an optionally signed digit sequence.
+ Either the whole-number part or the fraction part has to be present; for decimal floating
+ constants, either the period or the exponent part has to be present.
+
+Semantics
+
+3
+ The significand part is interpreted as a (decimal or hexadecimal) rational number; the
+ digit sequence in the exponent part is interpreted as a decimal integer. For decimal
+ floating constants, the exponent indicates the power of 10 by which the significand part is
+ to be scaled. For hexadecimal floating constants, the exponent indicates the power of 2
+ by which the significand part is to be scaled. For decimal floating constants, and also for
+ hexadecimal floating constants when FLT_RADIX is not a power of 2, the result is either
+ the nearest representable value, or the larger or smaller representable value immediately
+ adjacent to the nearest representable value, chosen in an implementation-defined manner.
+ For hexadecimal floating constants when FLT_RADIX is a power of 2, the result is
+ correctly rounded.
+
+4
+ An unsuffixed floating constant has type double. If suffixed by the letter f or F, it has
+ type float. If suffixed by the letter l or L, it has type long double.
+
+5
+ Floating constants are converted to internal format as if at translation-time. The
+ conversion of a floating constant shall not raise an exceptional condition or a floating-
+ point exception at execution time. All floating constants of the same source form75) shall
+ convert to the same internal format with the same value.
+
+Recommended practice
+
+6
+ The implementation should produce a diagnostic message if a hexadecimal constant
+ cannot be represented exactly in its evaluation format; the implementation should then
+ proceed with the translation of the program.
+
+7
+ The translation-time conversion of floating constants should match the execution-time
+ conversion of character strings by library functions, such as strtod, given matching
+ inputs suitable for both conversions, the same result format, and default execution-time
+ rounding.76)
+
+
+
+Footnotes
+
+75) 1.23, 1.230, 123e-2, 123e-02, and 1.23L are all different source forms and thus need not
+ convert to the same internal format and value.
+
+
+76) The specification for the library functions recommends more accurate conversion than required for
+ floating constants (see 7.22.1.3).
+
+
+Contents
+
+6.4.4.3 Enumeration constants
+
+
+Syntax
+
+1
+
+
+ enumeration-constant:
+ identifier
+
+
+Semantics
+
+2
+ An identifier declared as an enumeration constant has type int.
+
+ Forward references: enumeration specifiers (6.7.2.2).
+
+
+Contents
+
+6.4.4.4 Character constants
+
+
+Syntax
+
+1
+
+
+ character-constant:
+ ' c-char-sequence '
+ L' c-char-sequence '
+ u' c-char-sequence '
+ U' c-char-sequence '
+ c-char-sequence:
+ c-char
+ c-char-sequence c-char
+ c-char:
+ any member of the source character set except
+ the single-quote ', backslash \, or new-line character
+ escape-sequence
+ escape-sequence:
+ simple-escape-sequence
+ octal-escape-sequence
+ hexadecimal-escape-sequence
+ universal-character-name
+ simple-escape-sequence: one of
+ \' \" \? \\
+ \a \b \f \n \r \t \v
+ octal-escape-sequence:
+ \ octal-digit
+ \ octal-digit octal-digit
+ \ octal-digit octal-digit octal-digit
+ hexadecimal-escape-sequence:
+ \x hexadecimal-digit
+ hexadecimal-escape-sequence hexadecimal-digit
+
+
+Description
+
+2
+ An integer character constant is a sequence of one or more multibyte characters enclosed
+ in single-quotes, as in 'x'. A wide character constant is the same, except prefixed by the
+ letter L, u, or U. With a few exceptions detailed later, the elements of the sequence are
+ any members of the source character set; they are mapped in an implementation-defined
+ manner to members of the execution character set.
+
+3
+ The single-quote ', the double-quote ", the question-mark ?, the backslash \, and
+ arbitrary integer values are representable according to the following table of escape
+ sequences:
+
+
+ single quote ' \'
+ double quote " \"
+ question mark ? \?
+ backslash \ \\
+ octal character \octal digits
+ hexadecimal character \x hexadecimal digits
+
+
+4
+ The double-quote " and question-mark ? are representable either by themselves or by the
+ escape sequences \" and \?, respectively, but the single-quote ' and the backslash \
+ shall be represented, respectively, by the escape sequences \' and \\.
+
+5
+ The octal digits that follow the backslash in an octal escape sequence are taken to be part
+ of the construction of a single character for an integer character constant or of a single
+ wide character for a wide character constant. The numerical value of the octal integer so
+ formed specifies the value of the desired character or wide character.
+
+6
+ The hexadecimal digits that follow the backslash and the letter x in a hexadecimal escape
+ sequence are taken to be part of the construction of a single character for an integer
+ character constant or of a single wide character for a wide character constant. The
+ numerical value of the hexadecimal integer so formed specifies the value of the desired
+ character or wide character.
+
+7
+ Each octal or hexadecimal escape sequence is the longest sequence of characters that can
+ constitute the escape sequence.
+
+8
+ In addition, characters not in the basic character set are representable by universal
+ character names and certain nongraphic characters are representable by escape sequences
+ consisting of the backslash \ followed by a lowercase letter: \a, \b, \f, \n, \r, \t,
+ and \v.77)
+
+
+Constraints
+
+9
+ The value of an octal or hexadecimal escape sequence shall be in the range of
+ representable values for the corresponding type:
+
+
+ Prefix Corresponding Type
+
+ none unsigned char
+
+ L the unsigned type corresponding to wchar_t
+
+ u char16_t
+
+ U char32_t
+
+
+Semantics
+
+10
+ An integer character constant has type int. The value of an integer character constant
+ containing a single character that maps to a single-byte execution character is the
+ numerical value of the representation of the mapped character interpreted as an integer.
+ The value of an integer character constant containing more than one character (e.g.,
+ 'ab'), or containing a character or escape sequence that does not map to a single-byte
+ execution character, is implementation-defined. If an integer character constant contains
+ a single character or escape sequence, its value is the one that results when an object with
+ type char whose value is that of the single character or escape sequence is converted to
+ type int.
+
+11
+ A wide character constant prefixed by the letter L has type wchar_t, an integer type
+ defined in the <stddef.h> header; a wide character constant prefixed by the letter u or
+ U has type char16_t or char32_t, respectively, unsigned integer types defined in the
+ <uchar.h> header. The value of a wide character constant containing a single
+ multibyte character that maps to a single member of the extended execution character set
+ is the wide character corresponding to that multibyte character, as defined by the
+ mbtowc, mbrtoc16, or mbrtoc32 function as appropriate for its type, with an
+ implementation-defined current locale. The value of a wide character constant containing
+ more than one multibyte character or a single multibyte character that maps to multiple
+ members of the extended execution character set, or containing a multibyte character or
+ escape sequence not represented in the extended execution character set, is
+ implementation-defined.
+
+12
+ EXAMPLE 1 The construction '\0' is commonly used to represent the null character.
+
+
+13
+ EXAMPLE 2 Consider implementations that use two's complement representation for integers and eight
+ bits for objects that have type char. In an implementation in which type char has the same range of
+ values as signed char, the integer character constant '\xFF' has the value -1; if type char has the
+ same range of values as unsigned char, the character constant '\xFF' has the value +255.
+
+
+
+
+
+
+14
+ EXAMPLE 3 Even if eight bits are used for objects that have type char, the construction '\x123'
+ specifies an integer character constant containing only one character, since a hexadecimal escape sequence
+ is terminated only by a non-hexadecimal character. To specify an integer character constant containing the
+ two characters whose values are '\x12' and '3', the construction '\0223' may be used, since an octal
+ escape sequence is terminated after three octal digits. (The value of this two-character integer character
+ constant is implementation-defined.)
+
+
+15
+ EXAMPLE 4 Even if 12 or more bits are used for objects that have type wchar_t, the construction
+ L'\1234' specifies the implementation-defined value that results from the combination of the values
+ 0123 and '4'.
+
+
+ Forward references: common definitions <stddef.h> (7.19), the mbtowc function
+ (7.22.7.2), Unicode utilities <uchar.h> (7.28).
+
+
+Footnotes
+
+77) The semantics of these characters were discussed in 5.2.2. If any other character follows a backslash,
+ the result is not a token and a diagnostic is required. See ''future language directions'' (6.11.4).
+
+
+Contents
+
+6.4.5 String literals
+
+
+Syntax
+
+1
+
+
+ string-literal:
+ encoding-prefixopt " s-char-sequenceopt "
+ encoding-prefix:
+ u8
+ u
+ U
+ L
+ s-char-sequence:
+ s-char
+ s-char-sequence s-char
+ s-char:
+ any member of the source character set except
+ the double-quote ", backslash \, or new-line character
+ escape-sequence
+
+
+Constraints
+
+2
+ A sequence of adjacent string literal tokens shall not include both a wide string literal and
+ a UTF-8 string literal.
+
+Description
+
+3
+ A character string literal is a sequence of zero or more multibyte characters enclosed in
+ double-quotes, as in "xyz". A UTF-8 string literal is the same, except prefixed by u8.
+ A wide string literal is the same, except prefixed by the letter L, u, or U.
+
+4
+ The same considerations apply to each element of the sequence in a string literal as if it
+ were in an integer character constant (for a character or UTF-8 string literal) or a wide
+ character constant (for a wide string literal), except that the single-quote ' is
+ representable either by itself or by the escape sequence \', but the double-quote " shall
+
+ be represented by the escape sequence \".
+
+Semantics
+
+5
+ In translation phase 6, the multibyte character sequences specified by any sequence of
+ adjacent character and identically-prefixed string literal tokens are concatenated into a
+ single multibyte character sequence. If any of the tokens has an encoding prefix, the
+ resulting multibyte character sequence is treated as having the same prefix; otherwise, it
+ is treated as a character string literal. Whether differently-prefixed wide string literal
+ tokens can be concatenated and, if so, the treatment of the resulting multibyte character
+ sequence are implementation-defined.
+
+6
+ In translation phase 7, a byte or code of value zero is appended to each multibyte
+ character sequence that results from a string literal or literals.78) The multibyte character
+ sequence is then used to initialize an array of static storage duration and length just
+ sufficient to contain the sequence. For character string literals, the array elements have
+ type char, and are initialized with the individual bytes of the multibyte character
+ sequence. For UTF-8 string literals, the array elements have type char, and are
+ initialized with the characters of the multibyte character sequence, as encoded in UTF-8.
+ For wide string literals prefixed by the letter L, the array elements have type wchar_t
+ and are initialized with the sequence of wide characters corresponding to the multibyte
+ character sequence, as defined by the mbstowcs function with an implementation-
+ defined current locale. For wide string literals prefixed by the letter u or U, the array
+ elements have type char16_t or char32_t, respectively, and are initialized with the
+ sequence of wide characters corresponding to the multibyte character sequence, as
+ defined by successive calls to the mbrtoc16, or mbrtoc32 function as appropriate for
+ its type, with an implementation-defined current locale. The value of a string literal
+ containing a multibyte character or escape sequence not represented in the execution
+ character set is implementation-defined.
+
+7
+ It is unspecified whether these arrays are distinct provided their elements have the
+ appropriate values. If the program attempts to modify such an array, the behavior is
+ undefined.
+
+8
+ EXAMPLE 1 This pair of adjacent character string literals
+
+
+ "\x12" "3"
+
+
+ produces a single character string literal containing the two characters whose values are '\x12' and '3',
+ because escape sequences are converted into single members of the execution character set just prior to
+ adjacent string literal concatenation.
+
+
+9
+ EXAMPLE 2 Each of the sequences of adjacent string literal tokens
+
+
+
+
+
+ "a" "b" L"c"
+ "a" L"b" "c"
+ L"a" "b" L"c"
+ L"a" L"b" L"c"
+
+
+ is equivalent to the string literal
+
+
+ L"abc"
+
+
+ Likewise, each of the sequences
+
+
+ "a" "b" u"c"
+ "a" u"b" "c"
+ u"a" "b" u"c"
+ u"a" u"b" u"c"
+
+
+ is equivalent to
+
+
+ u"abc"
+
+
+
+
+ Forward references: common definitions <stddef.h> (7.19), the mbstowcs
+ function (7.22.8.1), Unicode utilities <uchar.h> (7.28).
+
+
+Footnotes
+
+78) A string literal need not be a string (see 7.1.1), because a null character may be embedded in it by a
+ \0 escape sequence.
+
+
+Contents
+
+6.4.6 Punctuators
+
+
+Syntax
+
+1
+
+
+ punctuator: one of
+ [ ] ( ) { } . ->
+ ++ -- & * + - ~ !
+ / % << >> < > <= >= == != ^ | && ||
+ ? : ; ...
+ = *= /= %= += -= <<= >>= &= ^= |=
+ , # ##
+ <: :> <% %> %: %:%:
+
+
+Semantics
+
+2
+ A punctuator is a symbol that has independent syntactic and semantic significance.
+ Depending on context, it may specify an operation to be performed (which in turn may
+ yield a value or a function designator, produce a side effect, or some combination thereof)
+ in which case it is known as an operator (other forms of operator also exist in some
+ contexts). An operand is an entity on which an operator acts.
+
+
+3
+ In all aspects of the language, the six tokens79)
+
+
+ <: :> <% %> %: %:%:
+
+
+ behave, respectively, the same as the six tokens
+
+
+ [ ] { } # ##
+
+
+ except for their spelling.80)
+
+ Forward references: expressions (6.5), declarations (6.7), preprocessing directives
+ (6.10), statements (6.8).
+
+
+Footnotes
+
+79) These tokens are sometimes called ''digraphs''.
+
+
+80) Thus [ and <: behave differently when ''stringized'' (see 6.10.3.2), but can otherwise be freely
+ interchanged.
+
+
+Contents
+
+6.4.7 Header names
+
+
+Syntax
+
+1
+
+
+ header-name:
+ < h-char-sequence >
+ " q-char-sequence "
+ h-char-sequence:
+ h-char
+ h-char-sequence h-char
+ h-char:
+ any member of the source character set except
+ the new-line character and >
+ q-char-sequence:
+ q-char
+ q-char-sequence q-char
+ q-char:
+ any member of the source character set except
+ the new-line character and "
+
+
+Semantics
+
+2
+ The sequences in both forms of header names are mapped in an implementation-defined
+ manner to headers or external source file names as specified in 6.10.2.
+
+3
+ If the characters ', \, ", //, or /* occur in the sequence between the < and > delimiters,
+ the behavior is undefined. Similarly, if the characters ', \, //, or /* occur in the
+
+
+
+
+
+ sequence between the " delimiters, the behavior is undefined.81) Header name
+ preprocessing tokens are recognized only within #include preprocessing directives and
+ in implementation-defined locations within #pragma directives.82)
+
+4
+ EXAMPLE The following sequence of characters:
+
+
+ 0x3<1/a.h>1e2
+ #include <1/a.h>
+ #define const.member@$
+
+
+ forms the following sequence of preprocessing tokens (with each individual preprocessing token delimited
+ by a { on the left and a } on the right).
+
+
+ {0x3}{<}{1}{/}{a}{.}{h}{>}{1e2}
+ {#}{include} {<1/a.h>}
+ {#}{define} {const}{.}{member}{@}{$}
+
+
+
+
+ Forward references: source file inclusion (6.10.2).
+
+
+Footnotes
+
+81) Thus, sequences of characters that resemble escape sequences cause undefined behavior.
+
+
+82) For an example of a header name preprocessing token used in a #pragma directive, see 6.10.9.
+
+
+Contents
+
+6.4.8 Preprocessing numbers
+
+
+Syntax
+
+1
+
+
+ pp-number:
+ digit
+ . digit
+ pp-number digit
+ pp-number identifier-nondigit
+ pp-number e sign
+ pp-number E sign
+ pp-number p sign
+ pp-number P sign
+ pp-number .
+
+
+Description
+
+2
+ A preprocessing number begins with a digit optionally preceded by a period (.) and may
+ be followed by valid identifier characters and the character sequences e+, e-, E+, E-,
+ p+, p-, P+, or P-.
+
+3
+ Preprocessing number tokens lexically include all floating and integer constant tokens.
+
+Semantics
+
+4
+ A preprocessing number does not have type or a value; it acquires both after a successful
+ conversion (as part of translation phase 7) to a floating constant token or an integer
+ constant token.
+
+
+
+
+Contents
+
+6.4.9 Comments
+
+
+1
+ Except within a character constant, a string literal, or a comment, the characters /*
+ introduce a comment. The contents of such a comment are examined only to identify
+ multibyte characters and to find the characters */ that terminate it.83)
+
+2
+ Except within a character constant, a string literal, or a comment, the characters //
+ introduce a comment that includes all multibyte characters up to, but not including, the
+ next new-line character. The contents of such a comment are examined only to identify
+ multibyte characters and to find the terminating new-line character.
+
+3
+ EXAMPLE
+
+
+ "a//b" // four-character string literal
+ #include "//e" // undefined behavior
+ // */ // comment, not syntax error
+ f = g/**//h; // equivalent to f = g / h;
+ //\
+ i(); // part of a two-line comment
+ /\
+ / j(); // part of a two-line comment
+ #define glue(x,y) x##y
+ glue(/,/) k(); // syntax error, not comment
+ /*//*/ l(); // equivalent to l();
+ m = n//**/o
+ + p; // equivalent to m = n + p;
+
+
+
+
+
+
+
+
+Footnotes
+
+83) Thus, /* ... */ comments do not nest.
+
+
+Contents
+
+6.5 Expressions
+
+
+1
+ An expression is a sequence of operators and operands that specifies computation of a
+ value, or that designates an object or a function, or that generates side effects, or that
+ performs a combination thereof. The value computations of the operands of an operator
+ are sequenced before the value computation of the result of the operator.
+
+2
+ If a side effect on a scalar object is unsequenced relative to either a different side effect
+ on the same scalar object or a value computation using the value of the same scalar
+ object, the behavior is undefined. If there are multiple allowable orderings of the
+ subexpressions of an expression, the behavior is undefined if such an unsequenced side
+ effect occurs in any of the orderings.84)
+
+3
+ The grouping of operators and operands is indicated by the syntax.85) Except as specified
+ later, side effects and value computations of subexpressions are unsequenced.86)
+
+4
+ Some operators (the unary operator ~, and the binary operators <<, >>, &, ^, and |,
+ collectively described as bitwise operators) are required to have operands that have
+ integer type. These operators yield values that depend on the internal representations of
+ integers, and have implementation-defined and undefined aspects for signed types.
+
+5
+ If an exceptional condition occurs during the evaluation of an expression (that is, if the
+ result is not mathematically defined or not in the range of representable values for its
+ type), the behavior is undefined.
+
+
+
+
+
+6
+ The effective type of an object for an access to its stored value is the declared type of the
+ object, if any.87) If a value is stored into an object having no declared type through an
+ lvalue having a type that is not a character type, then the type of the lvalue becomes the
+ effective type of the object for that access and for subsequent accesses that do not modify
+ the stored value. If a value is copied into an object having no declared type using
+ memcpy or memmove, or is copied as an array of character type, then the effective type
+ of the modified object for that access and for subsequent accesses that do not modify the
+ value is the effective type of the object from which the value is copied, if it has one. For
+ all other accesses to an object having no declared type, the effective type of the object is
+ simply the type of the lvalue used for the access.
+
+7
+ An object shall have its stored value accessed only by an lvalue expression that has one of
+ the following types:88)
+
+
+- a type compatible with the effective type of the object,
+
+- a qualified version of a type compatible with the effective type of the object,
+
+- a type that is the signed or unsigned type corresponding to the effective type of the
+ object,
+
+- a type that is the signed or unsigned type corresponding to a qualified version of the
+ effective type of the object,
+
+- an aggregate or union type that includes one of the aforementioned types among its
+ members (including, recursively, a member of a subaggregate or contained union), or
+
+- a character type.
+
+
+8
+ A floating expression may be contracted, that is, evaluated as though it were a single
+ operation, thereby omitting rounding errors implied by the source code and the
+ expression evaluation method.89) The FP_CONTRACT pragma in <math.h> provides a
+ way to disallow contracted expressions. Otherwise, whether and how expressions are
+ contracted is implementation-defined.90)
+
+ Forward references: the FP_CONTRACT pragma (7.12.2), copying functions (7.24.2).
+
+
+
+
+Footnotes
+
+84) This paragraph renders undefined statement expressions such as
+
+
+ i = ++i + 1;
+ a[i++] = i;
+
+
+ while allowing
+
+
+ i = i + 1;
+ a[i] = i;
+
+
+
+
+
+85) The syntax specifies the precedence of operators in the evaluation of an expression, which is the same
+ as the order of the major subclauses of this subclause, highest precedence first. Thus, for example, the
+ expressions allowed as the operands of the binary + operator (6.5.6) are those expressions defined in
+ 6.5.1 through 6.5.6. The exceptions are cast expressions (6.5.4) as operands of unary operators
+ (6.5.3), and an operand contained between any of the following pairs of operators: grouping
+ parentheses () (6.5.1), subscripting brackets [] (6.5.2.1), function-call parentheses () (6.5.2.2), and
+ the conditional operator ? : (6.5.15).
+ Within each major subclause, the operators have the same precedence. Left- or right-associativity is
+ indicated in each subclause by the syntax for the expressions discussed therein.
+
+
+86) In an expression that is evaluated more than once during the execution of a program, unsequenced and
+ indeterminately sequenced evaluations of its subexpressions need not be performed consistently in
+ different evaluations.
+
+
+87) Allocated objects have no declared type.
+
+
+88) The intent of this list is to specify those circumstances in which an object may or may not be aliased.
+
+
+89) The intermediate operations in the contracted expression are evaluated as if to infinite range and
+ precision, while the final operation is rounded to the format determined by the expression evaluation
+ method. A contracted expression might also omit the raising of floating-point exceptions.
+
+
+90) This license is specifically intended to allow implementations to exploit fast machine instructions that
+ combine multiple C operators. As contractions potentially undermine predictability, and can even
+ decrease accuracy for containing expressions, their use needs to be well-defined and clearly
+ documented.
+
+
+Contents
+
+6.5.1 Primary expressions
+
+
+Syntax
+
+1
+
+
+ primary-expression:
+ identifier
+ constant
+ string-literal
+ ( expression )
+ generic-selection
+
+
+Semantics
+
+2
+ An identifier is a primary expression, provided it has been declared as designating an
+ object (in which case it is an lvalue) or a function (in which case it is a function
+ designator).91)
+
+3
+ A constant is a primary expression. Its type depends on its form and value, as detailed in
+ 6.4.4.
+
+4
+ A string literal is a primary expression. It is an lvalue with type as detailed in 6.4.5.
+
+5
+ A parenthesized expression is a primary expression. Its type and value are identical to
+ those of the unparenthesized expression. It is an lvalue, a function designator, or a void
+ expression if the unparenthesized expression is, respectively, an lvalue, a function
+ designator, or a void expression.
+
+6
+ A generic selection is a primary expression. Its type and value depend on the selected
+ generic association, as detailed in the following subclause.
+
+ Forward references: declarations (6.7).
+
+
+Footnotes
+
+91) Thus, an undeclared identifier is a violation of the syntax.
+
+
+Contents
+
+6.5.1.1 Generic selection
+
+
+Syntax
+
+1
+
+
+ generic-selection:
+ _Generic ( assignment-expression , generic-assoc-list )
+ generic-assoc-list:
+ generic-association
+ generic-assoc-list , generic-association
+ generic-association:
+ type-name : assignment-expression
+ default : assignment-expression
+
+
+
+
+
+
+
+Constraints
+
+2
+ A generic selection shall have no more than one default generic association. The type
+ name in a generic association shall specify a complete object type other than a variably
+ modified type. No two generic associations in the same generic selection shall specify
+ compatible types. The controlling expression of a generic selection shall have type
+ compatible with at most one of the types named in its generic association list. If a
+ generic selection has no default generic association, its controlling expression shall
+ have type compatible with exactly one of the types named in its generic association list.
+
+Semantics
+
+3
+ The controlling expression of a generic selection is not evaluated. If a generic selection
+ has a generic association with a type name that is compatible with the type of the
+ controlling expression, then the result expression of the generic selection is the
+ expression in that generic association. Otherwise, the result expression of the generic
+ selection is the expression in the default generic association. None of the expressions
+ from any other generic association of the generic selection is evaluated.
+
+4
+ The type and value of a generic selection are identical to those of its result expression. It
+ is an lvalue, a function designator, or a void expression if its result expression is,
+ respectively, an lvalue, a function designator, or a void expression.
+
+5
+ EXAMPLE The cbrt type-generic macro could be implemented as follows:
+
+
+ #define cbrt(X) _Generic((X), \
+ long double: cbrtl, \
+ default: cbrt, \
+ float: cbrtf \
+ )(X)
+
+
+
+
+
+Contents
+
+6.5.2 Postfix operators
+
+
+Syntax
+
+1
+
+
+ postfix-expression:
+ primary-expression
+ postfix-expression [ expression ]
+ postfix-expression ( argument-expression-listopt )
+ postfix-expression . identifier
+ postfix-expression -> identifier
+ postfix-expression ++
+ postfix-expression --
+ ( type-name ) { initializer-list }
+ ( type-name ) { initializer-list , }
+ argument-expression-list:
+ assignment-expression
+ argument-expression-list , assignment-expression
+
+
+Contents
+
+6.5.2.1 Array subscripting
+
+
+Constraints
+
+1
+ One of the expressions shall have type ''pointer to complete object type'', the other
+ expression shall have integer type, and the result has type ''type''.
+
+Semantics
+
+2
+ A postfix expression followed by an expression in square brackets [] is a subscripted
+ designation of an element of an array object. The definition of the subscript operator []
+ is that E1[E2] is identical to (*((E1)+(E2))). Because of the conversion rules that
+ apply to the binary + operator, if E1 is an array object (equivalently, a pointer to the
+ initial element of an array object) and E2 is an integer, E1[E2] designates the E2-th
+ element of E1 (counting from zero).
+
+3
+ Successive subscript operators designate an element of a multidimensional array object.
+ If E is an n-dimensional array (n >= 2) with dimensions i x j x . . . x k, then E (used as
+ other than an lvalue) is converted to a pointer to an (n - 1)-dimensional array with
+ dimensions j x . . . x k. If the unary * operator is applied to this pointer explicitly, or
+ implicitly as a result of subscripting, the result is the referenced (n - 1)-dimensional
+ array, which itself is converted into a pointer if used as other than an lvalue. It follows
+ from this that arrays are stored in row-major order (last subscript varies fastest).
+
+4
+ EXAMPLE Consider the array object defined by the declaration
+
+
+ int x[3][5];
+
+
+ Here x is a 3 x 5 array of ints; more precisely, x is an array of three element objects, each of which is an
+ array of five ints. In the expression x[i], which is equivalent to (*((x)+(i))), x is first converted to
+ a pointer to the initial array of five ints. Then i is adjusted according to the type of x, which conceptually
+ entails multiplying i by the size of the object to which the pointer points, namely an array of five int
+ objects. The results are added and indirection is applied to yield an array of five ints. When used in the
+ expression x[i][j], that array is in turn converted to a pointer to the first of the ints, so x[i][j]
+ yields an int.
+
+
+ Forward references: additive operators (6.5.6), address and indirection operators
+ (6.5.3.2), array declarators (6.7.6.2).
+
+
+Contents
+
+6.5.2.2 Function calls
+
+
+Constraints
+
+1
+ The expression that denotes the called function92) shall have type pointer to function
+ returning void or returning a complete object type other than an array type.
+
+2
+ If the expression that denotes the called function has a type that includes a prototype, the
+ number of arguments shall agree with the number of parameters. Each argument shall
+ have a type such that its value may be assigned to an object with the unqualified version
+ of the type of its corresponding parameter.
+
+Semantics
+
+3
+ A postfix expression followed by parentheses () containing a possibly empty, comma-
+ separated list of expressions is a function call. The postfix expression denotes the called
+ function. The list of expressions specifies the arguments to the function.
+
+4
+ An argument may be an expression of any complete object type. In preparing for the call
+ to a function, the arguments are evaluated, and each parameter is assigned the value of the
+ corresponding argument.93)
+
+5
+ If the expression that denotes the called function has type pointer to function returning an
+ object type, the function call expression has the same type as that object type, and has the
+ value determined as specified in 6.8.6.4. Otherwise, the function call has type void.
+
+6
+ If the expression that denotes the called function has a type that does not include a
+ prototype, the integer promotions are performed on each argument, and arguments that
+ have type float are promoted to double. These are called the default argument
+ promotions. If the number of arguments does not equal the number of parameters, the
+ behavior is undefined. If the function is defined with a type that includes a prototype, and
+ either the prototype ends with an ellipsis (, ...) or the types of the arguments after
+ promotion are not compatible with the types of the parameters, the behavior is undefined.
+ If the function is defined with a type that does not include a prototype, and the types of
+ the arguments after promotion are not compatible with those of the parameters after
+ promotion, the behavior is undefined, except for the following cases:
+
+
+- one promoted type is a signed integer type, the other promoted type is the
+ corresponding unsigned integer type, and the value is representable in both types;
+
+
+
+
+
+- both types are pointers to qualified or unqualified versions of a character type or
+ void.
+
+
+7
+ If the expression that denotes the called function has a type that does include a prototype,
+ the arguments are implicitly converted, as if by assignment, to the types of the
+ corresponding parameters, taking the type of each parameter to be the unqualified version
+ of its declared type. The ellipsis notation in a function prototype declarator causes
+ argument type conversion to stop after the last declared parameter. The default argument
+ promotions are performed on trailing arguments.
+
+8
+ No other conversions are performed implicitly; in particular, the number and types of
+ arguments are not compared with those of the parameters in a function definition that
+ does not include a function prototype declarator.
+
+9
+ If the function is defined with a type that is not compatible with the type (of the
+ expression) pointed to by the expression that denotes the called function, the behavior is
+ undefined.
+
+10
+ There is a sequence point after the evaluations of the function designator and the actual
+ arguments but before the actual call. Every evaluation in the calling function (including
+ other function calls) that is not otherwise specifically sequenced before or after the
+ execution of the body of the called function is indeterminately sequenced with respect to
+ the execution of the called function.94)
+
+11
+ Recursive function calls shall be permitted, both directly and indirectly through any chain
+ of other functions.
+
+12
+ EXAMPLE In the function call
+
+
+ (*pf[f1()]) (f2(), f3() + f4())
+
+
+ the functions f1, f2, f3, and f4 may be called in any order. All side effects have to be completed before
+ the function pointed to by pf[f1()] is called.
+
+
+ Forward references: function declarators (including prototypes) (6.7.6.3), function
+ definitions (6.9.1), the return statement (6.8.6.4), simple assignment (6.5.16.1).
+
+
+Footnotes
+
+92) Most often, this is the result of converting an identifier that is a function designator.
+
+
+93) A function may change the values of its parameters, but these changes cannot affect the values of the
+ arguments. On the other hand, it is possible to pass a pointer to an object, and the function may
+ change the value of the object pointed to. A parameter declared to have array or function type is
+ adjusted to have a pointer type as described in 6.9.1.
+
+
+94) In other words, function executions do not ''interleave'' with each other.
+
+
+Contents
+
+6.5.2.3 Structure and union members
+
+
+Constraints
+
+1
+ The first operand of the . operator shall have an atomic, qualified, or unqualified
+ structure or union type, and the second operand shall name a member of that type.
+
+2
+ The first operand of the -> operator shall have type ''pointer to atomic, qualified, or
+ unqualified structure'' or ''pointer to atomic, qualified, or unqualified union'', and the
+ second operand shall name a member of the type pointed to.
+
+
+
+
+Semantics
+
+3
+ A postfix expression followed by the . operator and an identifier designates a member of
+ a structure or union object. The value is that of the named member,95) and is an lvalue if
+ the first expression is an lvalue. If the first expression has qualified type, the result has
+ the so-qualified version of the type of the designated member.
+
+4
+ A postfix expression followed by the -> operator and an identifier designates a member
+ of a structure or union object. The value is that of the named member of the object to
+ which the first expression points, and is an lvalue.96) If the first expression is a pointer to
+ a qualified type, the result has the so-qualified version of the type of the designated
+ member.
+
+5
+ Accessing a member of an atomic structure or union object results in undefined
+ behavior.97)
+
+6
+ One special guarantee is made in order to simplify the use of unions: if a union contains
+ several structures that share a common initial sequence (see below), and if the union
+ object currently contains one of these structures, it is permitted to inspect the common
+ initial part of any of them anywhere that a declaration of the completed type of the union
+ is visible. Two structures share a common initial sequence if corresponding members
+ have compatible types (and, for bit-fields, the same widths) for a sequence of one or more
+ initial members.
+
+7
+ EXAMPLE 1 If f is a function returning a structure or union, and x is a member of that structure or
+ union, f().x is a valid postfix expression but is not an lvalue.
+
+
+8
+ EXAMPLE 2 In:
+
+
+ struct s { int i; const int ci; };
+ struct s s;
+ const struct s cs;
+ volatile struct s vs;
+
+
+ the various members have the types:
+
+
+
+
+
+
+ s.i int
+ s.ci const int
+ cs.i const int
+ cs.ci const int
+ vs.i volatile int
+ vs.ci volatile const int
+
+
+
+
+9
+ EXAMPLE 3 The following is a valid fragment:
+
+
+ union {
+ struct {
+ int alltypes;
+ } n;
+ struct {
+ int type;
+ int intnode;
+ } ni;
+ struct {
+ int type;
+ double doublenode;
+ } nf;
+ } u;
+ u.nf.type = 1;
+ u.nf.doublenode = 3.14;
+ /* ... */
+ if (u.n.alltypes == 1)
+ if (sin(u.nf.doublenode) == 0.0)
+ /* ... */
+
+
+ The following is not a valid fragment (because the union type is not visible within function f):
+
+
+ struct t1 { int m; };
+ struct t2 { int m; };
+ int f(struct t1 *p1, struct t2 *p2)
+ {
+ if (p1->m < 0)
+ p2->m = -p2->m;
+ return p1->m;
+ }
+ int g()
+ {
+ union {
+ struct t1 s1;
+ struct t2 s2;
+ } u;
+ /* ... */
+ return f(&u.s1, &u.s2);
+ }
+
+
+
+
+ Forward references: address and indirection operators (6.5.3.2), structure and union
+ specifiers (6.7.2.1).
+
+
+Footnotes
+
+95) If the member used to read the contents of a union object is not the same as the member last used to
+ store a value in the object, the appropriate part of the object representation of the value is reinterpreted
+ as an object representation in the new type as described in 6.2.6 (a process sometimes called ''type
+ punning''). This might be a trap representation.
+
+
+96) If &E is a valid pointer expression (where & is the ''address-of '' operator, which generates a pointer to
+ its operand), the expression (&E)->MOS is the same as E.MOS.
+
+
+97) For example, a data race would occur if access to the entire structure or union in one thread conflicts
+ with access to a member from another thread, where at least one access is a modification. Members
+ can be safely accessed using a non-atomic object which is assigned to or from the atomic object.
+
+
+Contents
+
+6.5.2.4 Postfix increment and decrement operators
+
+
+Constraints
+
+1
+ The operand of the postfix increment or decrement operator shall have atomic, qualified,
+ or unqualified real or pointer type, and shall be a modifiable lvalue.
+
+Semantics
+
+2
+ The result of the postfix ++ operator is the value of the operand. As a side effect, the
+ value of the operand object is incremented (that is, the value 1 of the appropriate type is
+ added to it). See the discussions of additive operators and compound assignment for
+ information on constraints, types, and conversions and the effects of operations on
+ pointers. The value computation of the result is sequenced before the side effect of
+ updating the stored value of the operand. With respect to an indeterminately-sequenced
+ function call, the operation of postfix ++ is a single evaluation. Postfix ++ on an object
+ with atomic type is a read-modify-write operation with memory_order_seq_cst
+ memory order semantics.98)
+
+3
+ The postfix -- operator is analogous to the postfix ++ operator, except that the value of
+ the operand is decremented (that is, the value 1 of the appropriate type is subtracted from
+ it).
+
+ Forward references: additive operators (6.5.6), compound assignment (6.5.16.2).
+
+
+Footnotes
+
+98) Where a pointer to an atomic object can be formed and E has integer type, E++ is equivalent to the
+ following code sequence where T is the type of E:
+
+
+ T *addr = &E;
+ T old = *addr;
+ T new;
+ do {
+ new = old + 1;
+ } while (!atomic_compare_exchange_strong(addr, &old, new));
+
+
+ with old being the result of the operation.
+ Special care must be taken if E has floating type; see 6.5.16.2.
+
+
+Contents
+
+6.5.2.5 Compound literals
+
+
+Constraints
+
+1
+ The type name shall specify a complete object type or an array of unknown size, but not a
+ variable length array type.
+
+2
+ All the constraints for initializer lists in 6.7.9 also apply to compound literals.
+
+Semantics
+
+3
+ A postfix expression that consists of a parenthesized type name followed by a brace-
+ enclosed list of initializers is a compound literal. It provides an unnamed object whose
+
+
+ value is given by the initializer list.99)
+
+4
+ If the type name specifies an array of unknown size, the size is determined by the
+ initializer list as specified in 6.7.9, and the type of the compound literal is that of the
+ completed array type. Otherwise (when the type name specifies an object type), the type
+ of the compound literal is that specified by the type name. In either case, the result is an
+ lvalue.
+
+5
+ The value of the compound literal is that of an unnamed object initialized by the
+ initializer list. If the compound literal occurs outside the body of a function, the object
+ has static storage duration; otherwise, it has automatic storage duration associated with
+ the enclosing block.
+
+6
+ All the semantic rules for initializer lists in 6.7.9 also apply to compound literals.100)
+
+7
+ String literals, and compound literals with const-qualified types, need not designate
+ distinct objects.101)
+
+8
+ EXAMPLE 1 The file scope definition
+
+
+ int *p = (int []){2, 4};
+
+
+ initializes p to point to the first element of an array of two ints, the first having the value two and the
+ second, four. The expressions in this compound literal are required to be constant. The unnamed object
+ has static storage duration.
+
+
+9
+ EXAMPLE 2 In contrast, in
+
+
+ void f(void)
+ {
+ int *p;
+ /*...*/
+ p = (int [2]){*p};
+ /*...*/
+ }
+
+
+ p is assigned the address of the first element of an array of two ints, the first having the value previously
+ pointed to by p and the second, zero. The expressions in this compound literal need not be constant. The
+ unnamed object has automatic storage duration.
+
+
+10
+ EXAMPLE 3 Initializers with designations can be combined with compound literals. Structure objects
+ created using compound literals can be passed to functions without depending on member order:
+
+
+ drawline((struct point){.x=1, .y=1},
+ (struct point){.x=3, .y=4});
+
+
+
+
+
+
+ Or, if drawline instead expected pointers to struct point:
+
+
+ drawline(&(struct point){.x=1, .y=1},
+ &(struct point){.x=3, .y=4});
+
+
+
+
+11
+ EXAMPLE 4 A read-only compound literal can be specified through constructions like:
+
+
+ (const float []){1e0, 1e1, 1e2, 1e3, 1e4, 1e5, 1e6}
+
+
+
+
+12
+ EXAMPLE 5 The following three expressions have different meanings:
+
+
+ "/tmp/fileXXXXXX"
+ (char []){"/tmp/fileXXXXXX"}
+ (const char []){"/tmp/fileXXXXXX"}
+
+
+ The first always has static storage duration and has type array of char, but need not be modifiable; the last
+ two have automatic storage duration when they occur within the body of a function, and the first of these
+ two is modifiable.
+
+
+13
+ EXAMPLE 6 Like string literals, const-qualified compound literals can be placed into read-only memory
+ and can even be shared. For example,
+
+
+ (const char []){"abc"} == "abc"
+
+
+ might yield 1 if the literals' storage is shared.
+
+
+14
+ EXAMPLE 7 Since compound literals are unnamed, a single compound literal cannot specify a circularly
+ linked object. For example, there is no way to write a self-referential compound literal that could be used
+ as the function argument in place of the named object endless_zeros below:
+
+
+ struct int_list { int car; struct int_list *cdr; };
+ struct int_list endless_zeros = {0, &endless_zeros};
+ eval(endless_zeros);
+
+
+
+
+15
+ EXAMPLE 8 Each compound literal creates only a single object in a given scope:
+
+
+ struct s { int i; };
+ int f (void)
+ {
+ struct s *p = 0, *q;
+ int j = 0;
+ again:
+ q = p, p = &((struct s){ j++ });
+ if (j < 2) goto again;
+ return p == q && q->i == 1;
+ }
+
+
+ The function f() always returns the value 1.
+
+16
+ Note that if an iteration statement were used instead of an explicit goto and a labeled statement, the
+ lifetime of the unnamed object would be the body of the loop only, and on entry next time around p would
+ have an indeterminate value, which would result in undefined behavior.
+
+
+ Forward references: type names (6.7.7), initialization (6.7.9).
+
+
+Footnotes
+
+99) Note that this differs from a cast expression. For example, a cast specifies a conversion to scalar types
+ or void only, and the result of a cast expression is not an lvalue.
+
+
+100) For example, subobjects without explicit initializers are initialized to zero.
+
+
+101) This allows implementations to share storage for string literals and constant compound literals with
+ the same or overlapping representations.
+
+
+Contents
+
+6.5.3 Unary operators
+
+
+Syntax
+
+1
+
+
+ unary-expression:
+ postfix-expression
+ ++ unary-expression
+ -- unary-expression
+ unary-operator cast-expression
+ sizeof unary-expression
+ sizeof ( type-name )
+ _Alignof ( type-name )
+ unary-operator: one of
+ & * + - ~ !
+
+
+Contents
+
+6.5.3.1 Prefix increment and decrement operators
+
+
+Constraints
+
+1
+ The operand of the prefix increment or decrement operator shall have atomic, qualified,
+ or unqualified real or pointer type, and shall be a modifiable lvalue.
+
+Semantics
+
+2
+ The value of the operand of the prefix ++ operator is incremented. The result is the new
+ value of the operand after incrementation. The expression ++E is equivalent to (E+=1).
+ See the discussions of additive operators and compound assignment for information on
+ constraints, types, side effects, and conversions and the effects of operations on pointers.
+
+3
+ The prefix -- operator is analogous to the prefix ++ operator, except that the value of the
+ operand is decremented.
+
+ Forward references: additive operators (6.5.6), compound assignment (6.5.16.2).
+
+
+Contents
+
+6.5.3.2 Address and indirection operators
+
+
+Constraints
+
+1
+ The operand of the unary & operator shall be either a function designator, the result of a
+ [] or unary * operator, or an lvalue that designates an object that is not a bit-field and is
+ not declared with the register storage-class specifier.
+
+2
+ The operand of the unary * operator shall have pointer type.
+
+Semantics
+
+3
+ The unary & operator yields the address of its operand. If the operand has type ''type'',
+ the result has type ''pointer to type''. If the operand is the result of a unary * operator,
+ neither that operator nor the & operator is evaluated and the result is as if both were
+ omitted, except that the constraints on the operators still apply and the result is not an
+
+ lvalue. Similarly, if the operand is the result of a [] operator, neither the & operator nor
+ the unary * that is implied by the [] is evaluated and the result is as if the & operator
+ were removed and the [] operator were changed to a + operator. Otherwise, the result is
+ a pointer to the object or function designated by its operand.
+
+4
+ The unary * operator denotes indirection. If the operand points to a function, the result is
+ a function designator; if it points to an object, the result is an lvalue designating the
+ object. If the operand has type ''pointer to type'', the result has type ''type''. If an
+ invalid value has been assigned to the pointer, the behavior of the unary * operator is
+ undefined.102)
+
+ Forward references: storage-class specifiers (6.7.1), structure and union specifiers
+ (6.7.2.1).
+
+
+Footnotes
+
+102) Thus, &*E is equivalent to E (even if E is a null pointer), and &(E1[E2]) to ((E1)+(E2)). It is
+ always true that if E is a function designator or an lvalue that is a valid operand of the unary &
+ operator, *&E is a function designator or an lvalue equal to E. If *P is an lvalue and T is the name of
+ an object pointer type, *(T)P is an lvalue that has a type compatible with that to which T points.
+ Among the invalid values for dereferencing a pointer by the unary * operator are a null pointer, an
+ address inappropriately aligned for the type of object pointed to, and the address of an object after the
+ end of its lifetime.
+
+
+Contents
+
+6.5.3.3 Unary arithmetic operators
+
+
+Constraints
+
+1
+ The operand of the unary + or - operator shall have arithmetic type; of the ~ operator,
+ integer type; of the ! operator, scalar type.
+
+Semantics
+
+2
+ The result of the unary + operator is the value of its (promoted) operand. The integer
+ promotions are performed on the operand, and the result has the promoted type.
+
+3
+ The result of the unary - operator is the negative of its (promoted) operand. The integer
+ promotions are performed on the operand, and the result has the promoted type.
+
+4
+ The result of the ~ operator is the bitwise complement of its (promoted) operand (that is,
+ each bit in the result is set if and only if the corresponding bit in the converted operand is
+ not set). The integer promotions are performed on the operand, and the result has the
+ promoted type. If the promoted type is an unsigned type, the expression ~E is equivalent
+ to the maximum value representable in that type minus E.
+
+5
+ The result of the logical negation operator ! is 0 if the value of its operand compares
+ unequal to 0, 1 if the value of its operand compares equal to 0. The result has type int.
+ The expression !E is equivalent to (0==E).
+
+
+
+
+
+Contents
+
+6.5.3.4 The sizeof and _Alignof operators
+
+
+Constraints
+
+1
+ The sizeof operator shall not be applied to an expression that has function type or an
+ incomplete type, to the parenthesized name of such a type, or to an expression that
+ designates a bit-field member. The _Alignof operator shall not be applied to a
+ function type or an incomplete type.
+
+Semantics
+
+2
+ The sizeof operator yields the size (in bytes) of its operand, which may be an
+ expression or the parenthesized name of a type. The size is determined from the type of
+ the operand. The result is an integer. If the type of the operand is a variable length array
+ type, the operand is evaluated; otherwise, the operand is not evaluated and the result is an
+ integer constant.
+
+3
+ The _Alignof operator yields the alignment requirement of its operand type. The
+ operand is not evaluated and the result is an integer constant. When applied to an array
+ type, the result is the alignment requirement of the element type.
+
+4
+ When sizeof is applied to an operand that has type char, unsigned char, or
+ signed char, (or a qualified version thereof) the result is 1. When applied to an
+ operand that has array type, the result is the total number of bytes in the array.103) When
+ applied to an operand that has structure or union type, the result is the total number of
+ bytes in such an object, including internal and trailing padding.
+
+5
+ The value of the result of both operators is implementation-defined, and its type (an
+ unsigned integer type) is size_t, defined in <stddef.h> (and other headers).
+
+6
+ EXAMPLE 1 A principal use of the sizeof operator is in communication with routines such as storage
+ allocators and I/O systems. A storage-allocation function might accept a size (in bytes) of an object to
+ allocate and return a pointer to void. For example:
+
+
+ extern void *alloc(size_t);
+ double *dp = alloc(sizeof *dp);
+
+
+ The implementation of the alloc function should ensure that its return value is aligned suitably for
+ conversion to a pointer to double.
+
+
+7
+ EXAMPLE 2 Another use of the sizeof operator is to compute the number of elements in an array:
+
+
+ sizeof array / sizeof array[0]
+
+
+
+
+8
+ EXAMPLE 3 In this example, the size of a variable length array is computed and returned from a
+ function:
+
+
+ #include <stddef.h>
+
+
+
+
+
+
+
+ size_t fsize3(int n)
+ {
+ char b[n+3]; // variable length array
+ return sizeof b; // execution time sizeof
+ }
+ int main()
+ {
+ size_t size;
+ size = fsize3(10); // fsize3 returns 13
+ return 0;
+ }
+
+
+
+
+ Forward references: common definitions <stddef.h> (7.19), declarations (6.7),
+ structure and union specifiers (6.7.2.1), type names (6.7.7), array declarators (6.7.6.2).
+
+
+Footnotes
+
+103) When applied to a parameter declared to have array or function type, the sizeof operator yields the
+ size of the adjusted (pointer) type (see 6.9.1).
+
+
+Contents
+
+6.5.4 Cast operators
+
+
+Syntax
+
+1
+
+
+ cast-expression:
+ unary-expression
+ ( type-name ) cast-expression
+
+
+Constraints
+
+2
+ Unless the type name specifies a void type, the type name shall specify atomic, qualified,
+ or unqualified scalar type, and the operand shall have scalar type.
+
+3
+ Conversions that involve pointers, other than where permitted by the constraints of
+ 6.5.16.1, shall be specified by means of an explicit cast.
+
+4
+ A pointer type shall not be converted to any floating type. A floating type shall not be
+ converted to any pointer type.
+
+Semantics
+
+5
+ Preceding an expression by a parenthesized type name converts the value of the
+ expression to the named type. This construction is called a cast.104) A cast that specifies
+ no conversion has no effect on the type or value of an expression.
+
+6
+ If the value of the expression is represented with greater range or precision than required
+ by the type named by the cast (6.3.1.8), then the cast specifies a conversion even if the
+ type of the expression is the same as the named type and removes any extra range and
+ precision.
+
+ Forward references: equality operators (6.5.9), function declarators (including
+ prototypes) (6.7.6.3), simple assignment (6.5.16.1), type names (6.7.7).
+
+
+
+Footnotes
+
+104) A cast does not yield an lvalue. Thus, a cast to a qualified type has the same effect as a cast to the
+ unqualified version of the type.
+
+
+Contents
+
+6.5.5 Multiplicative operators
+
+
+Syntax
+
+1
+
+
+ multiplicative-expression:
+ cast-expression
+ multiplicative-expression * cast-expression
+ multiplicative-expression / cast-expression
+ multiplicative-expression % cast-expression
+
+
+Constraints
+
+2
+ Each of the operands shall have arithmetic type. The operands of the % operator shall
+ have integer type.
+
+Semantics
+
+3
+ The usual arithmetic conversions are performed on the operands.
+
+4
+ The result of the binary * operator is the product of the operands.
+
+5
+ The result of the / operator is the quotient from the division of the first operand by the
+ second; the result of the % operator is the remainder. In both operations, if the value of
+ the second operand is zero, the behavior is undefined.
+
+6
+ When integers are divided, the result of the / operator is the algebraic quotient with any
+ fractional part discarded.105) If the quotient a/b is representable, the expression
+ (a/b)*b + a%b shall equal a; otherwise, the behavior of both a/b and a%b is
+ undefined.
+
+
+Footnotes
+
+105) This is often called ''truncation toward zero''.
+
+
+Contents
+
+6.5.6 Additive operators
+
+
+Syntax
+
+1
+
+
+ additive-expression:
+ multiplicative-expression
+ additive-expression + multiplicative-expression
+ additive-expression - multiplicative-expression
+
+
+Constraints
+
+2
+ For addition, either both operands shall have arithmetic type, or one operand shall be a
+ pointer to a complete object type and the other shall have integer type. (Incrementing is
+ equivalent to adding 1.)
+
+3
+ For subtraction, one of the following shall hold:
+
+
+
+
+
+
+- both operands have arithmetic type;
+
+- both operands are pointers to qualified or unqualified versions of compatible complete
+ object types; or
+
+- the left operand is a pointer to a complete object type and the right operand has
+ integer type.
+
+ (Decrementing is equivalent to subtracting 1.)
+
+Semantics
+
+4
+ If both operands have arithmetic type, the usual arithmetic conversions are performed on
+ them.
+
+5
+ The result of the binary + operator is the sum of the operands.
+
+6
+ The result of the binary - operator is the difference resulting from the subtraction of the
+ second operand from the first.
+
+7
+ For the purposes of these operators, a pointer to an object that is not an element of an
+ array behaves the same as a pointer to the first element of an array of length one with the
+ type of the object as its element type.
+
+8
+ When an expression that has integer type is added to or subtracted from a pointer, the
+ result has the type of the pointer operand. If the pointer operand points to an element of
+ an array object, and the array is large enough, the result points to an element offset from
+ the original element such that the difference of the subscripts of the resulting and original
+ array elements equals the integer expression. In other words, if the expression P points to
+ the i-th element of an array object, the expressions (P)+N (equivalently, N+(P)) and
+ (P)-N (where N has the value n) point to, respectively, the i+n-th and i-n-th elements of
+ the array object, provided they exist. Moreover, if the expression P points to the last
+ element of an array object, the expression (P)+1 points one past the last element of the
+ array object, and if the expression Q points one past the last element of an array object,
+ the expression (Q)-1 points to the last element of the array object. If both the pointer
+ operand and the result point to elements of the same array object, or one past the last
+ element of the array object, the evaluation shall not produce an overflow; otherwise, the
+ behavior is undefined. If the result points one past the last element of the array object, it
+ shall not be used as the operand of a unary * operator that is evaluated.
+
+9
+ When two pointers are subtracted, both shall point to elements of the same array object,
+ or one past the last element of the array object; the result is the difference of the
+ subscripts of the two array elements. The size of the result is implementation-defined,
+ and its type (a signed integer type) is ptrdiff_t defined in the <stddef.h> header.
+ If the result is not representable in an object of that type, the behavior is undefined. In
+ other words, if the expressions P and Q point to, respectively, the i-th and j-th elements of
+ an array object, the expression (P)-(Q) has the value i-j provided the value fits in an
+
+ object of type ptrdiff_t. Moreover, if the expression P points either to an element of
+ an array object or one past the last element of an array object, and the expression Q points
+ to the last element of the same array object, the expression ((Q)+1)-(P) has the same
+ value as ((Q)-(P))+1 and as -((P)-((Q)+1)), and has the value zero if the
+ expression P points one past the last element of the array object, even though the
+ expression (Q)+1 does not point to an element of the array object.106)
+
+10
+ EXAMPLE Pointer arithmetic is well defined with pointers to variable length array types.
+
+
+ {
+ int n = 4, m = 3;
+ int a[n][m];
+ int (*p)[m] = a; // p == &a[0]
+ p += 1; // p == &a[1]
+ (*p)[2] = 99; // a[1][2] == 99
+ n = p - a; // n == 1
+ }
+
+
+11
+ If array a in the above example were declared to be an array of known constant size, and pointer p were
+ declared to be a pointer to an array of the same known constant size (pointing to a), the results would be
+ the same.
+
+
+ Forward references: array declarators (6.7.6.2), common definitions <stddef.h>
+ (7.19).
+
+
+Footnotes
+
+106) Another way to approach pointer arithmetic is first to convert the pointer(s) to character pointer(s): In
+ this scheme the integer expression added to or subtracted from the converted pointer is first multiplied
+ by the size of the object originally pointed to, and the resulting pointer is converted back to the
+ original type. For pointer subtraction, the result of the difference between the character pointers is
+ similarly divided by the size of the object originally pointed to.
+ When viewed in this way, an implementation need only provide one extra byte (which may overlap
+ another object in the program) just after the end of the object in order to satisfy the ''one past the last
+ element'' requirements.
+
+
+Contents
+
+6.5.7 Bitwise shift operators
+
+
+Syntax
+
+1
+
+
+ shift-expression:
+ additive-expression
+ shift-expression << additive-expression
+ shift-expression >> additive-expression
+
+
+Constraints
+
+2
+ Each of the operands shall have integer type.
+
+Semantics
+
+3
+ The integer promotions are performed on each of the operands. The type of the result is
+ that of the promoted left operand. If the value of the right operand is negative or is
+
+
+ greater than or equal to the width of the promoted left operand, the behavior is undefined.
+
+4
+ The result of E1 << E2 is E1 left-shifted E2 bit positions; vacated bits are filled with
+ zeros. If E1 has an unsigned type, the value of the result is E1 x 2E2 , reduced modulo
+ one more than the maximum value representable in the result type. If E1 has a signed
+ type and nonnegative value, and E1 x 2E2 is representable in the result type, then that is
+ the resulting value; otherwise, the behavior is undefined.
+
+5
+ The result of E1 >> E2 is E1 right-shifted E2 bit positions. If E1 has an unsigned type
+ or if E1 has a signed type and a nonnegative value, the value of the result is the integral
+ part of the quotient of E1 / 2E2 . If E1 has a signed type and a negative value, the
+ resulting value is implementation-defined.
+
+
+Contents
+
+6.5.8 Relational operators
+
+
+Syntax
+
+1
+
+
+ relational-expression:
+ shift-expression
+ relational-expression < shift-expression
+ relational-expression > shift-expression
+ relational-expression <= shift-expression
+ relational-expression >= shift-expression
+
+
+Constraints
+
+2
+ One of the following shall hold:
+
+
+- both operands have real type; or
+
+- both operands are pointers to qualified or unqualified versions of compatible object
+ types.
+
+
+Semantics
+
+3
+ If both of the operands have arithmetic type, the usual arithmetic conversions are
+ performed.
+
+4
+ For the purposes of these operators, a pointer to an object that is not an element of an
+ array behaves the same as a pointer to the first element of an array of length one with the
+ type of the object as its element type.
+
+5
+ When two pointers are compared, the result depends on the relative locations in the
+ address space of the objects pointed to. If two pointers to object types both point to the
+ same object, or both point one past the last element of the same array object, they
+ compare equal. If the objects pointed to are members of the same aggregate object,
+ pointers to structure members declared later compare greater than pointers to members
+ declared earlier in the structure, and pointers to array elements with larger subscript
+ values compare greater than pointers to elements of the same array with lower subscript
+
+ values. All pointers to members of the same union object compare equal. If the
+ expression P points to an element of an array object and the expression Q points to the
+ last element of the same array object, the pointer expression Q+1 compares greater than
+ P. In all other cases, the behavior is undefined.
+
+6
+ Each of the operators < (less than), > (greater than), <= (less than or equal to), and >=
+ (greater than or equal to) shall yield 1 if the specified relation is true and 0 if it is
+ false.107) The result has type int.
+
+
+Footnotes
+
+107) The expression a<b<c is not interpreted as in ordinary mathematics. As the syntax indicates, it
+ means (a<b)<c; in other words, ''if a is less than b, compare 1 to c; otherwise, compare 0 to c''.
+
+
+Contents
+
+6.5.9 Equality operators
+
+
+Syntax
+
+1
+
+
+ equality-expression:
+ relational-expression
+ equality-expression == relational-expression
+ equality-expression != relational-expression
+
+
+Constraints
+
+2
+ One of the following shall hold:
+
+
+- both operands have arithmetic type;
+
+- both operands are pointers to qualified or unqualified versions of compatible types;
+
+- one operand is a pointer to an object type and the other is a pointer to a qualified or
+ unqualified version of void; or
+
+- one operand is a pointer and the other is a null pointer constant.
+
+
+Semantics
+
+3
+ The == (equal to) and != (not equal to) operators are analogous to the relational
+ operators except for their lower precedence.108) Each of the operators yields 1 if the
+ specified relation is true and 0 if it is false. The result has type int. For any pair of
+ operands, exactly one of the relations is true.
+
+4
+ If both of the operands have arithmetic type, the usual arithmetic conversions are
+ performed. Values of complex types are equal if and only if both their real parts are equal
+ and also their imaginary parts are equal. Any two values of arithmetic types from
+ different type domains are equal if and only if the results of their conversions to the
+ (complex) result type determined by the usual arithmetic conversions are equal.
+
+
+
+
+
+5
+ Otherwise, at least one operand is a pointer. If one operand is a pointer and the other is a
+ null pointer constant, the null pointer constant is converted to the type of the pointer. If
+ one operand is a pointer to an object type and the other is a pointer to a qualified or
+ unqualified version of void, the former is converted to the type of the latter.
+
+6
+ Two pointers compare equal if and only if both are null pointers, both are pointers to the
+ same object (including a pointer to an object and a subobject at its beginning) or function,
+ both are pointers to one past the last element of the same array object, or one is a pointer
+ to one past the end of one array object and the other is a pointer to the start of a different
+ array object that happens to immediately follow the first array object in the address
+ space.109)
+
+7
+ For the purposes of these operators, a pointer to an object that is not an element of an
+ array behaves the same as a pointer to the first element of an array of length one with the
+ type of the object as its element type.
+
+
+Footnotes
+
+108) Because of the precedences, a<b == c<d is 1 whenever a<b and c<d have the same truth-value.
+
+
+109) Two objects may be adjacent in memory because they are adjacent elements of a larger array or
+ adjacent members of a structure with no padding between them, or because the implementation chose
+ to place them so, even though they are unrelated. If prior invalid pointer operations (such as accesses
+ outside array bounds) produced undefined behavior, subsequent comparisons also produce undefined
+ behavior.
+
+
+Contents
+
+6.5.10 Bitwise AND operator
+
+
+Syntax
+
+1
+
+
+ AND-expression:
+ equality-expression
+ AND-expression & equality-expression
+
+
+Constraints
+
+2
+ Each of the operands shall have integer type.
+
+Semantics
+
+3
+ The usual arithmetic conversions are performed on the operands.
+
+4
+ The result of the binary & operator is the bitwise AND of the operands (that is, each bit in
+ the result is set if and only if each of the corresponding bits in the converted operands is
+ set).
+
+
+
+
+
+
+Contents
+
+6.5.11 Bitwise exclusive OR operator
+
+
+Syntax
+
+1
+
+
+ exclusive-OR-expression:
+ AND-expression
+ exclusive-OR-expression ^ AND-expression
+
+
+Constraints
+
+2
+ Each of the operands shall have integer type.
+
+Semantics
+
+3
+ The usual arithmetic conversions are performed on the operands.
+
+4
+ The result of the ^ operator is the bitwise exclusive OR of the operands (that is, each bit
+ in the result is set if and only if exactly one of the corresponding bits in the converted
+ operands is set).
+
+
+Contents
+
+6.5.12 Bitwise inclusive OR operator
+
+
+Syntax
+
+1
+
+
+ inclusive-OR-expression:
+ exclusive-OR-expression
+ inclusive-OR-expression | exclusive-OR-expression
+
+
+Constraints
+
+2
+ Each of the operands shall have integer type.
+
+Semantics
+
+3
+ The usual arithmetic conversions are performed on the operands.
+
+4
+ The result of the | operator is the bitwise inclusive OR of the operands (that is, each bit in
+ the result is set if and only if at least one of the corresponding bits in the converted
+ operands is set).
+
+
+Contents
+
+6.5.13 Logical AND operator
+
+
+Syntax
+
+1
+
+
+ logical-AND-expression:
+ inclusive-OR-expression
+ logical-AND-expression && inclusive-OR-expression
+
+
+Constraints
+
+2
+ Each of the operands shall have scalar type.
+
+Semantics
+
+3
+ The && operator shall yield 1 if both of its operands compare unequal to 0; otherwise, it
+ yields 0. The result has type int.
+
+4
+ Unlike the bitwise binary & operator, the && operator guarantees left-to-right evaluation;
+ if the second operand is evaluated, there is a sequence point between the evaluations of
+ the first and second operands. If the first operand compares equal to 0, the second
+ operand is not evaluated.
+
+
+Contents
+
+6.5.14 Logical OR operator
+
+
+Syntax
+
+1
+
+
+ logical-OR-expression:
+ logical-AND-expression
+ logical-OR-expression || logical-AND-expression
+
+
+Constraints
+
+2
+ Each of the operands shall have scalar type.
+
+Semantics
+
+3
+ The || operator shall yield 1 if either of its operands compare unequal to 0; otherwise, it
+ yields 0. The result has type int.
+
+4
+ Unlike the bitwise | operator, the || operator guarantees left-to-right evaluation; if the
+ second operand is evaluated, there is a sequence point between the evaluations of the first
+ and second operands. If the first operand compares unequal to 0, the second operand is
+ not evaluated.
+
+
+Contents
+
+6.5.15 Conditional operator
+
+
+Syntax
+
+1
+
+
+ conditional-expression:
+ logical-OR-expression
+ logical-OR-expression ? expression : conditional-expression
+
+
+Constraints
+
+2
+ The first operand shall have scalar type.
+
+3
+ One of the following shall hold for the second and third operands:
+
+
+- both operands have arithmetic type;
+
+- both operands have the same structure or union type;
+
+- both operands have void type;
+
+- both operands are pointers to qualified or unqualified versions of compatible types;
+
+- one operand is a pointer and the other is a null pointer constant; or
+
+- one operand is a pointer to an object type and the other is a pointer to a qualified or
+ unqualified version of void.
+
+
+Semantics
+
+4
+ The first operand is evaluated; there is a sequence point between its evaluation and the
+ evaluation of the second or third operand (whichever is evaluated). The second operand
+ is evaluated only if the first compares unequal to 0; the third operand is evaluated only if
+ the first compares equal to 0; the result is the value of the second or third operand
+ (whichever is evaluated), converted to the type described below.110)
+
+5
+ If both the second and third operands have arithmetic type, the result type that would be
+ determined by the usual arithmetic conversions, were they applied to those two operands,
+ is the type of the result. If both the operands have structure or union type, the result has
+ that type. If both operands have void type, the result has void type.
+
+6
+ If both the second and third operands are pointers or one is a null pointer constant and the
+ other is a pointer, the result type is a pointer to a type qualified with all the type qualifiers
+ of the types referenced by both operands. Furthermore, if both operands are pointers to
+ compatible types or to differently qualified versions of compatible types, the result type is
+ a pointer to an appropriately qualified version of the composite type; if one operand is a
+ null pointer constant, the result has the type of the other operand; otherwise, one operand
+ is a pointer to void or a qualified version of void, in which case the result type is a
+ pointer to an appropriately qualified version of void.
+
+
+
+7
+ EXAMPLE The common type that results when the second and third operands are pointers is determined
+ in two independent stages. The appropriate qualifiers, for example, do not depend on whether the two
+ pointers have compatible types.
+
+8
+ Given the declarations
+
+
+ const void *c_vp;
+ void *vp;
+ const int *c_ip;
+ volatile int *v_ip;
+ int *ip;
+ const char *c_cp;
+
+
+ the third column in the following table is the common type that is the result of a conditional expression in
+ which the first two columns are the second and third operands (in either order):
+
+
+ c_vp c_ip const void *
+ v_ip 0 volatile int *
+ c_ip v_ip const volatile int *
+ vp c_cp const void *
+ ip c_ip const int *
+ vp ip void *
+
+
+
+
+
+Footnotes
+
+110) A conditional expression does not yield an lvalue.
+
+
+Contents
+
+6.5.16 Assignment operators
+
+
+Syntax
+
+1
+
+
+ assignment-expression:
+ conditional-expression
+ unary-expression assignment-operator assignment-expression
+ assignment-operator: one of
+ = *= /= %= += -= <<= >>= &= ^= |=
+
+
+Constraints
+
+2
+ An assignment operator shall have a modifiable lvalue as its left operand.
+
+Semantics
+
+3
+ An assignment operator stores a value in the object designated by the left operand. An
+ assignment expression has the value of the left operand after the assignment,111) but is not
+ an lvalue. The type of an assignment expression is the type the left operand would have
+ after lvalue conversion. The side effect of updating the stored value of the left operand is
+ sequenced after the value computations of the left and right operands. The evaluations of
+ the operands are unsequenced.
+
+
+
+
+
+
+Footnotes
+
+111) The implementation is permitted to read the object to determine the value but is not required to, even
+ when the object has volatile-qualified type.
+
+
+Contents
+
+6.5.16.1 Simple assignment
+
+
+Constraints
+
+1
+ One of the following shall hold:112)
+
+
+- the left operand has atomic, qualified, or unqualified arithmetic type, and the right has
+ arithmetic type;
+
+- the left operand has an atomic, qualified, or unqualified version of a structure or union
+ type compatible with the type of the right;
+
+- the left operand has atomic, qualified, or unqualified pointer type, and (considering
+ the type the left operand would have after lvalue conversion) both operands are
+ pointers to qualified or unqualified versions of compatible types, and the type pointed
+ to by the left has all the qualifiers of the type pointed to by the right;
+
+- the left operand has atomic, qualified, or unqualified pointer type, and (considering
+ the type the left operand would have after lvalue conversion) one operand is a pointer
+ to an object type, and the other is a pointer to a qualified or unqualified version of
+ void, and the type pointed to by the left has all the qualifiers of the type pointed to
+ by the right;
+
+- the left operand is an atomic, qualified, or unqualified pointer, and the right is a null
+ pointer constant; or
+
+- the left operand has type atomic, qualified, or unqualified _Bool, and the right is a
+ pointer.
+
+
+Semantics
+
+2
+ In simple assignment (=), the value of the right operand is converted to the type of the
+ assignment expression and replaces the value stored in the object designated by the left
+ operand.
+
+3
+ If the value being stored in an object is read from another object that overlaps in any way
+ the storage of the first object, then the overlap shall be exact and the two objects shall
+ have qualified or unqualified versions of a compatible type; otherwise, the behavior is
+ undefined.
+
+4
+ EXAMPLE 1 In the program fragment
+
+
+
+
+
+
+ int f(void);
+ char c;
+ /* ... */
+ if ((c = f()) == -1)
+ /* ... */
+
+
+ the int value returned by the function may be truncated when stored in the char, and then converted back
+ to int width prior to the comparison. In an implementation in which ''plain'' char has the same range of
+ values as unsigned char (and char is narrower than int), the result of the conversion cannot be
+ negative, so the operands of the comparison can never compare equal. Therefore, for full portability, the
+ variable c should be declared as int.
+
+
+5
+ EXAMPLE 2 In the fragment:
+
+
+ char c;
+ int i;
+ long l;
+ l = (c = i);
+
+
+ the value of i is converted to the type of the assignment expression c = i, that is, char type. The value
+ of the expression enclosed in parentheses is then converted to the type of the outer assignment expression,
+ that is, long int type.
+
+
+6
+ EXAMPLE 3 Consider the fragment:
+
+
+ const char **cpp;
+ char *p;
+ const char c = 'A';
+ cpp = &p; // constraint violation
+ *cpp = &c; // valid
+ *p = 0; // valid
+
+
+ The first assignment is unsafe because it would allow the following valid code to attempt to change the
+ value of the const object c.
+
+
+
+Footnotes
+
+112) The asymmetric appearance of these constraints with respect to type qualifiers is due to the conversion
+ (specified in 6.3.2.1) that changes lvalues to ''the value of the expression'' and thus removes any type
+ qualifiers that were applied to the type category of the expression (for example, it removes const but
+ not volatile from the type int volatile * const).
+
+
+Contents
+
+6.5.16.2 Compound assignment
+
+
+Constraints
+
+1
+ For the operators += and -= only, either the left operand shall be an atomic, qualified, or
+ unqualified pointer to a complete object type, and the right shall have integer type; or the
+ left operand shall have atomic, qualified, or unqualified arithmetic type, and the right
+ shall have arithmetic type.
+
+2
+ For the other operators, the left operand shall have atomic, qualified, or unqualified
+ arithmetic type, and (considering the type the left operand would have after lvalue
+ conversion) each operand shall have arithmetic type consistent with those allowed by the
+ corresponding binary operator.
+
+Semantics
+
+3
+ A compound assignment of the form E1 op = E2 is equivalent to the simple assignment
+ expression E1 = E1 op (E2), except that the lvalue E1 is evaluated only once, and with
+ respect to an indeterminately-sequenced function call, the operation of a compound
+
+ assignment is a single evaluation. If E1 has an atomic type, compound assignment is a
+ read-modify-write operation with memory_order_seq_cst memory order
+ semantics.113)
+
+
+
+
+
+
+Footnotes
+
+113) Where a pointer to an atomic object can be formed and E1 and E2 have integer type, this is equivalent
+ to the following code sequence where T1 is the type of E1 and T2 is the type of E2:
+
+
+ T1 *addr = &E1;
+ T2 val = (E2);
+ T1 old = *addr;
+ T1 new;
+ do {
+ new = old op val;
+ } while (!atomic_compare_exchange_strong(addr, &old, new));
+
+
+ with new being the result of the operation.
+ If E1 or E2 has floating type, then exceptional conditions or floating-point exceptions encountered
+ during discarded evaluations of new should also be discarded in order to satisfy the equivalence of E1
+ op = E2 and E1 = E1 op (E2). For example, if annex F is in effect, the floating types involved have
+ IEC 60559 formats, and FLT_EVAL_METHOD is 0, the equivalent code would be:
+
+
+ #include <fenv.h>
+ #pragma STDC FENV_ACCESS ON
+ /* ... */
+ fenv_t fenv;
+ T1 *addr = &E1;
+ T2 val = E2;
+ T1 old = *addr;
+ T1 new;
+ feholdexcept(&fenv);
+ for (;;) {
+ new = old op val;
+ if (atomic_compare_exchange_strong(addr, &old, new))
+ break;
+ feclearexcept(FE_ALL_EXCEPT);
+ }
+ feupdateenv(&fenv);
+
+
+ If FLT_EVAL_METHOD is not 0, then T2 must be a type with the range and precision to which E2 is
+ evaluated in order to satisfy the equivalence.
+
+
+Contents
+
+6.5.17 Comma operator
+
+
+Syntax
+
+1
+
+
+ expression:
+ assignment-expression
+ expression , assignment-expression
+
+
+Semantics
+
+2
+ The left operand of a comma operator is evaluated as a void expression; there is a
+ sequence point between its evaluation and that of the right operand. Then the right
+ operand is evaluated; the result has its type and value.114)
+
+3
+ EXAMPLE As indicated by the syntax, the comma operator (as described in this subclause) cannot
+ appear in contexts where a comma is used to separate items in a list (such as arguments to functions or lists
+ of initializers). On the other hand, it can be used within a parenthesized expression or within the second
+ expression of a conditional operator in such contexts. In the function call
+
+
+ f(a, (t=3, t+2), c)
+
+
+ the function has three arguments, the second of which has the value 5.
+
+
+ Forward references: initialization (6.7.9).
+
+
+
+
+
+
+Footnotes
+
+114) A comma operator does not yield an lvalue.
+
+
+Contents
+
+6.6 Constant expressions
+
+
+Syntax
+
+1
+
+
+ constant-expression:
+ conditional-expression
+
+
+Description
+
+2
+ A constant expression can be evaluated during translation rather than runtime, and
+ accordingly may be used in any place that a constant may be.
+
+Constraints
+
+3
+ Constant expressions shall not contain assignment, increment, decrement, function-call,
+ or comma operators, except when they are contained within a subexpression that is not
+ evaluated.115)
+
+4
+ Each constant expression shall evaluate to a constant that is in the range of representable
+ values for its type.
+
+Semantics
+
+5
+ An expression that evaluates to a constant is required in several contexts. If a floating
+ expression is evaluated in the translation environment, the arithmetic range and precision
+ shall be at least as great as if the expression were being evaluated in the execution
+ environment.116)
+
+6
+ An integer constant expression117) shall have integer type and shall only have operands
+ that are integer constants, enumeration constants, character constants, sizeof
+ expressions whose results are integer constants, _Alignof expressions, and floating
+ constants that are the immediate operands of casts. Cast operators in an integer constant
+ expression shall only convert arithmetic types to integer types, except as part of an
+ operand to the sizeof or _Alignof operator.
+
+7
+ More latitude is permitted for constant expressions in initializers. Such a constant
+ expression shall be, or evaluate to, one of the following:
+
+
+- an arithmetic constant expression,
+
+
+
+
+
+- a null pointer constant,
+
+- an address constant, or
+
+- an address constant for a complete object type plus or minus an integer constant
+ expression.
+
+
+8
+ An arithmetic constant expression shall have arithmetic type and shall only have
+ operands that are integer constants, floating constants, enumeration constants, character
+ constants, sizeof expressions whose results are integer constants, and _Alignof
+ expressions. Cast operators in an arithmetic constant expression shall only convert
+ arithmetic types to arithmetic types, except as part of an operand to a sizeof or
+ _Alignof operator.
+
+9
+ An address constant is a null pointer, a pointer to an lvalue designating an object of static
+ storage duration, or a pointer to a function designator; it shall be created explicitly using
+ the unary & operator or an integer constant cast to pointer type, or implicitly by the use of
+ an expression of array or function type. The array-subscript [] and member-access .
+ and -> operators, the address & and indirection * unary operators, and pointer casts may
+ be used in the creation of an address constant, but the value of an object shall not be
+ accessed by use of these operators.
+
+10
+ An implementation may accept other forms of constant expressions.
+
+11
+ The semantic rules for the evaluation of a constant expression are the same as for
+ nonconstant expressions.118)
+
+ Forward references: array declarators (6.7.6.2), initialization (6.7.9).
+
+
+
+
+
+
+Footnotes
+
+115) The operand of a sizeof or _Alignof operator is usually not evaluated (6.5.3.4).
+
+
+116) The use of evaluation formats as characterized by FLT_EVAL_METHOD also applies to evaluation in
+ the translation environment.
+
+
+117) An integer constant expression is required in a number of contexts such as the size of a bit-field
+ member of a structure, the value of an enumeration constant, and the size of a non-variable length
+ array. Further constraints that apply to the integer constant expressions used in conditional-inclusion
+ preprocessing directives are discussed in 6.10.1.
+
+
+118) Thus, in the following initialization,
+
+
+ static int i = 2 || 1 / 0;
+
+
+ the expression is a valid integer constant expression with value one.
+
+
+Contents
+
+6.7 Declarations
+
+
+Syntax
+
+1
+
+
+ declaration:
+ declaration-specifiers init-declarator-listopt ;
+ static_assert-declaration
+ declaration-specifiers:
+ storage-class-specifier declaration-specifiersopt
+ type-specifier declaration-specifiersopt
+ type-qualifier declaration-specifiersopt
+ function-specifier declaration-specifiersopt
+ alignment-specifier declaration-specifiersopt
+ init-declarator-list:
+ init-declarator
+ init-declarator-list , init-declarator
+ init-declarator:
+ declarator
+ declarator = initializer
+
+
+Constraints
+
+2
+ A declaration other than a static_assert declaration shall declare at least a declarator
+ (other than the parameters of a function or the members of a structure or union), a tag, or
+ the members of an enumeration.
+
+3
+ If an identifier has no linkage, there shall be no more than one declaration of the identifier
+ (in a declarator or type specifier) with the same scope and in the same name space, except
+ that:
+
+
+- a typedef name may be redefined to denote the same type as it currently does,
+ provided that type is not a variably modified type;
+
+- tags may be redeclared as specified in 6.7.2.3.
+
+
+4
+ All declarations in the same scope that refer to the same object or function shall specify
+ compatible types.
+
+Semantics
+
+5
+ A declaration specifies the interpretation and attributes of a set of identifiers. A definition
+ of an identifier is a declaration for that identifier that:
+
+
+- for an object, causes storage to be reserved for that object;
+
+- for a function, includes the function body;119)
+
+
+- for an enumeration constant, is the (only) declaration of the identifier;
+
+- for a typedef name, is the first (or only) declaration of the identifier.
+
+
+6
+ The declaration specifiers consist of a sequence of specifiers that indicate the linkage,
+ storage duration, and part of the type of the entities that the declarators denote. The init-
+ declarator-list is a comma-separated sequence of declarators, each of which may have
+ additional type information, or an initializer, or both. The declarators contain the
+ identifiers (if any) being declared.
+
+7
+ If an identifier for an object is declared with no linkage, the type for the object shall be
+ complete by the end of its declarator, or by the end of its init-declarator if it has an
+ initializer; in the case of function parameters (including in prototypes), it is the adjusted
+ type (see 6.7.6.3) that is required to be complete.
+
+ Forward references: declarators (6.7.6), enumeration specifiers (6.7.2.2), initialization
+ (6.7.9), type names (6.7.7), type qualifiers (6.7.3).
+
+
+Footnotes
+
+119) Function definitions have a different syntax, described in 6.9.1.
+
+
+Contents
+
+6.7.1 Storage-class specifiers
+
+
+Syntax
+
+1
+
+
+ storage-class-specifier:
+ typedef
+ extern
+ static
+ _Thread_local
+ auto
+ register
+
+
+Constraints
+
+2
+ At most, one storage-class specifier may be given in the declaration specifiers in a
+ declaration, except that _Thread_local may appear with static or extern.120)
+
+3
+ In the declaration of an object with block scope, if the declaration specifiers include
+ _Thread_local, they shall also include either static or extern. If
+ _Thread_local appears in any declaration of an object, it shall be present in every
+ declaration of that object.
+
+4
+ _Thread_local shall not appear in the declaration specifiers of a function declaration.
+
+
+
+
+
+
+Semantics
+
+5
+ The typedef specifier is called a ''storage-class specifier'' for syntactic convenience
+ only; it is discussed in 6.7.8. The meanings of the various linkages and storage durations
+ were discussed in 6.2.2 and 6.2.4.
+
+6
+ A declaration of an identifier for an object with storage-class specifier register
+ suggests that access to the object be as fast as possible. The extent to which such
+ suggestions are effective is implementation-defined.121)
+
+7
+ The declaration of an identifier for a function that has block scope shall have no explicit
+ storage-class specifier other than extern.
+
+8
+ If an aggregate or union object is declared with a storage-class specifier other than
+ typedef, the properties resulting from the storage-class specifier, except with respect to
+ linkage, also apply to the members of the object, and so on recursively for any aggregate
+ or union member objects.
+
+ Forward references: type definitions (6.7.8).
+
+
+
+
+
+
+Footnotes
+
+120) See ''future language directions'' (6.11.5).
+
+
+121) The implementation may treat any register declaration simply as an auto declaration. However,
+ whether or not addressable storage is actually used, the address of any part of an object declared with
+ storage-class specifier register cannot be computed, either explicitly (by use of the unary &
+ operator as discussed in 6.5.3.2) or implicitly (by converting an array name to a pointer as discussed in
+ 6.3.2.1). Thus, the only operators that can be applied to an array declared with storage-class specifier
+ register are sizeof and _Alignof.
+
+
+Contents
+
+6.7.2 Type specifiers
+
+
+Syntax
+
+1
+
+
+ type-specifier:
+ void
+ char
+ short
+ int
+ long
+ float
+ double
+ signed
+ unsigned
+ _Bool
+ _Complex
+ atomic-type-specifier
+ struct-or-union-specifier
+ enum-specifier
+ typedef-name
+
+
+Constraints
+
+2
+ At least one type specifier shall be given in the declaration specifiers in each declaration,
+ and in the specifier-qualifier list in each struct declaration and type name. Each list of
+ type specifiers shall be one of the following multisets (delimited by commas, when there
+ is more than one multiset per item); the type specifiers may occur in any order, possibly
+ intermixed with the other declaration specifiers.
+
+
+- void
+
+- char
+
+- signed char
+
+- unsigned char
+
+- short, signed short, short int, or signed short int
+
+- unsigned short, or unsigned short int
+
+- int, signed, or signed int
+
+- unsigned, or unsigned int
+
+- long, signed long, long int, or signed long int
+
+- unsigned long, or unsigned long int
+
+
+- long long, signed long long, long long int, or
+ signed long long int
+
+- unsigned long long, or unsigned long long int
+
+- float
+
+- double
+
+- long double
+
+- _Bool
+
+- float _Complex
+
+- double _Complex
+
+- long double _Complex
+
+- atomic type specifier
+
+- struct or union specifier
+
+- enum specifier
+
+- typedef name
+
+
+3
+ The type specifier _Complex shall not be used if the implementation does not support
+ complex types (see 6.10.8.3).
+
+Semantics
+
+4
+ Specifiers for structures, unions, enumerations, and atomic types are discussed in 6.7.2.1
+ through 6.7.2.4. Declarations of typedef names are discussed in 6.7.8. The
+ characteristics of the other types are discussed in 6.2.5.
+
+5
+ Each of the comma-separated multisets designates the same type, except that for bit-
+ fields, it is implementation-defined whether the specifier int designates the same type as
+ signed int or the same type as unsigned int.
+
+ Forward references: atomic type specifiers (6.7.2.4), enumeration specifiers (6.7.2.2),
+ structure and union specifiers (6.7.2.1), tags (6.7.2.3), type definitions (6.7.8).
+
+
+Contents
+
+6.7.2.1 Structure and union specifiers
+
+
+Syntax
+
+1
+
+
+ struct-or-union-specifier:
+ struct-or-union identifieropt { struct-declaration-list }
+ struct-or-union identifier
+ struct-or-union:
+ struct
+ union
+ struct-declaration-list:
+ struct-declaration
+ struct-declaration-list struct-declaration
+ struct-declaration:
+ specifier-qualifier-list struct-declarator-listopt ;
+ static_assert-declaration
+ specifier-qualifier-list:
+ type-specifier specifier-qualifier-listopt
+ type-qualifier specifier-qualifier-listopt
+ struct-declarator-list:
+ struct-declarator
+ struct-declarator-list , struct-declarator
+ struct-declarator:
+ declarator
+ declaratoropt : constant-expression
+
+
+Constraints
+
+2
+ A struct-declaration that does not declare an anonymous structure or anonymous union
+ shall contain a struct-declarator-list.
+
+3
+ A structure or union shall not contain a member with incomplete or function type (hence,
+ a structure shall not contain an instance of itself, but may contain a pointer to an instance
+ of itself), except that the last member of a structure with more than one named member
+ may have incomplete array type; such a structure (and any union containing, possibly
+ recursively, a member that is such a structure) shall not be a member of a structure or an
+ element of an array.
+
+4
+ The expression that specifies the width of a bit-field shall be an integer constant
+ expression with a nonnegative value that does not exceed the width of an object of the
+ type that would be specified were the colon and expression omitted.122) If the value is
+ zero, the declaration shall have no declarator.
+
+5
+ A bit-field shall have a type that is a qualified or unqualified version of _Bool, signed
+ int, unsigned int, or some other implementation-defined type. It is
+ implementation-defined whether atomic types are permitted.
+
+
+
+Semantics
+
+6
+ As discussed in 6.2.5, a structure is a type consisting of a sequence of members, whose
+ storage is allocated in an ordered sequence, and a union is a type consisting of a sequence
+ of members whose storage overlap.
+
+7
+ Structure and union specifiers have the same form. The keywords struct and union
+ indicate that the type being specified is, respectively, a structure type or a union type.
+
+8
+ The presence of a struct-declaration-list in a struct-or-union-specifier declares a new type,
+ within a translation unit. The struct-declaration-list is a sequence of declarations for the
+ members of the structure or union. If the struct-declaration-list does not contain any
+ named members, either directly or via an anonymous structure or anonymous union, the
+ behavior is undefined. The type is incomplete until immediately after the } that
+ terminates the list, and complete thereafter.
+
+9
+ A member of a structure or union may have any complete object type other than a
+ variably modified type.123) In addition, a member may be declared to consist of a
+ specified number of bits (including a sign bit, if any). Such a member is called a
+ bit-field;124) its width is preceded by a colon.
+
+10
+ A bit-field is interpreted as having a signed or unsigned integer type consisting of the
+ specified number of bits.125) If the value 0 or 1 is stored into a nonzero-width bit-field of
+ type _Bool, the value of the bit-field shall compare equal to the value stored; a _Bool
+ bit-field has the semantics of a _Bool.
+
+11
+ An implementation may allocate any addressable storage unit large enough to hold a bit-
+ field. If enough space remains, a bit-field that immediately follows another bit-field in a
+ structure shall be packed into adjacent bits of the same unit. If insufficient space remains,
+ whether a bit-field that does not fit is put into the next unit or overlaps adjacent units is
+ implementation-defined. The order of allocation of bit-fields within a unit (high-order to
+ low-order or low-order to high-order) is implementation-defined. The alignment of the
+ addressable storage unit is unspecified.
+
+12
+ A bit-field declaration with no declarator, but only a colon and a width, indicates an
+ unnamed bit-field.126) As a special case, a bit-field structure member with a width of 0
+
+
+
+ indicates that no further bit-field is to be packed into the unit in which the previous bit-
+ field, if any, was placed.
+
+13
+ An unnamed member whose type specifier is a structure specifier with no tag is called an
+ anonymous structure; an unnamed member whose type specifier is a union specifier with
+ no tag is called an anonymous union. The members of an anonymous structure or union
+ are considered to be members of the containing structure or union. This applies
+ recursively if the containing structure or union is also anonymous.
+
+14
+ Each non-bit-field member of a structure or union object is aligned in an implementation-
+ defined manner appropriate to its type.
+
+15
+ Within a structure object, the non-bit-field members and the units in which bit-fields
+ reside have addresses that increase in the order in which they are declared. A pointer to a
+ structure object, suitably converted, points to its initial member (or if that member is a
+ bit-field, then to the unit in which it resides), and vice versa. There may be unnamed
+ padding within a structure object, but not at its beginning.
+
+16
+ The size of a union is sufficient to contain the largest of its members. The value of at
+ most one of the members can be stored in a union object at any time. A pointer to a
+ union object, suitably converted, points to each of its members (or if a member is a bit-
+ field, then to the unit in which it resides), and vice versa.
+
+17
+ There may be unnamed padding at the end of a structure or union.
+
+18
+ As a special case, the last element of a structure with more than one named member may
+ have an incomplete array type; this is called a flexible array member. In most situations,
+ the flexible array member is ignored. In particular, the size of the structure is as if the
+ flexible array member were omitted except that it may have more trailing padding than
+ the omission would imply. However, when a . (or ->) operator has a left operand that is
+ (a pointer to) a structure with a flexible array member and the right operand names that
+ member, it behaves as if that member were replaced with the longest array (with the same
+ element type) that would not make the structure larger than the object being accessed; the
+ offset of the array shall remain that of the flexible array member, even if this would differ
+ from that of the replacement array. If this array would have no elements, it behaves as if
+ it had one element but the behavior is undefined if any attempt is made to access that
+ element or to generate a pointer one past it.
+
+19
+ EXAMPLE 1 The following illustrates anonymous structures and unions:
+
+
+ struct v {
+ union { // anonymous union
+ struct { int i, j; }; // anonymous structure
+ struct { long k, l; } w;
+ };
+ int m;
+ } v1;
+ v1.i = 2; // valid
+ v1.k = 3; // invalid: inner structure is not anonymous
+ v1.w.k = 5; // valid
+
+
+
+
+20
+ EXAMPLE 2 After the declaration:
+
+
+ struct s { int n; double d[]; };
+
+
+ the structure struct s has a flexible array member d. A typical way to use this is:
+
+
+ int m = /* some value */;
+ struct s *p = malloc(sizeof (struct s) + sizeof (double [m]));
+
+
+ and assuming that the call to malloc succeeds, the object pointed to by p behaves, for most purposes, as if
+ p had been declared as:
+
+
+ struct { int n; double d[m]; } *p;
+
+
+ (there are circumstances in which this equivalence is broken; in particular, the offsets of member d might
+ not be the same).
+
+21
+ Following the above declaration:
+
+
+ struct s t1 = { 0 }; // valid
+ struct s t2 = { 1, { 4.2 }}; // invalid
+ t1.n = 4; // valid
+ t1.d[0] = 4.2; // might be undefined behavior
+
+
+ The initialization of t2 is invalid (and violates a constraint) because struct s is treated as if it did not
+ contain member d. The assignment to t1.d[0] is probably undefined behavior, but it is possible that
+
+
+ sizeof (struct s) >= offsetof(struct s, d) + sizeof (double)
+
+
+ in which case the assignment would be legitimate. Nevertheless, it cannot appear in strictly conforming
+ code.
+
+22
+ After the further declaration:
+
+
+ struct ss { int n; };
+
+
+ the expressions:
+
+
+ sizeof (struct s) >= sizeof (struct ss)
+ sizeof (struct s) >= offsetof(struct s, d)
+
+
+ are always equal to 1.
+
+23
+ If sizeof (double) is 8, then after the following code is executed:
+
+
+ struct s *s1;
+ struct s *s2;
+ s1 = malloc(sizeof (struct s) + 64);
+ s2 = malloc(sizeof (struct s) + 46);
+
+
+ and assuming that the calls to malloc succeed, the objects pointed to by s1 and s2 behave, for most
+ purposes, as if the identifiers had been declared as:
+
+
+ struct { int n; double d[8]; } *s1;
+ struct { int n; double d[5]; } *s2;
+
+
+24
+ Following the further successful assignments:
+
+
+ s1 = malloc(sizeof (struct s) + 10);
+ s2 = malloc(sizeof (struct s) + 6);
+
+
+ they then behave as if the declarations were:
+
+
+ struct { int n; double d[1]; } *s1, *s2;
+
+
+ and:
+
+
+ double *dp;
+ dp = &(s1->d[0]); // valid
+ *dp = 42; // valid
+ dp = &(s2->d[0]); // valid
+ *dp = 42; // undefined behavior
+
+
+25
+ The assignment:
+
+
+ *s1 = *s2;
+
+
+ only copies the member n; if any of the array elements are within the first sizeof (struct s) bytes
+ of the structure, they might be copied or simply overwritten with indeterminate values.
+
+
+26
+ EXAMPLE 3 Because members of anonymous structures and unions are considered to be members of the
+ containing structure or union, struct s in the following example has more than one named member and
+ thus the use of a flexible array member is valid:
+
+
+ struct s {
+ struct { int i; };
+ int a[];
+ };
+
+
+
+
+ Forward references: declarators (6.7.6), tags (6.7.2.3).
+
+
+Footnotes
+
+122) While the number of bits in a _Bool object is at least CHAR_BIT, the width (number of sign and
+ value bits) of a _Bool may be just 1 bit.
+
+
+123) A structure or union cannot contain a member with a variably modified type because member names
+ are not ordinary identifiers as defined in 6.2.3.
+
+
+124) The unary & (address-of) operator cannot be applied to a bit-field object; thus, there are no pointers to
+ or arrays of bit-field objects.
+
+
+125) As specified in 6.7.2 above, if the actual type specifier used is int or a typedef-name defined as int,
+ then it is implementation-defined whether the bit-field is signed or unsigned.
+
+
+126) An unnamed bit-field structure member is useful for padding to conform to externally imposed
+ layouts.
+
+
+Contents
+
+6.7.2.2 Enumeration specifiers
+
+
+Syntax
+
+1
+
+
+ enum-specifier:
+ enum identifieropt { enumerator-list }
+ enum identifieropt { enumerator-list , }
+ enum identifier
+ enumerator-list:
+ enumerator
+ enumerator-list , enumerator
+ enumerator:
+ enumeration-constant
+ enumeration-constant = constant-expression
+
+
+Constraints
+
+2
+ The expression that defines the value of an enumeration constant shall be an integer
+ constant expression that has a value representable as an int.
+
+
+Semantics
+
+3
+ The identifiers in an enumerator list are declared as constants that have type int and
+ may appear wherever such are permitted.127) An enumerator with = defines its
+ enumeration constant as the value of the constant expression. If the first enumerator has
+ no =, the value of its enumeration constant is 0. Each subsequent enumerator with no =
+ defines its enumeration constant as the value of the constant expression obtained by
+ adding 1 to the value of the previous enumeration constant. (The use of enumerators with
+ = may produce enumeration constants with values that duplicate other values in the same
+ enumeration.) The enumerators of an enumeration are also known as its members.
+
+4
+ Each enumerated type shall be compatible with char, a signed integer type, or an
+ unsigned integer type. The choice of type is implementation-defined,128) but shall be
+ capable of representing the values of all the members of the enumeration. The
+ enumerated type is incomplete until immediately after the } that terminates the list of
+ enumerator declarations, and complete thereafter.
+
+5
+ EXAMPLE The following fragment:
+
+
+ enum hue { chartreuse, burgundy, claret=20, winedark };
+ enum hue col, *cp;
+ col = claret;
+ cp = &col;
+ if (*cp != burgundy)
+ /* ... */
+
+
+ makes hue the tag of an enumeration, and then declares col as an object that has that type and cp as a
+ pointer to an object that has that type. The enumerated values are in the set { 0, 1, 20, 21 }.
+
+
+ Forward references: tags (6.7.2.3).
+
+
+Footnotes
+
+127) Thus, the identifiers of enumeration constants declared in the same scope shall all be distinct from
+ each other and from other identifiers declared in ordinary declarators.
+
+
+128) An implementation may delay the choice of which integer type until all enumeration constants have
+ been seen.
+
+
+Contents
+
+6.7.2.3 Tags
+
+
+Constraints
+
+1
+ A specific type shall have its content defined at most once.
+
+2
+ Where two declarations that use the same tag declare the same type, they shall both use
+ the same choice of struct, union, or enum.
+
+3
+ A type specifier of the form
+
+
+ enum identifier
+
+
+ without an enumerator list shall only appear after the type it specifies is complete.
+
+
+
+
+Semantics
+
+4
+ All declarations of structure, union, or enumerated types that have the same scope and
+ use the same tag declare the same type. Irrespective of whether there is a tag or what
+ other declarations of the type are in the same translation unit, the type is incomplete129)
+ until immediately after the closing brace of the list defining the content, and complete
+ thereafter.
+
+5
+ Two declarations of structure, union, or enumerated types which are in different scopes or
+ use different tags declare distinct types. Each declaration of a structure, union, or
+ enumerated type which does not include a tag declares a distinct type.
+
+6
+ A type specifier of the form
+
+
+ struct-or-union identifieropt { struct-declaration-list }
+
+
+ or
+
+
+ enum identifieropt { enumerator-list }
+
+
+ or
+
+
+ enum identifieropt { enumerator-list , }
+
+
+ declares a structure, union, or enumerated type. The list defines the structure content,
+ union content, or enumeration content. If an identifier is provided,130) the type specifier
+ also declares the identifier to be the tag of that type.
+
+7
+ A declaration of the form
+
+
+ struct-or-union identifier ;
+
+
+ specifies a structure or union type and declares the identifier as a tag of that type.131)
+
+8
+ If a type specifier of the form
+
+
+ struct-or-union identifier
+
+
+ occurs other than as part of one of the above forms, and no other declaration of the
+ identifier as a tag is visible, then it declares an incomplete structure or union type, and
+ declares the identifier as the tag of that type.131)
+
+
+
+
+
+9
+ If a type specifier of the form
+
+
+ struct-or-union identifier
+
+
+ or
+
+
+ enum identifier
+
+
+ occurs other than as part of one of the above forms, and a declaration of the identifier as a
+ tag is visible, then it specifies the same type as that other declaration, and does not
+ redeclare the tag.
+
+10
+ EXAMPLE 1 This mechanism allows declaration of a self-referential structure.
+
+
+ struct tnode {
+ int count;
+ struct tnode *left, *right;
+ };
+
+
+ specifies a structure that contains an integer and two pointers to objects of the same type. Once this
+ declaration has been given, the declaration
+
+
+ struct tnode s, *sp;
+
+
+ declares s to be an object of the given type and sp to be a pointer to an object of the given type. With
+ these declarations, the expression sp->left refers to the left struct tnode pointer of the object to
+ which sp points; the expression s.right->count designates the count member of the right struct
+ tnode pointed to from s.
+
+11
+ The following alternative formulation uses the typedef mechanism:
+
+
+ typedef struct tnode TNODE;
+ struct tnode {
+ int count;
+ TNODE *left, *right;
+ };
+ TNODE s, *sp;
+
+
+
+
+12
+ EXAMPLE 2 To illustrate the use of prior declaration of a tag to specify a pair of mutually referential
+ structures, the declarations
+
+
+ struct s1 { struct s2 *s2p; /* ... */ }; // D1
+ struct s2 { struct s1 *s1p; /* ... */ }; // D2
+
+
+ specify a pair of structures that contain pointers to each other. Note, however, that if s2 were already
+ declared as a tag in an enclosing scope, the declaration D1 would refer to it, not to the tag s2 declared in
+ D2. To eliminate this context sensitivity, the declaration
+
+
+ struct s2;
+
+
+ may be inserted ahead of D1. This declares a new tag s2 in the inner scope; the declaration D2 then
+ completes the specification of the new type.
+
+
+ Forward references: declarators (6.7.6), type definitions (6.7.8).
+
+
+Footnotes
+
+129) An incomplete type may only by used when the size of an object of that type is not needed. It is not
+ needed, for example, when a typedef name is declared to be a specifier for a structure or union, or
+ when a pointer to or a function returning a structure or union is being declared. (See incomplete types
+ in 6.2.5.) The specification has to be complete before such a function is called or defined.
+
+
+130) If there is no identifier, the type can, within the translation unit, only be referred to by the declaration
+ of which it is a part. Of course, when the declaration is of a typedef name, subsequent declarations
+ can make use of that typedef name to declare objects having the specified structure, union, or
+ enumerated type.
+
+
+131) A similar construction with enum does not exist.
+
+
+Contents
+
+6.7.2.4 Atomic type specifiers
+
+
+Syntax
+
+1
+
+
+ atomic-type-specifier:
+ _Atomic ( type-name )
+
+
+Constraints
+
+2
+ Atomic type specifiers shall not be used if the implementation does not support atomic
+ types (see 6.10.8.3).
+
+3
+ The type name in an atomic type specifier shall not refer to an array type, a function type,
+ an atomic type, or a qualified type.
+
+Semantics
+
+4
+ The properties associated with atomic types are meaningful only for expressions that are
+ lvalues. If the _Atomic keyword is immediately followed by a left parenthesis, it is
+ interpreted as a type specifier (with a type name), not as a type qualifier.
+
+
+Contents
+
+6.7.3 Type qualifiers
+
+
+Syntax
+
+1
+
+
+ type-qualifier:
+ const
+ restrict
+ volatile
+ _Atomic
+
+
+Constraints
+
+2
+ Types other than pointer types whose referenced type is an object type shall not be
+ restrict-qualified.
+
+3
+ The type modified by the _Atomic qualifier shall not be an array type or a function
+ type.
+
+Semantics
+
+4
+ The properties associated with qualified types are meaningful only for expressions that
+ are lvalues.132)
+
+5
+ If the same qualifier appears more than once in the same specifier-qualifier-list, either
+ directly or via one or more typedefs, the behavior is the same as if it appeared only
+ once. If other qualifiers appear along with the _Atomic qualifier in a specifier-qualifier-
+
+
+ list, the resulting type is the so-qualified atomic type.
+
+6
+ If an attempt is made to modify an object defined with a const-qualified type through use
+ of an lvalue with non-const-qualified type, the behavior is undefined. If an attempt is
+ made to refer to an object defined with a volatile-qualified type through use of an lvalue
+ with non-volatile-qualified type, the behavior is undefined.133)
+
+7
+ An object that has volatile-qualified type may be modified in ways unknown to the
+ implementation or have other unknown side effects. Therefore any expression referring
+ to such an object shall be evaluated strictly according to the rules of the abstract machine,
+ as described in 5.1.2.3. Furthermore, at every sequence point the value last stored in the
+ object shall agree with that prescribed by the abstract machine, except as modified by the
+ unknown factors mentioned previously.134) What constitutes an access to an object that
+ has volatile-qualified type is implementation-defined.
+
+8
+ An object that is accessed through a restrict-qualified pointer has a special association
+ with that pointer. This association, defined in 6.7.3.1 below, requires that all accesses to
+ that object use, directly or indirectly, the value of that particular pointer.135) The intended
+ use of the restrict qualifier (like the register storage class) is to promote
+ optimization, and deleting all instances of the qualifier from all preprocessing translation
+ units composing a conforming program does not change its meaning (i.e., observable
+ behavior).
+
+9
+ If the specification of an array type includes any type qualifiers, the element type is so-
+ qualified, not the array type. If the specification of a function type includes any type
+ qualifiers, the behavior is undefined.136)
+
+10
+ For two qualified types to be compatible, both shall have the identically qualified version
+ of a compatible type; the order of type qualifiers within a list of specifiers or qualifiers
+ does not affect the specified type.
+
+11
+ EXAMPLE 1 An object declared
+
+
+ extern const volatile int real_time_clock;
+
+
+
+
+
+
+ may be modifiable by hardware, but cannot be assigned to, incremented, or decremented.
+
+
+12
+ EXAMPLE 2 The following declarations and expressions illustrate the behavior when type qualifiers
+ modify an aggregate type:
+
+
+ const struct s { int mem; } cs = { 1 };
+ struct s ncs; // the object ncs is modifiable
+ typedef int A[2][3];
+ const A a = {{4, 5, 6}, {7, 8, 9}}; // array of array of const int
+ int *pi;
+ const int *pci;
+ ncs = cs; // valid
+ cs = ncs; // violates modifiable lvalue constraint for =
+ pi = &ncs.mem; // valid
+ pi = &cs.mem; // violates type constraints for =
+ pci = &cs.mem; // valid
+ pi = a[0]; // invalid: a[0] has type ''const int *''
+
+
+
+
+13
+ EXAMPLE 3 The declaration
+
+
+ _Atomic volatile int *p;
+
+
+ specifies that p has the type ''pointer to volatile atomic int'', a pointer to a volatile-qualified atomic type.
+
+
+
+Footnotes
+
+132) The implementation may place a const object that is not volatile in a read-only region of
+ storage. Moreover, the implementation need not allocate storage for such an object if its address is
+ never used.
+
+
+133) This applies to those objects that behave as if they were defined with qualified types, even if they are
+ never actually defined as objects in the program (such as an object at a memory-mapped input/output
+ address).
+
+
+134) A volatile declaration may be used to describe an object corresponding to a memory-mapped
+ input/output port or an object accessed by an asynchronously interrupting function. Actions on
+ objects so declared shall not be ''optimized out'' by an implementation or reordered except as
+ permitted by the rules for evaluating expressions.
+
+
+135) For example, a statement that assigns a value returned by malloc to a single pointer establishes this
+ association between the allocated object and the pointer.
+
+
+136) Both of these can occur through the use of typedefs.
+
+
+Contents
+
+6.7.3.1 Formal definition of restrict
+
+
+1
+ Let D be a declaration of an ordinary identifier that provides a means of designating an
+ object P as a restrict-qualified pointer to type T.
+
+2
+ If D appears inside a block and does not have storage class extern, let B denote the
+ block. If D appears in the list of parameter declarations of a function definition, let B
+ denote the associated block. Otherwise, let B denote the block of main (or the block of
+ whatever function is called at program startup in a freestanding environment).
+
+3
+ In what follows, a pointer expression E is said to be based on object P if (at some
+ sequence point in the execution of B prior to the evaluation of E) modifying P to point to
+ a copy of the array object into which it formerly pointed would change the value of E.137)
+ Note that ''based'' is defined only for expressions with pointer types.
+
+4
+ During each execution of B, let L be any lvalue that has &L based on P. If L is used to
+ access the value of the object X that it designates, and X is also modified (by any means),
+ then the following requirements apply: T shall not be const-qualified. Every other lvalue
+ used to access the value of X shall also have its address based on P. Every access that
+ modifies X shall be considered also to modify P, for the purposes of this subclause. If P
+ is assigned the value of a pointer expression E that is based on another restricted pointer
+
+
+
+ object P2, associated with block B2, then either the execution of B2 shall begin before
+ the execution of B, or the execution of B2 shall end prior to the assignment. If these
+ requirements are not met, then the behavior is undefined.
+
+5
+ Here an execution of B means that portion of the execution of the program that would
+ correspond to the lifetime of an object with scalar type and automatic storage duration
+ associated with B.
+
+6
+ A translator is free to ignore any or all aliasing implications of uses of restrict.
+
+7
+ EXAMPLE 1 The file scope declarations
+
+
+ int * restrict a;
+ int * restrict b;
+ extern int c[];
+
+
+ assert that if an object is accessed using one of a, b, or c, and that object is modified anywhere in the
+ program, then it is never accessed using either of the other two.
+
+
+8
+ EXAMPLE 2 The function parameter declarations in the following example
+
+
+ void f(int n, int * restrict p, int * restrict q)
+ {
+ while (n-- > 0)
+ *p++ = *q++;
+ }
+
+
+ assert that, during each execution of the function, if an object is accessed through one of the pointer
+ parameters, then it is not also accessed through the other.
+
+9
+ The benefit of the restrict qualifiers is that they enable a translator to make an effective dependence
+ analysis of function f without examining any of the calls of f in the program. The cost is that the
+ programmer has to examine all of those calls to ensure that none give undefined behavior. For example, the
+ second call of f in g has undefined behavior because each of d[1] through d[49] is accessed through
+ both p and q.
+
+
+ void g(void)
+ {
+ extern int d[100];
+ f(50, d + 50, d); // valid
+ f(50, d + 1, d); // undefined behavior
+ }
+
+
+
+
+10
+ EXAMPLE 3 The function parameter declarations
+
+
+ void h(int n, int * restrict p, int * restrict q, int * restrict r)
+ {
+ int i;
+ for (i = 0; i < n; i++)
+ p[i] = q[i] + r[i];
+ }
+
+
+ illustrate how an unmodified object can be aliased through two restricted pointers. In particular, if a and b
+ are disjoint arrays, a call of the form h(100, a, b, b) has defined behavior, because array b is not
+ modified within function h.
+
+
+11
+ EXAMPLE 4 The rule limiting assignments between restricted pointers does not distinguish between a
+ function call and an equivalent nested block. With one exception, only ''outer-to-inner'' assignments
+ between restricted pointers declared in nested blocks have defined behavior.
+
+
+ {
+ int * restrict p1;
+ int * restrict q1;
+ p1 = q1; // undefined behavior
+ {
+ int * restrict p2 = p1; // valid
+ int * restrict q2 = q1; // valid
+ p1 = q2; // undefined behavior
+ p2 = q2; // undefined behavior
+ }
+ }
+
+
+12
+ The one exception allows the value of a restricted pointer to be carried out of the block in which it (or, more
+ precisely, the ordinary identifier used to designate it) is declared when that block finishes execution. For
+ example, this permits new_vector to return a vector.
+
+
+ typedef struct { int n; float * restrict v; } vector;
+ vector new_vector(int n)
+ {
+ vector t;
+ t.n = n;
+ t.v = malloc(n * sizeof (float));
+ return t;
+ }
+
+
+
+
+
+Footnotes
+
+137) In other words, E depends on the value of P itself rather than on the value of an object referenced
+ indirectly through P. For example, if identifier p has type (int **restrict), then the pointer
+ expressions p and p+1 are based on the restricted pointer object designated by p, but the pointer
+ expressions *p and p[1] are not.
+
+
+Contents
+
+6.7.4 Function specifiers
+
+
+Syntax
+
+1
+
+
+ function-specifier:
+ inline
+ _Noreturn
+
+
+Constraints
+
+2
+ Function specifiers shall be used only in the declaration of an identifier for a function.
+
+3
+ An inline definition of a function with external linkage shall not contain a definition of a
+ modifiable object with static or thread storage duration, and shall not contain a reference
+ to an identifier with internal linkage.
+
+4
+ In a hosted environment, no function specifier(s) shall appear in a declaration of main.
+
+Semantics
+
+5
+ A function specifier may appear more than once; the behavior is the same as if it
+ appeared only once.
+
+6
+ A function declared with an inline function specifier is an inline function. Making a
+ function an inline function suggests that calls to the function be as fast as possible.138)
+
+ The extent to which such suggestions are effective is implementation-defined.139)
+
+7
+ Any function with internal linkage can be an inline function. For a function with external
+ linkage, the following restrictions apply: If a function is declared with an inline
+ function specifier, then it shall also be defined in the same translation unit. If all of the
+ file scope declarations for a function in a translation unit include the inline function
+ specifier without extern, then the definition in that translation unit is an inline
+ definition. An inline definition does not provide an external definition for the function,
+ and does not forbid an external definition in another translation unit. An inline definition
+ provides an alternative to an external definition, which a translator may use to implement
+ any call to the function in the same translation unit. It is unspecified whether a call to the
+ function uses the inline definition or the external definition.140)
+
+8
+ A function declared with a _Noreturn function specifier shall not return to its caller.
+
+Recommended practice
+
+9
+ The implementation should produce a diagnostic message for a function declared with a
+ _Noreturn function specifier that appears to be capable of returning to its caller.
+
+10
+ EXAMPLE 1 The declaration of an inline function with external linkage can result in either an external
+ definition, or a definition available for use only within the translation unit. A file scope declaration with
+ extern creates an external definition. The following example shows an entire translation unit.
+
+
+ inline double fahr(double t)
+ {
+ return (9.0 * t) / 5.0 + 32.0;
+ }
+ inline double cels(double t)
+ {
+ return (5.0 * (t - 32.0)) / 9.0;
+ }
+ extern double fahr(double); // creates an external definition
+
+
+
+
+
+
+
+
+ double convert(int is_fahr, double temp)
+ {
+ /* A translator may perform inline substitutions */
+ return is_fahr ? cels(temp) : fahr(temp);
+ }
+
+
+11
+ Note that the definition of fahr is an external definition because fahr is also declared with extern, but
+ the definition of cels is an inline definition. Because cels has external linkage and is referenced, an
+ external definition has to appear in another translation unit (see 6.9); the inline definition and the external
+ definition are distinct and either may be used for the call.
+
+
+12
+ EXAMPLE 2
+
+
+ _Noreturn void f () {
+ abort(); // ok
+ }
+ _Noreturn void g (int i) { // causes undefined behavior if i <= 0
+ if (i > 0) abort();
+ }
+
+
+
+
+ Forward references: function definitions (6.9.1).
+
+
+Footnotes
+
+138) By using, for example, an alternative to the usual function call mechanism, such as ''inline
+ substitution''. Inline substitution is not textual substitution, nor does it create a new function.
+ Therefore, for example, the expansion of a macro used within the body of the function uses the
+ definition it had at the point the function body appears, and not where the function is called; and
+ identifiers refer to the declarations in scope where the body occurs. Likewise, the function has a
+ single address, regardless of the number of inline definitions that occur in addition to the external
+ definition.
+
+
+139) For example, an implementation might never perform inline substitution, or might only perform inline
+ substitutions to calls in the scope of an inline declaration.
+
+
+140) Since an inline definition is distinct from the corresponding external definition and from any other
+ corresponding inline definitions in other translation units, all corresponding objects with static storage
+ duration are also distinct in each of the definitions.
+
+
+Contents
+
+6.7.5 Alignment specifier
+
+
+Syntax
+
+1
+
+
+ alignment-specifier:
+ _Alignas ( type-name )
+ _Alignas ( constant-expression )
+
+
+Constraints
+
+2
+ An alignment attribute shall not be specified in a declaration of a typedef, or a bit-field, or
+ a function, or a parameter, or an object declared with the register storage-class
+ specifier.
+
+3
+ The constant expression shall be an integer constant expression. It shall evaluate to a
+ valid fundamental alignment, or to a valid extended alignment supported by the
+ implementation in the context in which it appears, or to zero.
+
+4
+ The combined effect of all alignment attributes in a declaration shall not specify an
+ alignment that is less strict than the alignment that would otherwise be required for the
+ type of the object or member being declared.
+
+Semantics
+
+5
+ The first form is equivalent to _Alignas (_Alignof (type-name)).
+
+6
+ The alignment requirement of the declared object or member is taken to be the specified
+ alignment. An alignment specification of zero has no effect.141) When multiple
+ alignment specifiers occur in a declaration, the effective alignment requirement is the
+ strictest specified alignment.
+
+
+7
+ If the definition of an object has an alignment specifier, any other declaration of that
+ object shall either specify equivalent alignment or have no alignment specifier. If the
+ definition of an object does not have an alignment specifier, any other declaration of that
+ object shall also have no alignment specifier. If declarations of an object in different
+ translation units have different alignment specifiers, the behavior is undefined.
+
+
+Footnotes
+
+141) An alignment specification of zero also does not affect other alignment specifications in the same
+ declaration.
+
+
+Contents
+
+6.7.6 Declarators
+
+
+Syntax
+
+1
+
+
+ declarator:
+ pointeropt direct-declarator
+ direct-declarator:
+ identifier
+ ( declarator )
+ direct-declarator [ type-qualifier-listopt assignment-expressionopt ]
+ direct-declarator [ static type-qualifier-listopt assignment-expression ]
+ direct-declarator [ type-qualifier-list static assignment-expression ]
+ direct-declarator [ type-qualifier-listopt * ]
+ direct-declarator ( parameter-type-list )
+ direct-declarator ( identifier-listopt )
+ pointer:
+ * type-qualifier-listopt
+ * type-qualifier-listopt pointer
+ type-qualifier-list:
+ type-qualifier
+ type-qualifier-list type-qualifier
+ parameter-type-list:
+ parameter-list
+ parameter-list , ...
+ parameter-list:
+ parameter-declaration
+ parameter-list , parameter-declaration
+ parameter-declaration:
+ declaration-specifiers declarator
+ declaration-specifiers abstract-declaratoropt
+
+
+
+
+
+
+
+ identifier-list:
+ identifier
+ identifier-list , identifier
+
+
+Semantics
+
+2
+ Each declarator declares one identifier, and asserts that when an operand of the same
+ form as the declarator appears in an expression, it designates a function or object with the
+ scope, storage duration, and type indicated by the declaration specifiers.
+
+3
+ A full declarator is a declarator that is not part of another declarator. The end of a full
+ declarator is a sequence point. If, in the nested sequence of declarators in a full
+ declarator, there is a declarator specifying a variable length array type, the type specified
+ by the full declarator is said to be variably modified. Furthermore, any type derived by
+ declarator type derivation from a variably modified type is itself variably modified.
+
+4
+ In the following subclauses, consider a declaration
+
+
+ T D1
+
+
+ where T contains the declaration specifiers that specify a type T (such as int) and D1 is
+ a declarator that contains an identifier ident. The type specified for the identifier ident in
+ the various forms of declarator is described inductively using this notation.
+
+5
+ If, in the declaration ''T D1'', D1 has the form
+
+
+ identifier
+
+
+ then the type specified for ident is T .
+
+6
+ If, in the declaration ''T D1'', D1 has the form
+
+
+ ( D )
+
+
+ then ident has the type specified by the declaration ''T D''. Thus, a declarator in
+ parentheses is identical to the unparenthesized declarator, but the binding of complicated
+ declarators may be altered by parentheses.
+
+Implementation limits
+
+7
+ As discussed in 5.2.4.1, an implementation may limit the number of pointer, array, and
+ function declarators that modify an arithmetic, structure, union, or void type, either
+ directly or via one or more typedefs.
+
+ Forward references: array declarators (6.7.6.2), type definitions (6.7.8).
+
+
+Contents
+
+6.7.6.1 Pointer declarators
+
+
+Semantics
+
+1
+ If, in the declaration ''T D1'', D1 has the form
+
+
+ * type-qualifier-listopt D
+
+
+ and the type specified for ident in the declaration ''T D'' is ''derived-declarator-type-list
+ T '', then the type specified for ident is ''derived-declarator-type-list type-qualifier-list
+ pointer to T ''. For each type qualifier in the list, ident is a so-qualified pointer.
+
+2
+ For two pointer types to be compatible, both shall be identically qualified and both shall
+ be pointers to compatible types.
+
+3
+ EXAMPLE The following pair of declarations demonstrates the difference between a ''variable pointer
+ to a constant value'' and a ''constant pointer to a variable value''.
+
+
+ const int *ptr_to_constant;
+ int *const constant_ptr;
+
+
+ The contents of any object pointed to by ptr_to_constant shall not be modified through that pointer,
+ but ptr_to_constant itself may be changed to point to another object. Similarly, the contents of the
+ int pointed to by constant_ptr may be modified, but constant_ptr itself shall always point to the
+ same location.
+
+4
+ The declaration of the constant pointer constant_ptr may be clarified by including a definition for the
+ type ''pointer to int''.
+
+
+ typedef int *int_ptr;
+ const int_ptr constant_ptr;
+
+
+ declares constant_ptr as an object that has type ''const-qualified pointer to int''.
+
+
+
+Contents
+
+6.7.6.2 Array declarators
+
+
+Constraints
+
+1
+ In addition to optional type qualifiers and the keyword static, the [ and ] may delimit
+ an expression or *. If they delimit an expression (which specifies the size of an array), the
+ expression shall have an integer type. If the expression is a constant expression, it shall
+ have a value greater than zero. The element type shall not be an incomplete or function
+ type. The optional type qualifiers and the keyword static shall appear only in a
+ declaration of a function parameter with an array type, and then only in the outermost
+ array type derivation.
+
+2
+ If an identifier is declared as having a variably modified type, it shall be an ordinary
+ identifier (as defined in 6.2.3), have no linkage, and have either block scope or function
+ prototype scope. If an identifier is declared to be an object with static or thread storage
+ duration, it shall not have a variable length array type.
+
+
+Semantics
+
+3
+ If, in the declaration ''T D1'', D1 has one of the forms:
+
+
+ D[ type-qualifier-listopt assignment-expressionopt ]
+ D[ static type-qualifier-listopt assignment-expression ]
+ D[ type-qualifier-list static assignment-expression ]
+ D[ type-qualifier-listopt * ]
+
+
+ and the type specified for ident in the declaration ''T D'' is ''derived-declarator-type-list
+ T '', then the type specified for ident is ''derived-declarator-type-list array of T ''.142)
+ (See 6.7.6.3 for the meaning of the optional type qualifiers and the keyword static.)
+
+4
+ If the size is not present, the array type is an incomplete type. If the size is * instead of
+ being an expression, the array type is a variable length array type of unspecified size,
+ which can only be used in declarations or type names with function prototype scope;143)
+ such arrays are nonetheless complete types. If the size is an integer constant expression
+ and the element type has a known constant size, the array type is not a variable length
+ array type; otherwise, the array type is a variable length array type. (Variable length
+ arrays are a conditional feature that implementations need not support; see 6.10.8.3.)
+
+5
+ If the size is an expression that is not an integer constant expression: if it occurs in a
+ declaration at function prototype scope, it is treated as if it were replaced by *; otherwise,
+ each time it is evaluated it shall have a value greater than zero. The size of each instance
+ of a variable length array type does not change during its lifetime. Where a size
+ expression is part of the operand of a sizeof operator and changing the value of the
+ size expression would not affect the result of the operator, it is unspecified whether or not
+ the size expression is evaluated.
+
+6
+ For two array types to be compatible, both shall have compatible element types, and if
+ both size specifiers are present, and are integer constant expressions, then both size
+ specifiers shall have the same constant value. If the two array types are used in a context
+ which requires them to be compatible, it is undefined behavior if the two size specifiers
+ evaluate to unequal values.
+
+7
+ EXAMPLE 1
+
+
+ float fa[11], *afp[17];
+
+
+ declares an array of float numbers and an array of pointers to float numbers.
+
+
+8
+ EXAMPLE 2 Note the distinction between the declarations
+
+
+
+
+
+
+ extern int *x;
+ extern int y[];
+
+
+ The first declares x to be a pointer to int; the second declares y to be an array of int of unspecified size
+ (an incomplete type), the storage for which is defined elsewhere.
+
+
+9
+ EXAMPLE 3 The following declarations demonstrate the compatibility rules for variably modified types.
+
+
+ extern int n;
+ extern int m;
+ void fcompat(void)
+ {
+ int a[n][6][m];
+ int (*p)[4][n+1];
+ int c[n][n][6][m];
+ int (*r)[n][n][n+1];
+ p = a; // invalid: not compatible because 4 != 6
+ r = c; // compatible, but defined behavior only if
+ // n == 6 and m == n+1
+ }
+
+
+
+
+10
+ EXAMPLE 4 All declarations of variably modified (VM) types have to be at either block scope or
+ function prototype scope. Array objects declared with the _Thread_local, static, or extern
+ storage-class specifier cannot have a variable length array (VLA) type. However, an object declared with
+ the static storage-class specifier can have a VM type (that is, a pointer to a VLA type). Finally, all
+ identifiers declared with a VM type have to be ordinary identifiers and cannot, therefore, be members of
+ structures or unions.
+
+
+ extern int n;
+ int A[n]; // invalid: file scope VLA
+ extern int (*p2)[n]; // invalid: file scope VM
+ int B[100]; // valid: file scope but not VM
+ void fvla(int m, int C[m][m]); // valid: VLA with prototype scope
+ void fvla(int m, int C[m][m]) // valid: adjusted to auto pointer to VLA
+ {
+ typedef int VLA[m][m]; // valid: block scope typedef VLA
+ struct tag {
+ int (*y)[n]; // invalid: y not ordinary identifier
+ int z[n]; // invalid: z not ordinary identifier
+ };
+ int D[m]; // valid: auto VLA
+ static int E[m]; // invalid: static block scope VLA
+ extern int F[m]; // invalid: F has linkage and is VLA
+ int (*s)[m]; // valid: auto pointer to VLA
+ extern int (*r)[m]; // invalid: r has linkage and points to VLA
+ static int (*q)[m] = &B; // valid: q is a static block pointer to VLA
+ }
+
+
+
+
+ Forward references: function declarators (6.7.6.3), function definitions (6.9.1),
+ initialization (6.7.9).
+
+
+Footnotes
+
+142) When several ''array of'' specifications are adjacent, a multidimensional array is declared.
+
+
+143) Thus, * can be used only in function declarations that are not definitions (see 6.7.6.3).
+
+
+Contents
+
+6.7.6.3 Function declarators (including prototypes)
+
+
+Constraints
+
+1
+ A function declarator shall not specify a return type that is a function type or an array
+ type.
+
+2
+ The only storage-class specifier that shall occur in a parameter declaration is register.
+
+3
+ An identifier list in a function declarator that is not part of a definition of that function
+ shall be empty.
+
+4
+ After adjustment, the parameters in a parameter type list in a function declarator that is
+ part of a definition of that function shall not have incomplete type.
+
+Semantics
+
+5
+ If, in the declaration ''T D1'', D1 has the form
+
+
+ D( parameter-type-list )
+
+
+ or
+
+
+ D( identifier-listopt )
+
+
+ and the type specified for ident in the declaration ''T D'' is ''derived-declarator-type-list
+ T '', then the type specified for ident is ''derived-declarator-type-list function returning
+ T ''.
+
+6
+ A parameter type list specifies the types of, and may declare identifiers for, the
+ parameters of the function.
+
+7
+ A declaration of a parameter as ''array of type'' shall be adjusted to ''qualified pointer to
+ type'', where the type qualifiers (if any) are those specified within the [ and ] of the
+ array type derivation. If the keyword static also appears within the [ and ] of the
+ array type derivation, then for each call to the function, the value of the corresponding
+ actual argument shall provide access to the first element of an array with at least as many
+ elements as specified by the size expression.
+
+8
+ A declaration of a parameter as ''function returning type'' shall be adjusted to ''pointer to
+ function returning type'', as in 6.3.2.1.
+
+9
+ If the list terminates with an ellipsis (, ...), no information about the number or types
+ of the parameters after the comma is supplied.144)
+
+10
+ The special case of an unnamed parameter of type void as the only item in the list
+ specifies that the function has no parameters.
+
+
+
+
+
+11
+ If, in a parameter declaration, an identifier can be treated either as a typedef name or as a
+ parameter name, it shall be taken as a typedef name.
+
+12
+ If the function declarator is not part of a definition of that function, parameters may have
+ incomplete type and may use the [*] notation in their sequences of declarator specifiers
+ to specify variable length array types.
+
+13
+ The storage-class specifier in the declaration specifiers for a parameter declaration, if
+ present, is ignored unless the declared parameter is one of the members of the parameter
+ type list for a function definition.
+
+14
+ An identifier list declares only the identifiers of the parameters of the function. An empty
+ list in a function declarator that is part of a definition of that function specifies that the
+ function has no parameters. The empty list in a function declarator that is not part of a
+ definition of that function specifies that no information about the number or types of the
+ parameters is supplied.145)
+
+15
+ For two function types to be compatible, both shall specify compatible return types.146)
+ Moreover, the parameter type lists, if both are present, shall agree in the number of
+ parameters and in use of the ellipsis terminator; corresponding parameters shall have
+ compatible types. If one type has a parameter type list and the other type is specified by a
+ function declarator that is not part of a function definition and that contains an empty
+ identifier list, the parameter list shall not have an ellipsis terminator and the type of each
+ parameter shall be compatible with the type that results from the application of the
+ default argument promotions. If one type has a parameter type list and the other type is
+ specified by a function definition that contains a (possibly empty) identifier list, both shall
+ agree in the number of parameters, and the type of each prototype parameter shall be
+ compatible with the type that results from the application of the default argument
+ promotions to the type of the corresponding identifier. (In the determination of type
+ compatibility and of a composite type, each parameter declared with function or array
+ type is taken as having the adjusted type and each parameter declared with qualified type
+ is taken as having the unqualified version of its declared type.)
+
+16
+ EXAMPLE 1 The declaration
+
+
+ int f(void), *fip(), (*pfi)();
+
+
+ declares a function f with no parameters returning an int, a function fip with no parameter specification
+ returning a pointer to an int, and a pointer pfi to a function with no parameter specification returning an
+ int. It is especially useful to compare the last two. The binding of *fip() is *(fip()), so that the
+ declaration suggests, and the same construction in an expression requires, the calling of a function fip,
+ and then using indirection through the pointer result to yield an int. In the declarator (*pfi)(), the
+ extra parentheses are necessary to indicate that indirection through a pointer to a function yields a function
+
+
+
+ designator, which is then used to call the function; it returns an int.
+
+17
+ If the declaration occurs outside of any function, the identifiers have file scope and external linkage. If the
+ declaration occurs inside a function, the identifiers of the functions f and fip have block scope and either
+ internal or external linkage (depending on what file scope declarations for these identifiers are visible), and
+ the identifier of the pointer pfi has block scope and no linkage.
+
+
+18
+ EXAMPLE 2 The declaration
+
+
+ int (*apfi[3])(int *x, int *y);
+
+
+ declares an array apfi of three pointers to functions returning int. Each of these functions has two
+ parameters that are pointers to int. The identifiers x and y are declared for descriptive purposes only and
+ go out of scope at the end of the declaration of apfi.
+
+
+19
+ EXAMPLE 3 The declaration
+
+
+ int (*fpfi(int (*)(long), int))(int, ...);
+
+
+ declares a function fpfi that returns a pointer to a function returning an int. The function fpfi has two
+ parameters: a pointer to a function returning an int (with one parameter of type long int), and an int.
+ The pointer returned by fpfi points to a function that has one int parameter and accepts zero or more
+ additional arguments of any type.
+
+
+20
+ EXAMPLE 4 The following prototype has a variably modified parameter.
+
+
+ void addscalar(int n, int m,
+ double a[n][n*m+300], double x);
+ int main()
+ {
+ double b[4][308];
+ addscalar(4, 2, b, 2.17);
+ return 0;
+ }
+ void addscalar(int n, int m,
+ double a[n][n*m+300], double x)
+ {
+ for (int i = 0; i < n; i++)
+ for (int j = 0, k = n*m+300; j < k; j++)
+ // a is a pointer to a VLA with n*m+300 elements
+ a[i][j] += x;
+ }
+
+
+
+
+21
+ EXAMPLE 5 The following are all compatible function prototype declarators.
+
+
+ double maximum(int n, int m, double a[n][m]);
+ double maximum(int n, int m, double a[*][*]);
+ double maximum(int n, int m, double a[ ][*]);
+ double maximum(int n, int m, double a[ ][m]);
+
+
+ as are:
+
+
+ void f(double (* restrict a)[5]);
+ void f(double a[restrict][5]);
+ void f(double a[restrict 3][5]);
+ void f(double a[restrict static 3][5]);
+
+
+ (Note that the last declaration also specifies that the argument corresponding to a in any call to f must be a
+ non-null pointer to the first of at least three arrays of 5 doubles, which the others do not.)
+
+
+ Forward references: function definitions (6.9.1), type names (6.7.7).
+
+
+Footnotes
+
+144) The macros defined in the <stdarg.h> header (7.16) may be used to access arguments that
+ correspond to the ellipsis.
+
+
+145) See ''future language directions'' (6.11.6).
+
+
+146) If both function types are ''old style'', parameter types are not compared.
+
+
+Contents
+
+6.7.7 Type names
+
+
+Syntax
+
+1
+
+
+ type-name:
+ specifier-qualifier-list abstract-declaratoropt
+ abstract-declarator:
+ pointer
+ pointeropt direct-abstract-declarator
+ direct-abstract-declarator:
+ ( abstract-declarator )
+ direct-abstract-declaratoropt [ type-qualifier-listopt
+ assignment-expressionopt ]
+ direct-abstract-declaratoropt [ static type-qualifier-listopt
+ assignment-expression ]
+ direct-abstract-declaratoropt [ type-qualifier-list static
+ assignment-expression ]
+ direct-abstract-declaratoropt [ * ]
+ direct-abstract-declaratoropt ( parameter-type-listopt )
+
+
+Semantics
+
+2
+ In several contexts, it is necessary to specify a type. This is accomplished using a type
+ name, which is syntactically a declaration for a function or an object of that type that
+ omits the identifier.147)
+
+3
+ EXAMPLE The constructions
+
+
+ (a) int
+ (b) int *
+ (c) int *[3]
+ (d) int (*)[3]
+ (e) int (*)[*]
+ (f) int *()
+ (g) int (*)(void)
+ (h) int (*const [])(unsigned int, ...)
+
+
+ name respectively the types (a) int, (b) pointer to int, (c) array of three pointers to int, (d) pointer to an
+ array of three ints, (e) pointer to a variable length array of an unspecified number of ints, (f) function
+ with no parameter specification returning a pointer to int, (g) pointer to function with no parameters
+
+
+
+ returning an int, and (h) array of an unspecified number of constant pointers to functions, each with one
+ parameter that has type unsigned int and an unspecified number of other parameters, returning an
+ int.
+
+
+
+Footnotes
+
+147) As indicated by the syntax, empty parentheses in a type name are interpreted as ''function with no
+ parameter specification'', rather than redundant parentheses around the omitted identifier.
+
+
+Contents
+
+6.7.8 Type definitions
+
+
+Syntax
+
+1
+
+
+ typedef-name:
+ identifier
+
+
+Constraints
+
+2
+ If a typedef name specifies a variably modified type then it shall have block scope.
+
+Semantics
+
+3
+ In a declaration whose storage-class specifier is typedef, each declarator defines an
+ identifier to be a typedef name that denotes the type specified for the identifier in the way
+ described in 6.7.6. Any array size expressions associated with variable length array
+ declarators are evaluated each time the declaration of the typedef name is reached in the
+ order of execution. A typedef declaration does not introduce a new type, only a
+ synonym for the type so specified. That is, in the following declarations:
+
+
+ typedef T type_ident;
+ type_ident D;
+
+
+ type_ident is defined as a typedef name with the type specified by the declaration
+ specifiers in T (known as T ), and the identifier in D has the type ''derived-declarator-
+ type-list T '' where the derived-declarator-type-list is specified by the declarators of D. A
+ typedef name shares the same name space as other identifiers declared in ordinary
+ declarators.
+
+4
+ EXAMPLE 1 After
+
+
+ typedef int MILES, KLICKSP();
+ typedef struct { double hi, lo; } range;
+
+
+ the constructions
+
+
+ MILES distance;
+ extern KLICKSP *metricp;
+ range x;
+ range z, *zp;
+
+
+ are all valid declarations. The type of distance is int, that of metricp is ''pointer to function with no
+ parameter specification returning int'', and that of x and z is the specified structure; zp is a pointer to
+ such a structure. The object distance has a type compatible with any other int object.
+
+
+5
+ EXAMPLE 2 After the declarations
+
+
+ typedef struct s1 { int x; } t1, *tp1;
+ typedef struct s2 { int x; } t2, *tp2;
+
+
+ type t1 and the type pointed to by tp1 are compatible. Type t1 is also compatible with type struct
+
+ s1, but not compatible with the types struct s2, t2, the type pointed to by tp2, or int.
+
+
+6
+ EXAMPLE 3 The following obscure constructions
+
+
+ typedef signed int t;
+ typedef int plain;
+ struct tag {
+ unsigned t:4;
+ const t:5;
+ plain r:5;
+ };
+
+
+ declare a typedef name t with type signed int, a typedef name plain with type int, and a structure
+ with three bit-field members, one named t that contains values in the range [0, 15], an unnamed const-
+ qualified bit-field which (if it could be accessed) would contain values in either the range [-15, +15] or
+ [-16, +15], and one named r that contains values in one of the ranges [0, 31], [-15, +15], or [-16, +15].
+ (The choice of range is implementation-defined.) The first two bit-field declarations differ in that
+ unsigned is a type specifier (which forces t to be the name of a structure member), while const is a
+ type qualifier (which modifies t which is still visible as a typedef name). If these declarations are followed
+ in an inner scope by
+
+
+ t f(t (t));
+ long t;
+
+
+ then a function f is declared with type ''function returning signed int with one unnamed parameter
+ with type pointer to function returning signed int with one unnamed parameter with type signed
+ int'', and an identifier t with type long int.
+
+
+7
+ EXAMPLE 4 On the other hand, typedef names can be used to improve code readability. All three of the
+ following declarations of the signal function specify exactly the same type, the first without making use
+ of any typedef names.
+
+
+ typedef void fv(int), (*pfv)(int);
+ void (*signal(int, void (*)(int)))(int);
+ fv *signal(int, fv *);
+ pfv signal(int, pfv);
+
+
+
+
+8
+ EXAMPLE 5 If a typedef name denotes a variable length array type, the length of the array is fixed at the
+ time the typedef name is defined, not each time it is used:
+
+
+ void copyt(int n)
+ {
+ typedef int B[n]; // B is n ints, n evaluated now
+ n += 1;
+ B a; // a is n ints, n without += 1
+ int b[n]; // a and b are different sizes
+ for (int i = 1; i < n; i++)
+ a[i-1] = b[i];
+ }
+
+
+Contents
+
+6.7.9 Initialization
+
+
+Syntax
+
+1
+
+
+ initializer:
+ assignment-expression
+ { initializer-list }
+ { initializer-list , }
+ initializer-list:
+ designationopt initializer
+ initializer-list , designationopt initializer
+ designation:
+ designator-list =
+ designator-list:
+ designator
+ designator-list designator
+ designator:
+ [ constant-expression ]
+ . identifier
+
+
+Constraints
+
+2
+ No initializer shall attempt to provide a value for an object not contained within the entity
+ being initialized.
+
+3
+ The type of the entity to be initialized shall be an array of unknown size or a complete
+ object type that is not a variable length array type.
+
+4
+ All the expressions in an initializer for an object that has static or thread storage duration
+ shall be constant expressions or string literals.
+
+5
+ If the declaration of an identifier has block scope, and the identifier has external or
+ internal linkage, the declaration shall have no initializer for the identifier.
+
+6
+ If a designator has the form
+
+
+ [ constant-expression ]
+
+
+ then the current object (defined below) shall have array type and the expression shall be
+ an integer constant expression. If the array is of unknown size, any nonnegative value is
+ valid.
+
+7
+ If a designator has the form
+
+
+ . identifier
+
+
+ then the current object (defined below) shall have structure or union type and the
+ identifier shall be the name of a member of that type.
+
+
+Semantics
+
+8
+ An initializer specifies the initial value stored in an object.
+
+9
+ Except where explicitly stated otherwise, for the purposes of this subclause unnamed
+ members of objects of structure and union type do not participate in initialization.
+ Unnamed members of structure objects have indeterminate value even after initialization.
+
+10
+ If an object that has automatic storage duration is not initialized explicitly, its value is
+ indeterminate. If an object that has static or thread storage duration is not initialized
+ explicitly, then:
+
+
+- if it has pointer type, it is initialized to a null pointer;
+
+- if it has arithmetic type, it is initialized to (positive or unsigned) zero;
+
+- if it is an aggregate, every member is initialized (recursively) according to these rules,
+ and any padding is initialized to zero bits;
+
+- if it is a union, the first named member is initialized (recursively) according to these
+ rules, and any padding is initialized to zero bits;
+
+
+11
+ The initializer for a scalar shall be a single expression, optionally enclosed in braces. The
+ initial value of the object is that of the expression (after conversion); the same type
+ constraints and conversions as for simple assignment apply, taking the type of the scalar
+ to be the unqualified version of its declared type.
+
+12
+ The rest of this subclause deals with initializers for objects that have aggregate or union
+ type.
+
+13
+ The initializer for a structure or union object that has automatic storage duration shall be
+ either an initializer list as described below, or a single expression that has compatible
+ structure or union type. In the latter case, the initial value of the object, including
+ unnamed members, is that of the expression.
+
+14
+ An array of character type may be initialized by a character string literal or UTF-8 string
+ literal, optionally enclosed in braces. Successive bytes of the string literal (including the
+ terminating null character if there is room or if the array is of unknown size) initialize the
+ elements of the array.
+
+15
+ An array with element type compatible with a qualified or unqualified version of
+ wchar_t, char16_t, or char32_t may be initialized by a wide string literal with
+ the corresponding encoding prefix (L, u, or U, respectively), optionally enclosed in
+ braces. Successive wide characters of the wide string literal (including the terminating
+ null wide character if there is room or if the array is of unknown size) initialize the
+ elements of the array.
+
+16
+ Otherwise, the initializer for an object that has aggregate or union type shall be a brace-
+ enclosed list of initializers for the elements or named members.
+
+
+17
+ Each brace-enclosed initializer list has an associated current object. When no
+ designations are present, subobjects of the current object are initialized in order according
+ to the type of the current object: array elements in increasing subscript order, structure
+ members in declaration order, and the first named member of a union.148) In contrast, a
+ designation causes the following initializer to begin initialization of the subobject
+ described by the designator. Initialization then continues forward in order, beginning
+ with the next subobject after that described by the designator.149)
+
+18
+ Each designator list begins its description with the current object associated with the
+ closest surrounding brace pair. Each item in the designator list (in order) specifies a
+ particular member of its current object and changes the current object for the next
+ designator (if any) to be that member.150) The current object that results at the end of the
+ designator list is the subobject to be initialized by the following initializer.
+
+19
+ The initialization shall occur in initializer list order, each initializer provided for a
+ particular subobject overriding any previously listed initializer for the same subobject;151)
+ all subobjects that are not initialized explicitly shall be initialized implicitly the same as
+ objects that have static storage duration.
+
+20
+ If the aggregate or union contains elements or members that are aggregates or unions,
+ these rules apply recursively to the subaggregates or contained unions. If the initializer of
+ a subaggregate or contained union begins with a left brace, the initializers enclosed by
+ that brace and its matching right brace initialize the elements or members of the
+ subaggregate or the contained union. Otherwise, only enough initializers from the list are
+ taken to account for the elements or members of the subaggregate or the first member of
+ the contained union; any remaining initializers are left to initialize the next element or
+ member of the aggregate of which the current subaggregate or contained union is a part.
+
+21
+ If there are fewer initializers in a brace-enclosed list than there are elements or members
+ of an aggregate, or fewer characters in a string literal used to initialize an array of known
+ size than there are elements in the array, the remainder of the aggregate shall be
+ initialized implicitly the same as objects that have static storage duration.
+
+
+
+
+
+22
+ If an array of unknown size is initialized, its size is determined by the largest indexed
+ element with an explicit initializer. The array type is completed at the end of its
+ initializer list.
+
+23
+ The evaluations of the initialization list expressions are indeterminately sequenced with
+ respect to one another and thus the order in which any side effects occur is
+ unspecified.152)
+
+24
+ EXAMPLE 1 Provided that <complex.h> has been #included, the declarations
+
+
+ int i = 3.5;
+ double complex c = 5 + 3 * I;
+
+
+ define and initialize i with the value 3 and c with the value 5.0 + i3.0.
+
+
+25
+ EXAMPLE 2 The declaration
+
+
+ int x[] = { 1, 3, 5 };
+
+
+ defines and initializes x as a one-dimensional array object that has three elements, as no size was specified
+ and there are three initializers.
+
+
+26
+ EXAMPLE 3 The declaration
+
+
+ int y[4][3] = {
+ { 1, 3, 5 },
+ { 2, 4, 6 },
+ { 3, 5, 7 },
+ };
+
+
+ is a definition with a fully bracketed initialization: 1, 3, and 5 initialize the first row of y (the array object
+ y[0]), namely y[0][0], y[0][1], and y[0][2]. Likewise the next two lines initialize y[1] and
+ y[2]. The initializer ends early, so y[3] is initialized with zeros. Precisely the same effect could have
+ been achieved by
+
+
+ int y[4][3] = {
+ 1, 3, 5, 2, 4, 6, 3, 5, 7
+ };
+
+
+ The initializer for y[0] does not begin with a left brace, so three items from the list are used. Likewise the
+ next three are taken successively for y[1] and y[2].
+
+
+27
+ EXAMPLE 4 The declaration
+
+
+ int z[4][3] = {
+ { 1 }, { 2 }, { 3 }, { 4 }
+ };
+
+
+ initializes the first column of z as specified and initializes the rest with zeros.
+
+
+28
+ EXAMPLE 5 The declaration
+
+
+ struct { int a[3], b; } w[] = { { 1 }, 2 };
+
+
+ is a definition with an inconsistently bracketed initialization. It defines an array with two element
+
+
+
+
+ structures: w[0].a[0] is 1 and w[1].a[0] is 2; all the other elements are zero.
+
+
+29
+ EXAMPLE 6 The declaration
+
+
+ short q[4][3][2] = {
+ { 1 },
+ { 2, 3 },
+ { 4, 5, 6 }
+ };
+
+
+ contains an incompletely but consistently bracketed initialization. It defines a three-dimensional array
+ object: q[0][0][0] is 1, q[1][0][0] is 2, q[1][0][1] is 3, and 4, 5, and 6 initialize
+ q[2][0][0], q[2][0][1], and q[2][1][0], respectively; all the rest are zero. The initializer for
+ q[0][0] does not begin with a left brace, so up to six items from the current list may be used. There is
+ only one, so the values for the remaining five elements are initialized with zero. Likewise, the initializers
+ for q[1][0] and q[2][0] do not begin with a left brace, so each uses up to six items, initializing their
+ respective two-dimensional subaggregates. If there had been more than six items in any of the lists, a
+ diagnostic message would have been issued. The same initialization result could have been achieved by:
+
+
+ short q[4][3][2] = {
+ 1, 0, 0, 0, 0, 0,
+ 2, 3, 0, 0, 0, 0,
+ 4, 5, 6
+ };
+
+
+ or by:
+
+
+ short q[4][3][2] = {
+ {
+ { 1 },
+ },
+ {
+ { 2, 3 },
+ },
+ {
+ { 4, 5 },
+ { 6 },
+ }
+ };
+
+
+ in a fully bracketed form.
+
+30
+ Note that the fully bracketed and minimally bracketed forms of initialization are, in general, less likely to
+ cause confusion.
+
+
+31
+ EXAMPLE 7 One form of initialization that completes array types involves typedef names. Given the
+ declaration
+
+
+ typedef int A[]; // OK - declared with block scope
+
+
+ the declaration
+
+
+ A a = { 1, 2 }, b = { 3, 4, 5 };
+
+
+ is identical to
+
+
+ int a[] = { 1, 2 }, b[] = { 3, 4, 5 };
+
+
+ due to the rules for incomplete types.
+
+
+32
+ EXAMPLE 8 The declaration
+
+
+ char s[] = "abc", t[3] = "abc";
+
+
+ defines ''plain'' char array objects s and t whose elements are initialized with character string literals.
+ This declaration is identical to
+
+
+ char s[] = { 'a', 'b', 'c', '\0' },
+ t[] = { 'a', 'b', 'c' };
+
+
+ The contents of the arrays are modifiable. On the other hand, the declaration
+
+
+ char *p = "abc";
+
+
+ defines p with type ''pointer to char'' and initializes it to point to an object with type ''array of char''
+ with length 4 whose elements are initialized with a character string literal. If an attempt is made to use p to
+ modify the contents of the array, the behavior is undefined.
+
+
+33
+ EXAMPLE 9 Arrays can be initialized to correspond to the elements of an enumeration by using
+ designators:
+
+
+ enum { member_one, member_two };
+ const char *nm[] = {
+ [member_two] = "member two",
+ [member_one] = "member one",
+ };
+
+
+
+
+34
+ EXAMPLE 10 Structure members can be initialized to nonzero values without depending on their order:
+
+
+ div_t answer = { .quot = 2, .rem = -1 };
+
+
+
+
+35
+ EXAMPLE 11 Designators can be used to provide explicit initialization when unadorned initializer lists
+ might be misunderstood:
+
+
+ struct { int a[3], b; } w[] =
+ { [0].a = {1}, [1].a[0] = 2 };
+
+
+
+
+36
+ EXAMPLE 12 Space can be ''allocated'' from both ends of an array by using a single designator:
+
+
+ int a[MAX] = {
+ 1, 3, 5, 7, 9, [MAX-5] = 8, 6, 4, 2, 0
+ };
+
+
+37
+ In the above, if MAX is greater than ten, there will be some zero-valued elements in the middle; if it is less
+ than ten, some of the values provided by the first five initializers will be overridden by the second five.
+
+
+38
+ EXAMPLE 13 Any member of a union can be initialized:
+
+
+ union { /* ... */ } u = { .any_member = 42 };
+
+
+
+
+ Forward references: common definitions <stddef.h> (7.19).
+
+
+Footnotes
+
+148) If the initializer list for a subaggregate or contained union does not begin with a left brace, its
+ subobjects are initialized as usual, but the subaggregate or contained union does not become the
+ current object: current objects are associated only with brace-enclosed initializer lists.
+
+
+149) After a union member is initialized, the next object is not the next member of the union; instead, it is
+ the next subobject of an object containing the union.
+
+
+150) Thus, a designator can only specify a strict subobject of the aggregate or union that is associated with
+ the surrounding brace pair. Note, too, that each separate designator list is independent.
+
+
+151) Any initializer for the subobject which is overridden and so not used to initialize that subobject might
+ not be evaluated at all.
+
+
+152) In particular, the evaluation order need not be the same as the order of subobject initialization.
+
+
+Contents
+
+6.7.10 Static assertions
+
+
+Syntax
+
+1
+
+
+ static_assert-declaration:
+ _Static_assert ( constant-expression , string-literal ) ;
+
+
+Constraints
+
+2
+ The constant expression shall compare unequal to 0.
+
+Semantics
+
+3
+ The constant expression shall be an integer constant expression. If the value of the
+ constant expression compares unequal to 0, the declaration has no effect. Otherwise, the
+ constraint is violated and the implementation shall produce a diagnostic message that
+ includes the text of the string literal, except that characters not in the basic source
+ character set are not required to appear in the message.
+
+ Forward references: diagnostics (7.2).
+
+
+Contents
+
+6.8 Statements and blocks
+
+
+Syntax
+
+1
+
+
+ statement:
+ labeled-statement
+ compound-statement
+ expression-statement
+ selection-statement
+ iteration-statement
+ jump-statement
+
+
+Semantics
+
+2
+ A statement specifies an action to be performed. Except as indicated, statements are
+ executed in sequence.
+
+3
+ A block allows a set of declarations and statements to be grouped into one syntactic unit.
+ The initializers of objects that have automatic storage duration, and the variable length
+ array declarators of ordinary identifiers with block scope, are evaluated and the values are
+ stored in the objects (including storing an indeterminate value in objects without an
+ initializer) each time the declaration is reached in the order of execution, as if it were a
+ statement, and within each declaration in the order that declarators appear.
+
+4
+ A full expression is an expression that is not part of another expression or of a declarator.
+ Each of the following is a full expression: an initializer that is not part of a compound
+ literal; the expression in an expression statement; the controlling expression of a selection
+ statement (if or switch); the controlling expression of a while or do statement; each
+ of the (optional) expressions of a for statement; the (optional) expression in a return
+ statement. There is a sequence point between the evaluation of a full expression and the
+ evaluation of the next full expression to be evaluated.
+
+ Forward references: expression and null statements (6.8.3), selection statements
+ (6.8.4), iteration statements (6.8.5), the return statement (6.8.6.4).
+
+
+Contents
+
+6.8.1 Labeled statements
+
+
+Syntax
+
+1
+
+
+ labeled-statement:
+ identifier : statement
+ case constant-expression : statement
+ default : statement
+
+
+Constraints
+
+2
+ A case or default label shall appear only in a switch statement. Further
+ constraints on such labels are discussed under the switch statement.
+
+
+3
+ Label names shall be unique within a function.
+
+Semantics
+
+4
+ Any statement may be preceded by a prefix that declares an identifier as a label name.
+ Labels in themselves do not alter the flow of control, which continues unimpeded across
+ them.
+
+ Forward references: the goto statement (6.8.6.1), the switch statement (6.8.4.2).
+
+
+Contents
+
+6.8.2 Compound statement
+
+
+Syntax
+
+1
+
+
+ compound-statement:
+ { block-item-listopt }
+ block-item-list:
+ block-item
+ block-item-list block-item
+ block-item:
+ declaration
+ statement
+
+
+Semantics
+
+2
+ A compound statement is a block.
+
+
+Contents
+
+6.8.3 Expression and null statements
+
+
+Syntax
+
+1
+
+
+ expression-statement:
+ expressionopt ;
+
+
+Semantics
+
+2
+ The expression in an expression statement is evaluated as a void expression for its side
+ effects.153)
+
+3
+ A null statement (consisting of just a semicolon) performs no operations.
+
+4
+ EXAMPLE 1 If a function call is evaluated as an expression statement for its side effects only, the
+ discarding of its value may be made explicit by converting the expression to a void expression by means of
+ a cast:
+
+
+ int p(int);
+ /* ... */
+ (void)p(0);
+
+
+
+
+
+
+
+5
+ EXAMPLE 2 In the program fragment
+
+
+ char *s;
+ /* ... */
+ while (*s++ != '\0')
+ ;
+
+
+ a null statement is used to supply an empty loop body to the iteration statement.
+
+
+6
+ EXAMPLE 3 A null statement may also be used to carry a label just before the closing } of a compound
+ statement.
+
+
+ while (loop1) {
+ /* ... */
+ while (loop2) {
+ /* ... */
+ if (want_out)
+ goto end_loop1;
+ /* ... */
+ }
+ /* ... */
+ end_loop1: ;
+ }
+
+
+
+
+ Forward references: iteration statements (6.8.5).
+
+
+Footnotes
+
+153) Such as assignments, and function calls which have side effects.
+
+
+Contents
+
+6.8.4 Selection statements
+
+
+Syntax
+
+1
+
+
+ selection-statement:
+ if ( expression ) statement
+ if ( expression ) statement else statement
+ switch ( expression ) statement
+
+
+Semantics
+
+2
+ A selection statement selects among a set of statements depending on the value of a
+ controlling expression.
+
+3
+ A selection statement is a block whose scope is a strict subset of the scope of its
+ enclosing block. Each associated substatement is also a block whose scope is a strict
+ subset of the scope of the selection statement.
+
+
+Contents
+
+6.8.4.1 The if statement
+
+
+Constraints
+
+1
+ The controlling expression of an if statement shall have scalar type.
+
+Semantics
+
+2
+ In both forms, the first substatement is executed if the expression compares unequal to 0.
+ In the else form, the second substatement is executed if the expression compares equal
+
+ to 0. If the first substatement is reached via a label, the second substatement is not
+ executed.
+
+3
+ An else is associated with the lexically nearest preceding if that is allowed by the
+ syntax.
+
+
+Contents
+
+6.8.4.2 The switch statement
+
+
+Constraints
+
+1
+ The controlling expression of a switch statement shall have integer type.
+
+2
+ If a switch statement has an associated case or default label within the scope of an
+ identifier with a variably modified type, the entire switch statement shall be within the
+ scope of that identifier.154)
+
+3
+ The expression of each case label shall be an integer constant expression and no two of
+ the case constant expressions in the same switch statement shall have the same value
+ after conversion. There may be at most one default label in a switch statement.
+ (Any enclosed switch statement may have a default label or case constant
+ expressions with values that duplicate case constant expressions in the enclosing
+ switch statement.)
+
+Semantics
+
+4
+ A switch statement causes control to jump to, into, or past the statement that is the
+ switch body, depending on the value of a controlling expression, and on the presence of a
+ default label and the values of any case labels on or in the switch body. A case or
+ default label is accessible only within the closest enclosing switch statement.
+
+5
+ The integer promotions are performed on the controlling expression. The constant
+ expression in each case label is converted to the promoted type of the controlling
+ expression. If a converted value matches that of the promoted controlling expression,
+ control jumps to the statement following the matched case label. Otherwise, if there is
+ a default label, control jumps to the labeled statement. If no converted case constant
+ expression matches and there is no default label, no part of the switch body is
+ executed.
+
+Implementation limits
+
+6
+ As discussed in 5.2.4.1, the implementation may limit the number of case values in a
+ switch statement.
+
+
+
+
+
+
+7
+ EXAMPLE In the artificial program fragment
+
+
+ switch (expr)
+ {
+ int i = 4;
+ f(i);
+ case 0:
+ i = 17;
+ /* falls through into default code */
+ default:
+ printf("%d\n", i);
+ }
+
+
+ the object whose identifier is i exists with automatic storage duration (within the block) but is never
+ initialized, and thus if the controlling expression has a nonzero value, the call to the printf function will
+ access an indeterminate value. Similarly, the call to the function f cannot be reached.
+
+
+
+Footnotes
+
+154) That is, the declaration either precedes the switch statement, or it follows the last case or
+ default label associated with the switch that is in the block containing the declaration.
+
+
+Contents
+
+6.8.5 Iteration statements
+
+
+Syntax
+
+1
+
+
+ iteration-statement:
+ while ( expression ) statement
+ do statement while ( expression ) ;
+ for ( expressionopt ; expressionopt ; expressionopt ) statement
+ for ( declaration expressionopt ; expressionopt ) statement
+
+
+Constraints
+
+2
+ The controlling expression of an iteration statement shall have scalar type.
+
+3
+ The declaration part of a for statement shall only declare identifiers for objects having
+ storage class auto or register.
+
+Semantics
+
+4
+ An iteration statement causes a statement called the loop body to be executed repeatedly
+ until the controlling expression compares equal to 0. The repetition occurs regardless of
+ whether the loop body is entered from the iteration statement or by a jump.155)
+
+5
+ An iteration statement is a block whose scope is a strict subset of the scope of its
+ enclosing block. The loop body is also a block whose scope is a strict subset of the scope
+ of the iteration statement.
+
+6
+ An iteration statement whose controlling expression is not a constant expression,156) that
+ performs no input/output operations, does not access volatile objects, and performs no
+ synchronization or atomic operations in its body, controlling expression, or (in the case of
+
+
+ a for statement) its expression-3, may be assumed by the implementation to
+ terminate.157)
+
+
+Footnotes
+
+155) Code jumped over is not executed. In particular, the controlling expression of a for or while
+ statement is not evaluated before entering the loop body, nor is clause-1 of a for statement.
+
+
+156) An omitted controlling expression is replaced by a nonzero constant, which is a constant expression.
+
+
+157) This is intended to allow compiler transformations such as removal of empty loops even when
+ termination cannot be proven.
+
+
+Contents
+
+6.8.5.1 The while statement
+
+
+1
+ The evaluation of the controlling expression takes place before each execution of the loop
+ body.
+
+
+Contents
+
+6.8.5.2 The do statement
+
+
+1
+ The evaluation of the controlling expression takes place after each execution of the loop
+ body.
+
+
+Contents
+
+6.8.5.3 The for statement
+
+
+1
+ The statement
+
+
+ for ( clause-1 ; expression-2 ; expression-3 ) statement
+
+
+ behaves as follows: The expression expression-2 is the controlling expression that is
+ evaluated before each execution of the loop body. The expression expression-3 is
+ evaluated as a void expression after each execution of the loop body. If clause-1 is a
+ declaration, the scope of any identifiers it declares is the remainder of the declaration and
+ the entire loop, including the other two expressions; it is reached in the order of execution
+ before the first evaluation of the controlling expression. If clause-1 is an expression, it is
+ evaluated as a void expression before the first evaluation of the controlling expression.158)
+
+2
+ Both clause-1 and expression-3 can be omitted. An omitted expression-2 is replaced by a
+ nonzero constant.
+
+
+Footnotes
+
+158) Thus, clause-1 specifies initialization for the loop, possibly declaring one or more variables for use in
+ the loop; the controlling expression, expression-2, specifies an evaluation made before each iteration,
+ such that execution of the loop continues until the expression compares equal to 0; and expression-3
+ specifies an operation (such as incrementing) that is performed after each iteration.
+
+
+Contents
+
+6.8.6 Jump statements
+
+
+Syntax
+
+1
+
+
+ jump-statement:
+ goto identifier ;
+ continue ;
+ break ;
+ return expressionopt ;
+
+
+
+
+
+
+
+
+Semantics
+
+2
+ A jump statement causes an unconditional jump to another place.
+
+
+Contents
+
+6.8.6.1 The goto statement
+
+
+Constraints
+
+1
+ The identifier in a goto statement shall name a label located somewhere in the enclosing
+ function. A goto statement shall not jump from outside the scope of an identifier having
+ a variably modified type to inside the scope of that identifier.
+
+Semantics
+
+2
+ A goto statement causes an unconditional jump to the statement prefixed by the named
+ label in the enclosing function.
+
+3
+ EXAMPLE 1 It is sometimes convenient to jump into the middle of a complicated set of statements. The
+ following outline presents one possible approach to a problem based on these three assumptions:
+
+
+- The general initialization code accesses objects only visible to the current function.
+
+- The general initialization code is too large to warrant duplication.
+
+- The code to determine the next operation is at the head of the loop. (To allow it to be reached by
+ continue statements, for example.)
+
+
+ /* ... */
+ goto first_time;
+ for (;;) {
+ // determine next operation
+ /* ... */
+ if (need to reinitialize) {
+ // reinitialize-only code
+ /* ... */
+ first_time:
+ // general initialization code
+ /* ... */
+ continue;
+ }
+ // handle other operations
+ /* ... */
+ }
+
+
+4
+ EXAMPLE 2 A goto statement is not allowed to jump past any declarations of objects with variably
+ modified types. A jump within the scope, however, is permitted.
+
+
+ goto lab3; // invalid: going INTO scope of VLA.
+ {
+ double a[n];
+ a[j] = 4.4;
+ lab3:
+ a[j] = 3.3;
+ goto lab4; // valid: going WITHIN scope of VLA.
+ a[j] = 5.5;
+ lab4:
+ a[j] = 6.6;
+ }
+ goto lab4; // invalid: going INTO scope of VLA.
+
+
+
+
+
+Contents
+
+6.8.6.2 The continue statement
+
+
+Constraints
+
+1
+ A continue statement shall appear only in or as a loop body.
+
+Semantics
+
+2
+ A continue statement causes a jump to the loop-continuation portion of the smallest
+ enclosing iteration statement; that is, to the end of the loop body. More precisely, in each
+ of the statements
+
+
+ while (/* ... */) { do { for (/* ... */) {
+ /* ... */ /* ... */ /* ... */
+ continue; continue; continue;
+ /* ... */ /* ... */ /* ... */
+ contin: ; contin: ; contin: ;
+ } } while (/* ... */); }
+
+
+ unless the continue statement shown is in an enclosed iteration statement (in which
+ case it is interpreted within that statement), it is equivalent to goto contin;.159)
+
+
+Footnotes
+
+159) Following the contin: label is a null statement.
+
+
+Contents
+
+6.8.6.3 The break statement
+
+
+Constraints
+
+1
+ A break statement shall appear only in or as a switch body or loop body.
+
+Semantics
+
+2
+ A break statement terminates execution of the smallest enclosing switch or iteration
+ statement.
+
+
+
+
+
+Contents
+
+6.8.6.4 The return statement
+
+
+Constraints
+
+1
+ A return statement with an expression shall not appear in a function whose return type
+ is void. A return statement without an expression shall only appear in a function
+ whose return type is void.
+
+Semantics
+
+2
+ A return statement terminates execution of the current function and returns control to
+ its caller. A function may have any number of return statements.
+
+3
+ If a return statement with an expression is executed, the value of the expression is
+ returned to the caller as the value of the function call expression. If the expression has a
+ type different from the return type of the function in which it appears, the value is
+ converted as if by assignment to an object having the return type of the function.160)
+
+4
+ EXAMPLE In:
+
+
+ struct s { double i; } f(void);
+ union {
+ struct {
+ int f1;
+ struct s f2;
+ } u1;
+ struct {
+ struct s f3;
+ int f4;
+ } u2;
+ } g;
+ struct s f(void)
+ {
+ return g.u1.f2;
+ }
+ /* ... */
+ g.u2.f3 = f();
+
+
+ there is no undefined behavior, although there would be if the assignment were done directly (without using
+ a function call to fetch the value).
+
+
+
+
+
+
+Footnotes
+
+160) The return statement is not an assignment. The overlap restriction of subclause 6.5.16.1 does not
+ apply to the case of function return. The representation of floating-point values may have wider range
+ or precision than implied by the type; a cast may be used to remove this extra range and precision.
+
+
+Contents
+
+6.9 External definitions
+
+
+Syntax
+
+1
+
+
+ translation-unit:
+ external-declaration
+ translation-unit external-declaration
+ external-declaration:
+ function-definition
+ declaration
+
+
+Constraints
+
+2
+ The storage-class specifiers auto and register shall not appear in the declaration
+ specifiers in an external declaration.
+
+3
+ There shall be no more than one external definition for each identifier declared with
+ internal linkage in a translation unit. Moreover, if an identifier declared with internal
+ linkage is used in an expression (other than as a part of the operand of a sizeof or
+ _Alignof operator whose result is an integer constant), there shall be exactly one
+ external definition for the identifier in the translation unit.
+
+Semantics
+
+4
+ As discussed in 5.1.1.1, the unit of program text after preprocessing is a translation unit,
+ which consists of a sequence of external declarations. These are described as ''external''
+ because they appear outside any function (and hence have file scope). As discussed in
+ 6.7, a declaration that also causes storage to be reserved for an object or a function named
+ by the identifier is a definition.
+
+5
+ An external definition is an external declaration that is also a definition of a function
+ (other than an inline definition) or an object. If an identifier declared with external
+ linkage is used in an expression (other than as part of the operand of a sizeof or
+ _Alignof operator whose result is an integer constant), somewhere in the entire
+ program there shall be exactly one external definition for the identifier; otherwise, there
+ shall be no more than one.161)
+
+
+
+
+
+
+Footnotes
+
+161) Thus, if an identifier declared with external linkage is not used in an expression, there need be no
+ external definition for it.
+
+
+Contents
+
+6.9.1 Function definitions
+
+
+Syntax
+
+1
+
+
+ function-definition:
+ declaration-specifiers declarator declaration-listopt compound-statement
+ declaration-list:
+ declaration
+ declaration-list declaration
+
+
+Constraints
+
+2
+ The identifier declared in a function definition (which is the name of the function) shall
+ have a function type, as specified by the declarator portion of the function definition.162)
+
+3
+ The return type of a function shall be void or a complete object type other than array
+ type.
+
+4
+ The storage-class specifier, if any, in the declaration specifiers shall be either extern or
+ static.
+
+5
+ If the declarator includes a parameter type list, the declaration of each parameter shall
+ include an identifier, except for the special case of a parameter list consisting of a single
+ parameter of type void, in which case there shall not be an identifier. No declaration list
+ shall follow.
+
+6
+ If the declarator includes an identifier list, each declaration in the declaration list shall
+ have at least one declarator, those declarators shall declare only identifiers from the
+ identifier list, and every identifier in the identifier list shall be declared. An identifier
+ declared as a typedef name shall not be redeclared as a parameter. The declarations in the
+ declaration list shall contain no storage-class specifier other than register and no
+ initializations.
+
+
+
+
+
+Semantics
+
+7
+ The declarator in a function definition specifies the name of the function being defined
+ and the identifiers of its parameters. If the declarator includes a parameter type list, the
+ list also specifies the types of all the parameters; such a declarator also serves as a
+ function prototype for later calls to the same function in the same translation unit. If the
+ declarator includes an identifier list,163) the types of the parameters shall be declared in a
+ following declaration list. In either case, the type of each parameter is adjusted as
+ described in 6.7.6.3 for a parameter type list; the resulting type shall be a complete object
+ type.
+
+8
+ If a function that accepts a variable number of arguments is defined without a parameter
+ type list that ends with the ellipsis notation, the behavior is undefined.
+
+9
+ Each parameter has automatic storage duration; its identifier is an lvalue.164) The layout
+ of the storage for parameters is unspecified.
+
+10
+ On entry to the function, the size expressions of each variably modified parameter are
+ evaluated and the value of each argument expression is converted to the type of the
+ corresponding parameter as if by assignment. (Array expressions and function
+ designators as arguments were converted to pointers before the call.)
+
+11
+ After all parameters have been assigned, the compound statement that constitutes the
+ body of the function definition is executed.
+
+12
+ If the } that terminates a function is reached, and the value of the function call is used by
+ the caller, the behavior is undefined.
+
+13
+ EXAMPLE 1 In the following:
+
+
+ extern int max(int a, int b)
+ {
+ return a > b ? a : b;
+ }
+
+
+ extern is the storage-class specifier and int is the type specifier; max(int a, int b) is the
+ function declarator; and
+
+
+ { return a > b ? a : b; }
+
+
+ is the function body. The following similar definition uses the identifier-list form for the parameter
+ declarations:
+
+
+
+
+
+
+ extern int max(a, b)
+ int a, b;
+ {
+ return a > b ? a : b;
+ }
+
+
+ Here int a, b; is the declaration list for the parameters. The difference between these two definitions is
+ that the first form acts as a prototype declaration that forces conversion of the arguments of subsequent calls
+ to the function, whereas the second form does not.
+
+
+14
+ EXAMPLE 2 To pass one function to another, one might say
+
+
+ int f(void);
+ /* ... */
+ g(f);
+
+
+ Then the definition of g might read
+
+
+ void g(int (*funcp)(void))
+ {
+ /* ... */
+ (*funcp)(); /* or funcp(); ... */
+ }
+
+
+ or, equivalently,
+
+
+ void g(int func(void))
+ {
+ /* ... */
+ func(); /* or (*func)(); ... */
+ }
+
+
+
+
+
+Footnotes
+
+162) The intent is that the type category in a function definition cannot be inherited from a typedef:
+
+
+ typedef int F(void); // type F is ''function with no parameters
+ // returning int''
+ F f, g; // f and g both have type compatible with F
+ F f { /* ... */ } // WRONG: syntax/constraint error
+ F g() { /* ... */ } // WRONG: declares that g returns a function
+ int f(void) { /* ... */ } // RIGHT: f has type compatible with F
+ int g() { /* ... */ } // RIGHT: g has type compatible with F
+ F *e(void) { /* ... */ } // e returns a pointer to a function
+ F *((e))(void) { /* ... */ } // same: parentheses irrelevant
+ int (*fp)(void); // fp points to a function that has type F
+ F *Fp; // Fp points to a function that has type F
+
+
+163) See ''future language directions'' (6.11.7).
+
+
+164) A parameter identifier cannot be redeclared in the function body except in an enclosed block.
+
+
+Contents
+
+6.9.2 External object definitions
+
+
+Semantics
+
+1
+ If the declaration of an identifier for an object has file scope and an initializer, the
+ declaration is an external definition for the identifier.
+
+2
+ A declaration of an identifier for an object that has file scope without an initializer, and
+ without a storage-class specifier or with the storage-class specifier static, constitutes a
+ tentative definition. If a translation unit contains one or more tentative definitions for an
+ identifier, and the translation unit contains no external definition for that identifier, then
+ the behavior is exactly as if the translation unit contains a file scope declaration of that
+ identifier, with the composite type as of the end of the translation unit, with an initializer
+ equal to 0.
+
+3
+ If the declaration of an identifier for an object is a tentative definition and has internal
+ linkage, the declared type shall not be an incomplete type.
+
+
+4
+ EXAMPLE 1
+
+
+ int i1 = 1; // definition, external linkage
+ static int i2 = 2; // definition, internal linkage
+ extern int i3 = 3; // definition, external linkage
+ int i4; // tentative definition, external linkage
+ static int i5; // tentative definition, internal linkage
+ int i1; // valid tentative definition, refers to previous
+ int i2; // 6.2.2 renders undefined, linkage disagreement
+ int i3; // valid tentative definition, refers to previous
+ int i4; // valid tentative definition, refers to previous
+ int i5; // 6.2.2 renders undefined, linkage disagreement
+ extern int i1; // refers to previous, whose linkage is external
+ extern int i2; // refers to previous, whose linkage is internal
+ extern int i3; // refers to previous, whose linkage is external
+ extern int i4; // refers to previous, whose linkage is external
+ extern int i5; // refers to previous, whose linkage is internal
+
+
+
+
+5
+ EXAMPLE 2 If at the end of the translation unit containing
+
+
+ int i[];
+
+
+ the array i still has incomplete type, the implicit initializer causes it to have one element, which is set to
+ zero on program startup.
+
+
+Contents
+
+6.10 Preprocessing directives
+
+
+Syntax
+
+1
+
+
+ preprocessing-file:
+ groupopt
+ group:
+ group-part
+ group group-part
+ group-part:
+ if-section
+ control-line
+ text-line
+ # non-directive
+ if-section:
+ if-group elif-groupsopt else-groupopt endif-line
+ if-group:
+ # if constant-expression new-line groupopt
+ # ifdef identifier new-line groupopt
+ # ifndef identifier new-line groupopt
+ elif-groups:
+ elif-group
+ elif-groups elif-group
+ elif-group:
+ # elif constant-expression new-line groupopt
+ else-group:
+ # else new-line groupopt
+ endif-line:
+ # endif new-line
+ control-line:
+ # include pp-tokens new-line
+ # define identifier replacement-list new-line
+ # define identifier lparen identifier-listopt )
+ replacement-list new-line
+ # define identifier lparen ... ) replacement-list new-line
+ # define identifier lparen identifier-list , ... )
+ replacement-list new-line
+ # undef identifier new-line
+ # line pp-tokens new-line
+ # error pp-tokensopt new-line
+ # pragma pp-tokensopt new-line
+ # new-line
+ text-line:
+ pp-tokensopt new-line
+ non-directive:
+ pp-tokens new-line
+ lparen:
+ a ( character not immediately preceded by white-space
+ replacement-list:
+ pp-tokensopt
+ pp-tokens:
+ preprocessing-token
+ pp-tokens preprocessing-token
+ new-line:
+ the new-line character
+
+
+Description
+
+2
+ A preprocessing directive consists of a sequence of preprocessing tokens that satisfies the
+ following constraints: The first token in the sequence is a # preprocessing token that (at
+ the start of translation phase 4) is either the first character in the source file (optionally
+ after white space containing no new-line characters) or that follows white space
+ containing at least one new-line character. The last token in the sequence is the first new-
+ line character that follows the first token in the sequence.165) A new-line character ends
+ the preprocessing directive even if it occurs within what would otherwise be an
+
+
+ invocation of a function-like macro.
+
+3
+ A text line shall not begin with a # preprocessing token. A non-directive shall not begin
+ with any of the directive names appearing in the syntax.
+
+4
+ When in a group that is skipped (6.10.1), the directive syntax is relaxed to allow any
+ sequence of preprocessing tokens to occur between the directive name and the following
+ new-line character.
+
+Constraints
+
+5
+ The only white-space characters that shall appear between preprocessing tokens within a
+ preprocessing directive (from just after the introducing # preprocessing token through
+ just before the terminating new-line character) are space and horizontal-tab (including
+ spaces that have replaced comments or possibly other white-space characters in
+ translation phase 3).
+
+Semantics
+
+6
+ The implementation can process and skip sections of source files conditionally, include
+ other source files, and replace macros. These capabilities are called preprocessing,
+ because conceptually they occur before translation of the resulting translation unit.
+
+7
+ The preprocessing tokens within a preprocessing directive are not subject to macro
+ expansion unless otherwise stated.
+
+8
+ EXAMPLE In:
+
+
+ #define EMPTY
+ EMPTY # include <file.h>
+
+
+ the sequence of preprocessing tokens on the second line is not a preprocessing directive, because it does not
+ begin with a # at the start of translation phase 4, even though it will do so after the macro EMPTY has been
+ replaced.
+
+
+
+Footnotes
+
+165) Thus, preprocessing directives are commonly called ''lines''. These ''lines'' have no other syntactic
+ significance, as all white space is equivalent except in certain situations during preprocessing (see the
+ # character string literal creation operator in 6.10.3.2, for example).
+
+
+Contents
+
+6.10.1 Conditional inclusion
+
+
+Constraints
+
+1
+ The expression that controls conditional inclusion shall be an integer constant expression
+ except that: identifiers (including those lexically identical to keywords) are interpreted as
+ described below;166) and it may contain unary operator expressions of the form
+
+
+ defined identifier
+
+
+ or
+
+
+ defined ( identifier )
+
+
+ which evaluate to 1 if the identifier is currently defined as a macro name (that is, if it is
+
+
+
+ predefined or if it has been the subject of a #define preprocessing directive without an
+ intervening #undef directive with the same subject identifier), 0 if it is not.
+
+2
+ Each preprocessing token that remains (in the list of preprocessing tokens that will
+ become the controlling expression) after all macro replacements have occurred shall be in
+ the lexical form of a token (6.4).
+
+Semantics
+
+3
+ Preprocessing directives of the forms
+
+
+ # if constant-expression new-line groupopt
+ # elif constant-expression new-line groupopt
+
+
+ check whether the controlling constant expression evaluates to nonzero.
+
+4
+ Prior to evaluation, macro invocations in the list of preprocessing tokens that will become
+ the controlling constant expression are replaced (except for those macro names modified
+ by the defined unary operator), just as in normal text. If the token defined is
+ generated as a result of this replacement process or use of the defined unary operator
+ does not match one of the two specified forms prior to macro replacement, the behavior is
+ undefined. After all replacements due to macro expansion and the defined unary
+ operator have been performed, all remaining identifiers (including those lexically
+ identical to keywords) are replaced with the pp-number 0, and then each preprocessing
+ token is converted into a token. The resulting tokens compose the controlling constant
+ expression which is evaluated according to the rules of 6.6. For the purposes of this
+ token conversion and evaluation, all signed integer types and all unsigned integer types
+ act as if they have the same representation as, respectively, the types intmax_t and
+ uintmax_t defined in the header <stdint.h>.167) This includes interpreting
+ character constants, which may involve converting escape sequences into execution
+ character set members. Whether the numeric value for these character constants matches
+ the value obtained when an identical character constant occurs in an expression (other
+ than within a #if or #elif directive) is implementation-defined.168) Also, whether a
+ single-character character constant may have a negative value is implementation-defined.
+
+
+
+
+
+
+5
+ Preprocessing directives of the forms
+
+
+ # ifdef identifier new-line groupopt
+ # ifndef identifier new-line groupopt
+
+
+ check whether the identifier is or is not currently defined as a macro name. Their
+ conditions are equivalent to #if defined identifier and #if !defined identifier
+ respectively.
+
+6
+ Each directive's condition is checked in order. If it evaluates to false (zero), the group
+ that it controls is skipped: directives are processed only through the name that determines
+ the directive in order to keep track of the level of nested conditionals; the rest of the
+ directives' preprocessing tokens are ignored, as are the other preprocessing tokens in the
+ group. Only the first group whose control condition evaluates to true (nonzero) is
+ processed. If none of the conditions evaluates to true, and there is a #else directive, the
+ group controlled by the #else is processed; lacking a #else directive, all the groups
+ until the #endif are skipped.169)
+
+ Forward references: macro replacement (6.10.3), source file inclusion (6.10.2), largest
+ integer types (7.20.1.5).
+
+
+Footnotes
+
+166) Because the controlling constant expression is evaluated during translation phase 4, all identifiers
+ either are or are not macro names -- there simply are no keywords, enumeration constants, etc.
+
+
+167) Thus, on an implementation where INT_MAX is 0x7FFF and UINT_MAX is 0xFFFF, the constant
+ 0x8000 is signed and positive within a #if expression even though it would be unsigned in
+ translation phase 7.
+
+
+168) Thus, the constant expression in the following #if directive and if statement is not guaranteed to
+ evaluate to the same value in these two contexts.
+
+
+ #if 'z' - 'a' == 25
+ if ('z' - 'a' == 25)
+
+
+169) As indicated by the syntax, a preprocessing token shall not follow a #else or #endif directive
+ before the terminating new-line character. However, comments may appear anywhere in a source file,
+ including within a preprocessing directive.
+
+
+Contents
+
+6.10.2 Source file inclusion
+
+
+Constraints
+
+1
+ A #include directive shall identify a header or source file that can be processed by the
+ implementation.
+
+Semantics
+
+2
+ A preprocessing directive of the form
+
+
+ # include <h-char-sequence> new-line
+
+
+ searches a sequence of implementation-defined places for a header identified uniquely by
+ the specified sequence between the < and > delimiters, and causes the replacement of that
+ directive by the entire contents of the header. How the places are specified or the header
+ identified is implementation-defined.
+
+3
+ A preprocessing directive of the form
+
+
+ # include "q-char-sequence" new-line
+
+
+ causes the replacement of that directive by the entire contents of the source file identified
+ by the specified sequence between the " delimiters. The named source file is searched
+
+
+
+ for in an implementation-defined manner. If this search is not supported, or if the search
+ fails, the directive is reprocessed as if it read
+
+
+ # include <h-char-sequence> new-line
+
+
+ with the identical contained sequence (including > characters, if any) from the original
+ directive.
+
+4
+ A preprocessing directive of the form
+
+
+ # include pp-tokens new-line
+
+
+ (that does not match one of the two previous forms) is permitted. The preprocessing
+ tokens after include in the directive are processed just as in normal text. (Each
+ identifier currently defined as a macro name is replaced by its replacement list of
+ preprocessing tokens.) The directive resulting after all replacements shall match one of
+ the two previous forms.170) The method by which a sequence of preprocessing tokens
+ between a < and a > preprocessing token pair or a pair of " characters is combined into a
+ single header name preprocessing token is implementation-defined.
+
+5
+ The implementation shall provide unique mappings for sequences consisting of one or
+ more nondigits or digits (6.4.2.1) followed by a period (.) and a single nondigit. The
+ first character shall not be a digit. The implementation may ignore distinctions of
+ alphabetical case and restrict the mapping to eight significant characters before the
+ period.
+
+6
+ A #include preprocessing directive may appear in a source file that has been read
+ because of a #include directive in another file, up to an implementation-defined
+ nesting limit (see 5.2.4.1).
+
+7
+ EXAMPLE 1 The most common uses of #include preprocessing directives are as in the following:
+
+
+ #include <stdio.h>
+ #include "myprog.h"
+
+
+
+
+
+
+
+
+8
+ EXAMPLE 2 This illustrates macro-replaced #include directives:
+
+
+ #if VERSION == 1
+ #define INCFILE "vers1.h"
+ #elif VERSION == 2
+ #define INCFILE "vers2.h" // and so on
+ #else
+ #define INCFILE "versN.h"
+ #endif
+ #include INCFILE
+
+
+
+
+ Forward references: macro replacement (6.10.3).
+
+
+Footnotes
+
+170) Note that adjacent string literals are not concatenated into a single string literal (see the translation
+ phases in 5.1.1.2); thus, an expansion that results in two string literals is an invalid directive.
+
+
+Contents
+
+6.10.3 Macro replacement
+
+
+Constraints
+
+1
+ Two replacement lists are identical if and only if the preprocessing tokens in both have
+ the same number, ordering, spelling, and white-space separation, where all white-space
+ separations are considered identical.
+
+2
+ An identifier currently defined as an object-like macro shall not be redefined by another
+ #define preprocessing directive unless the second definition is an object-like macro
+ definition and the two replacement lists are identical. Likewise, an identifier currently
+ defined as a function-like macro shall not be redefined by another #define
+ preprocessing directive unless the second definition is a function-like macro definition
+ that has the same number and spelling of parameters, and the two replacement lists are
+ identical.
+
+3
+ There shall be white-space between the identifier and the replacement list in the definition
+ of an object-like macro.
+
+4
+ If the identifier-list in the macro definition does not end with an ellipsis, the number of
+ arguments (including those arguments consisting of no preprocessing tokens) in an
+ invocation of a function-like macro shall equal the number of parameters in the macro
+ definition. Otherwise, there shall be more arguments in the invocation than there are
+ parameters in the macro definition (excluding the ...). There shall exist a )
+ preprocessing token that terminates the invocation.
+
+5
+ The identifier __VA_ARGS__ shall occur only in the replacement-list of a function-like
+ macro that uses the ellipsis notation in the parameters.
+
+6
+ A parameter identifier in a function-like macro shall be uniquely declared within its
+ scope.
+
+Semantics
+
+7
+ The identifier immediately following the define is called the macro name. There is one
+ name space for macro names. Any white-space characters preceding or following the
+ replacement list of preprocessing tokens are not considered part of the replacement list
+
+ for either form of macro.
+
+8
+ If a # preprocessing token, followed by an identifier, occurs lexically at the point at which
+ a preprocessing directive could begin, the identifier is not subject to macro replacement.
+
+9
+ A preprocessing directive of the form
+
+
+ # define identifier replacement-list new-line
+
+
+ defines an object-like macro that causes each subsequent instance of the macro name171)
+ to be replaced by the replacement list of preprocessing tokens that constitute the
+ remainder of the directive. The replacement list is then rescanned for more macro names
+ as specified below.
+
+10
+ A preprocessing directive of the form
+
+
+ # define identifier lparen identifier-listopt ) replacement-list new-line
+ # define identifier lparen ... ) replacement-list new-line
+ # define identifier lparen identifier-list , ... ) replacement-list new-line
+
+
+ defines a function-like macro with parameters, whose use is similar syntactically to a
+ function call. The parameters are specified by the optional list of identifiers, whose scope
+ extends from their declaration in the identifier list until the new-line character that
+ terminates the #define preprocessing directive. Each subsequent instance of the
+ function-like macro name followed by a ( as the next preprocessing token introduces the
+ sequence of preprocessing tokens that is replaced by the replacement list in the definition
+ (an invocation of the macro). The replaced sequence of preprocessing tokens is
+ terminated by the matching ) preprocessing token, skipping intervening matched pairs of
+ left and right parenthesis preprocessing tokens. Within the sequence of preprocessing
+ tokens making up an invocation of a function-like macro, new-line is considered a normal
+ white-space character.
+
+11
+ The sequence of preprocessing tokens bounded by the outside-most matching parentheses
+ forms the list of arguments for the function-like macro. The individual arguments within
+ the list are separated by comma preprocessing tokens, but comma preprocessing tokens
+ between matching inner parentheses do not separate arguments. If there are sequences of
+ preprocessing tokens within the list of arguments that would otherwise act as
+ preprocessing directives,172) the behavior is undefined.
+
+12
+ If there is a ... in the identifier-list in the macro definition, then the trailing arguments,
+ including any separating comma preprocessing tokens, are merged to form a single item:
+
+
+
+ the variable arguments. The number of arguments so combined is such that, following
+ merger, the number of arguments is one more than the number of parameters in the macro
+ definition (excluding the ...).
+
+
+Footnotes
+
+171) Since, by macro-replacement time, all character constants and string literals are preprocessing tokens,
+ not sequences possibly containing identifier-like subsequences (see 5.1.1.2, translation phases), they
+ are never scanned for macro names or parameters.
+
+
+172) Despite the name, a non-directive is a preprocessing directive.
+
+
+Contents
+
+6.10.3.1 Argument substitution
+
+
+1
+ After the arguments for the invocation of a function-like macro have been identified,
+ argument substitution takes place. A parameter in the replacement list, unless preceded
+ by a # or ## preprocessing token or followed by a ## preprocessing token (see below), is
+ replaced by the corresponding argument after all macros contained therein have been
+ expanded. Before being substituted, each argument's preprocessing tokens are
+ completely macro replaced as if they formed the rest of the preprocessing file; no other
+ preprocessing tokens are available.
+
+2
+ An identifier __VA_ARGS__ that occurs in the replacement list shall be treated as if it
+ were a parameter, and the variable arguments shall form the preprocessing tokens used to
+ replace it.
+
+
+Contents
+
+6.10.3.2 The # operator
+
+
+Constraints
+
+1
+ Each # preprocessing token in the replacement list for a function-like macro shall be
+ followed by a parameter as the next preprocessing token in the replacement list.
+
+Semantics
+
+2
+ If, in the replacement list, a parameter is immediately preceded by a # preprocessing
+ token, both are replaced by a single character string literal preprocessing token that
+ contains the spelling of the preprocessing token sequence for the corresponding
+ argument. Each occurrence of white space between the argument's preprocessing tokens
+ becomes a single space character in the character string literal. White space before the
+ first preprocessing token and after the last preprocessing token composing the argument
+ is deleted. Otherwise, the original spelling of each preprocessing token in the argument
+ is retained in the character string literal, except for special handling for producing the
+ spelling of string literals and character constants: a \ character is inserted before each "
+ and \ character of a character constant or string literal (including the delimiting "
+ characters), except that it is implementation-defined whether a \ character is inserted
+ before the \ character beginning a universal character name. If the replacement that
+ results is not a valid character string literal, the behavior is undefined. The character
+ string literal corresponding to an empty argument is "". The order of evaluation of # and
+ ## operators is unspecified.
+
+
+Contents
+
+6.10.3.3 The ## operator
+
+
+Constraints
+
+1
+ A ## preprocessing token shall not occur at the beginning or at the end of a replacement
+ list for either form of macro definition.
+
+Semantics
+
+2
+ If, in the replacement list of a function-like macro, a parameter is immediately preceded
+ or followed by a ## preprocessing token, the parameter is replaced by the corresponding
+ argument's preprocessing token sequence; however, if an argument consists of no
+ preprocessing tokens, the parameter is replaced by a placemarker preprocessing token
+ instead.173)
+
+3
+ For both object-like and function-like macro invocations, before the replacement list is
+ reexamined for more macro names to replace, each instance of a ## preprocessing token
+ in the replacement list (not from an argument) is deleted and the preceding preprocessing
+ token is concatenated with the following preprocessing token. Placemarker
+ preprocessing tokens are handled specially: concatenation of two placemarkers results in
+ a single placemarker preprocessing token, and concatenation of a placemarker with a
+ non-placemarker preprocessing token results in the non-placemarker preprocessing token.
+ If the result is not a valid preprocessing token, the behavior is undefined. The resulting
+ token is available for further macro replacement. The order of evaluation of ## operators
+ is unspecified.
+
+4
+ EXAMPLE In the following fragment:
+
+
+ #define hash_hash # ## #
+ #define mkstr(a) # a
+ #define in_between(a) mkstr(a)
+ #define join(c, d) in_between(c hash_hash d)
+ char p[] = join(x, y); // equivalent to
+ // char p[] = "x ## y";
+
+
+ The expansion produces, at various stages:
+
+
+ join(x, y)
+ in_between(x hash_hash y)
+ in_between(x ## y)
+ mkstr(x ## y)
+ "x ## y"
+
+
+ In other words, expanding hash_hash produces a new token, consisting of two adjacent sharp signs, but
+ this new token is not the ## operator.
+
+
+
+
+Footnotes
+
+173) Placemarker preprocessing tokens do not appear in the syntax because they are temporary entities that
+ exist only within translation phase 4.
+
+
+Contents
+
+6.10.3.4 Rescanning and further replacement
+
+
+1
+ After all parameters in the replacement list have been substituted and # and ##
+ processing has taken place, all placemarker preprocessing tokens are removed. The
+ resulting preprocessing token sequence is then rescanned, along with all subsequent
+ preprocessing tokens of the source file, for more macro names to replace.
+
+2
+ If the name of the macro being replaced is found during this scan of the replacement list
+ (not including the rest of the source file's preprocessing tokens), it is not replaced.
+ Furthermore, if any nested replacements encounter the name of the macro being replaced,
+ it is not replaced. These nonreplaced macro name preprocessing tokens are no longer
+ available for further replacement even if they are later (re)examined in contexts in which
+ that macro name preprocessing token would otherwise have been replaced.
+
+3
+ The resulting completely macro-replaced preprocessing token sequence is not processed
+ as a preprocessing directive even if it resembles one, but all pragma unary operator
+ expressions within it are then processed as specified in 6.10.9 below.
+
+4
+ EXAMPLE There are cases where it is not clear whether a replacement is nested or not. For example,
+ given the following macro definitions:
+
+
+ #define f(a) a*g
+ #define g(a) f(a)
+
+
+ the invocation
+
+
+ f(2)(9)
+
+
+ may expand to either
+
+
+ 2*f(9)
+
+
+ or
+
+
+ 2*9*g
+
+
+ Strictly conforming programs are not permitted to depend on such unspecified behavior.
+
+
+
+Contents
+
+6.10.3.5 Scope of macro definitions
+
+
+1
+ A macro definition lasts (independent of block structure) until a corresponding #undef
+ directive is encountered or (if none is encountered) until the end of the preprocessing
+ translation unit. Macro definitions have no significance after translation phase 4.
+
+2
+ A preprocessing directive of the form
+
+
+ # undef identifier new-line
+
+
+ causes the specified identifier no longer to be defined as a macro name. It is ignored if
+ the specified identifier is not currently defined as a macro name.
+
+3
+ EXAMPLE 1 The simplest use of this facility is to define a ''manifest constant'', as in
+
+
+ #define TABSIZE 100
+ int table[TABSIZE];
+
+
+
+
+4
+ EXAMPLE 2 The following defines a function-like macro whose value is the maximum of its arguments.
+ It has the advantages of working for any compatible types of the arguments and of generating in-line code
+ without the overhead of function calling. It has the disadvantages of evaluating one or the other of its
+ arguments a second time (including side effects) and generating more code than a function if invoked
+ several times. It also cannot have its address taken, as it has none.
+
+
+ #define max(a, b) ((a) > (b) ? (a) : (b))
+
+
+ The parentheses ensure that the arguments and the resulting expression are bound properly.
+
+
+5
+ EXAMPLE 3 To illustrate the rules for redefinition and reexamination, the sequence
+
+
+ #define x 3
+ #define f(a) f(x * (a))
+ #undef x
+ #define x 2
+ #define g f
+ #define z z[0]
+ #define h g(~
+ #define m(a) a(w)
+ #define w 0,1
+ #define t(a) a
+ #define p() int
+ #define q(x) x
+ #define r(x,y) x ## y
+ #define str(x) # x
+ f(y+1) + f(f(z)) % t(t(g)(0) + t)(1);
+ g(x+(3,4)-w) | h 5) & m
+ (f)^m(m);
+ p() i[q()] = { q(1), r(2,3), r(4,), r(,5), r(,) };
+ char c[2][6] = { str(hello), str() };
+
+
+ results in
+
+
+ f(2 * (y+1)) + f(2 * (f(2 * (z[0])))) % f(2 * (0)) + t(1);
+ f(2 * (2+(3,4)-0,1)) | f(2 * (~ 5)) & f(2 * (0,1))^m(0,1);
+ int i[] = { 1, 23, 4, 5, };
+ char c[2][6] = { "hello", "" };
+
+
+
+
+6
+ EXAMPLE 4 To illustrate the rules for creating character string literals and concatenating tokens, the
+ sequence
+
+
+ #define str(s) # s
+ #define xstr(s) str(s)
+ #define debug(s, t) printf("x" # s "= %d, x" # t "= %s", \
+ x ## s, x ## t)
+ #define INCFILE(n) vers ## n
+ #define glue(a, b) a ## b
+ #define xglue(a, b) glue(a, b)
+ #define HIGHLOW "hello"
+ #define LOW LOW ", world"
+ debug(1, 2);
+ fputs(str(strncmp("abc\0d", "abc", '\4') // this goes away
+ == 0) str(: @\n), s);
+ #include xstr(INCFILE(2).h)
+ glue(HIGH, LOW);
+ xglue(HIGH, LOW)
+
+
+ results in
+
+
+ printf("x" "1" "= %d, x" "2" "= %s", x1, x2);
+ fputs(
+ "strncmp(\"abc\\0d\", \"abc\", '\\4') == 0" ": @\n",
+ s);
+ #include "vers2.h" (after macro replacement, before file access)
+ "hello";
+ "hello" ", world"
+
+
+ or, after concatenation of the character string literals,
+
+
+ printf("x1= %d, x2= %s", x1, x2);
+ fputs(
+ "strncmp(\"abc\\0d\", \"abc\", '\\4') == 0: @\n",
+ s);
+ #include "vers2.h" (after macro replacement, before file access)
+ "hello";
+ "hello, world"
+
+
+ Space around the # and ## tokens in the macro definition is optional.
+
+
+7
+ EXAMPLE 5 To illustrate the rules for placemarker preprocessing tokens, the sequence
+
+
+ #define t(x,y,z) x ## y ## z
+ int j[] = { t(1,2,3), t(,4,5), t(6,,7), t(8,9,),
+ t(10,,), t(,11,), t(,,12), t(,,) };
+
+
+ results in
+
+
+ int j[] = { 123, 45, 67, 89,
+ 10, 11, 12, };
+
+
+
+
+8
+ EXAMPLE 6 To demonstrate the redefinition rules, the following sequence is valid.
+
+
+ #define OBJ_LIKE (1-1)
+ #define OBJ_LIKE /* white space */ (1-1) /* other */
+ #define FUNC_LIKE(a) ( a )
+ #define FUNC_LIKE( a )( /* note the white space */ \
+ a /* other stuff on this line
+ */ )
+
+
+ But the following redefinitions are invalid:
+
+
+ #define OBJ_LIKE (0) // different token sequence
+ #define OBJ_LIKE (1 - 1) // different white space
+ #define FUNC_LIKE(b) ( a ) // different parameter usage
+ #define FUNC_LIKE(b) ( b ) // different parameter spelling
+
+
+
+
+9
+ EXAMPLE 7 Finally, to show the variable argument list macro facilities:
+
+
+ #define debug(...) fprintf(stderr, __VA_ARGS__)
+ #define showlist(...) puts(#__VA_ARGS__)
+ #define report(test, ...) ((test)?puts(#test):\
+ printf(__VA_ARGS__))
+ debug("Flag");
+ debug("X = %d\n", x);
+ showlist(The first, second, and third items.);
+ report(x>y, "x is %d but y is %d", x, y);
+
+
+ results in
+
+
+ fprintf(stderr, "Flag" );
+ fprintf(stderr, "X = %d\n", x );
+ puts( "The first, second, and third items." );
+ ((x>y)?puts("x>y"):
+ printf("x is %d but y is %d", x, y));
+
+
+
+
+
+Contents
+
+6.10.4 Line control
+
+
+Constraints
+
+1
+ The string literal of a #line directive, if present, shall be a character string literal.
+
+Semantics
+
+2
+ The line number of the current source line is one greater than the number of new-line
+ characters read or introduced in translation phase 1 (5.1.1.2) while processing the source
+ file to the current token.
+
+3
+ A preprocessing directive of the form
+
+
+ # line digit-sequence new-line
+
+
+ causes the implementation to behave as if the following sequence of source lines begins
+ with a source line that has a line number as specified by the digit sequence (interpreted as
+ a decimal integer). The digit sequence shall not specify zero, nor a number greater than
+ 2147483647.
+
+4
+ A preprocessing directive of the form
+
+
+ # line digit-sequence "s-char-sequenceopt" new-line
+
+
+ sets the presumed line number similarly and changes the presumed name of the source
+ file to be the contents of the character string literal.
+
+5
+ A preprocessing directive of the form
+
+
+ # line pp-tokens new-line
+
+
+ (that does not match one of the two previous forms) is permitted. The preprocessing
+ tokens after line on the directive are processed just as in normal text (each identifier
+ currently defined as a macro name is replaced by its replacement list of preprocessing
+ tokens). The directive resulting after all replacements shall match one of the two
+ previous forms and is then processed as appropriate.
+
+
+Contents
+
+6.10.5 Error directive
+
+
+Semantics
+
+1
+ A preprocessing directive of the form
+
+
+ # error pp-tokensopt new-line
+
+
+ causes the implementation to produce a diagnostic message that includes the specified
+ sequence of preprocessing tokens.
+
+
+Contents
+
+6.10.6 Pragma directive
+
+
+Semantics
+
+1
+ A preprocessing directive of the form
+
+
+ # pragma pp-tokensopt new-line
+
+
+ where the preprocessing token STDC does not immediately follow pragma in the
+ directive (prior to any macro replacement)174) causes the implementation to behave in an
+ implementation-defined manner. The behavior might cause translation to fail or cause the
+ translator or the resulting program to behave in a non-conforming manner. Any such
+ pragma that is not recognized by the implementation is ignored.
+
+2
+ If the preprocessing token STDC does immediately follow pragma in the directive (prior
+ to any macro replacement), then no macro replacement is performed on the directive, and
+ the directive shall have one of the following forms175) whose meanings are described
+ elsewhere:
+
+
+ #pragma STDC FP_CONTRACT on-off-switch
+ #pragma STDC FENV_ACCESS on-off-switch
+ #pragma STDC CX_LIMITED_RANGE on-off-switch
+ on-off-switch: one of
+ ON OFF DEFAULT
+
+
+ Forward references: the FP_CONTRACT pragma (7.12.2), the FENV_ACCESS pragma
+ (7.6.1), the CX_LIMITED_RANGE pragma (7.3.4).
+
+
+
+
+
+
+Footnotes
+
+174) An implementation is not required to perform macro replacement in pragmas, but it is permitted
+ except for in standard pragmas (where STDC immediately follows pragma). If the result of macro
+ replacement in a non-standard pragma has the same form as a standard pragma, the behavior is still
+ implementation-defined; an implementation is permitted to behave as if it were the standard pragma,
+ but is not required to.
+
+
+175) See ''future language directions'' (6.11.8).
+
+
+Contents
+
+6.10.7 Null directive
+
+
+Semantics
+
+1
+ A preprocessing directive of the form
+
+
+ # new-line
+
+
+ has no effect.
+
+
+Contents
+
+6.10.8 Predefined macro names
+
+
+1
+ The values of the predefined macros listed in the following subclauses176) (except for
+ __FILE__ and __LINE__) remain constant throughout the translation unit.
+
+2
+ None of these macro names, nor the identifier defined, shall be the subject of a
+ #define or a #undef preprocessing directive. Any other predefined macro names
+ shall begin with a leading underscore followed by an uppercase letter or a second
+ underscore.
+
+3
+ The implementation shall not predefine the macro __cplusplus, nor shall it define it
+ in any standard header.
+
+ Forward references: standard headers (7.1.2).
+
+
+Footnotes
+
+176) See ''future language directions'' (6.11.9).
+
+
+Contents
+
+6.10.8.1 Mandatory macros
+
+
+1
+ The following macro names shall be defined by the implementation:
+
+ __DATE__ The date of translation of the preprocessing translation unit: a character
+ string literal of the form "Mmm dd yyyy", where the names of the
+ months are the same as those generated by the asctime function, and the
+ first character of dd is a space character if the value is less than 10. If the
+ date of translation is not available, an implementation-defined valid date
+ shall be supplied.
+ __FILE__ The presumed name of the current source file (a character string literal).177)
+ __LINE__ The presumed line number (within the current source file) of the current
+ source line (an integer constant).177)
+ __STDC__ The integer constant 1, intended to indicate a conforming implementation.
+ __STDC_HOSTED__ The integer constant 1 if the implementation is a hosted
+ implementation or the integer constant 0 if it is not.
+
+ __STDC_VERSION__ The integer constant 201ymmL.178)
+ __TIME__ The time of translation of the preprocessing translation unit: a character
+ string literal of the form "hh:mm:ss" as in the time generated by the
+ asctime function. If the time of translation is not available, an
+ implementation-defined valid time shall be supplied.
+
+
+ Forward references: the asctime function (7.27.3.1).
+
+
+Footnotes
+
+177) The presumed source file name and line number can be changed by the #line directive.
+
+
+178) This macro was not specified in ISO/IEC 9899:1990 and was specified as 199409L in
+ ISO/IEC 9899/AMD1:1995 and as 199901L in ISO/IEC 9899:1999. The intention is that this will
+ remain an integer constant of type long int that is increased with each revision of this International
+ Standard.
+
+
+Contents
+
+6.10.8.2 Environment macros
+
+
+1
+ The following macro names are conditionally defined by the implementation:
+
+ __STDC_ISO_10646__ An integer constant of the form yyyymmL (for example,
+ 199712L). If this symbol is defined, then every character in the Unicode
+ required set, when stored in an object of type wchar_t, has the same
+ value as the short identifier of that character. The Unicode required set
+ consists of all the characters that are defined by ISO/IEC 10646, along with
+ all amendments and technical corrigenda, as of the specified year and
+ month. If some other encoding is used, the macro shall not be defined and
+ the actual encoding used is implementation-defined.
+ __STDC_MB_MIGHT_NEQ_WC__ The integer constant 1, intended to indicate that, in
+ the encoding for wchar_t, a member of the basic character set need not
+ have a code value equal to its value when used as the lone character in an
+ integer character constant.
+ __STDC_UTF_16__ The integer constant 1, intended to indicate that values of type
+ char16_t are UTF-16 encoded. If some other encoding is used, the
+ macro shall not be defined and the actual encoding used is implementation-
+ defined.
+ __STDC_UTF_32__ The integer constant 1, intended to indicate that values of type
+ char32_t are UTF-32 encoded. If some other encoding is used, the
+ macro shall not be defined and the actual encoding used is implementation-
+ defined.
+
+
+ Forward references: common definitions (7.19), unicode utilities (7.28).
+
+
+
+
+
+
+Contents
+
+6.10.8.3 Conditional feature macros
+
+
+1
+ The following macro names are conditionally defined by the implementation:
+
+ __STDC_ANALYZABLE__ The integer constant 1, intended to indicate conformance to
+ the specifications in annex L (Analyzability).
+ __STDC_IEC_559__ The integer constant 1, intended to indicate conformance to the
+ specifications in annex F (IEC 60559 floating-point arithmetic).
+ __STDC_IEC_559_COMPLEX__ The integer constant 1, intended to indicate
+ adherence to the specifications in annex G (IEC 60559 compatible complex
+ arithmetic).
+ __STDC_LIB_EXT1__ The integer constant 201ymmL, intended to indicate support
+ for the extensions defined in annex K (Bounds-checking interfaces).179)
+ __STDC_NO_ATOMICS__ The integer constant 1, intended to indicate that the
+ implementation does not support atomic types (including the _Atomic
+ type qualifier) and the <stdatomic.h> header.
+ __STDC_NO_COMPLEX__ The integer constant 1, intended to indicate that the
+ implementation does not support complex types or the <complex.h>
+ header.
+ __STDC_NO_THREADS__ The integer constant 1, intended to indicate that the
+ implementation does not support the <threads.h> header.
+ __STDC_NO_VLA__ The integer constant 1, intended to indicate that the
+ implementation does not support variable length arrays or variably
+ modified types.
+
+
+2
+ An implementation that defines __STDC_NO_COMPLEX__ shall not define
+ __STDC_IEC_559_COMPLEX__.
+
+
+
+
+
+
+Footnotes
+
+179) The intention is that this will remain an integer constant of type long int that is increased with
+ each revision of this International Standard.
+
+
+Contents
+
+6.10.9 Pragma operator
+
+
+Semantics
+
+1
+ A unary operator expression of the form:
+
+
+ _Pragma ( string-literal )
+
+
+ is processed as follows: The string literal is destringized by deleting any encoding prefix,
+ deleting the leading and trailing double-quotes, replacing each escape sequence \" by a
+ double-quote, and replacing each escape sequence \\ by a single backslash. The
+ resulting sequence of characters is processed through translation phase 3 to produce
+ preprocessing tokens that are executed as if they were the pp-tokens in a pragma
+ directive. The original four preprocessing tokens in the unary operator expression are
+ removed.
+
+2
+ EXAMPLE A directive of the form:
+
+
+ #pragma listing on "..\listing.dir"
+
+
+ can also be expressed as:
+
+
+ _Pragma ( "listing on \"..\\listing.dir\"" )
+
+
+ The latter form is processed in the same way whether it appears literally as shown, or results from macro
+ replacement, as in:
+
+
+ #define LISTING(x) PRAGMA(listing on #x)
+ #define PRAGMA(x) _Pragma(#x)
+ LISTING ( ..\listing.dir )
+
+
+Contents
+
+6.11 Future language directions
+
+
+Contents
+
+6.11.1 Floating types
+
+
+1
+ Future standardization may include additional floating-point types, including those with
+ greater range, precision, or both than long double.
+
+
+Contents
+
+6.11.2 Linkages of identifiers
+
+
+1
+ Declaring an identifier with internal linkage at file scope without the static storage-
+ class specifier is an obsolescent feature.
+
+
+Contents
+
+6.11.3 External names
+
+
+1
+ Restriction of the significance of an external name to fewer than 255 characters
+ (considering each universal character name or extended source character as a single
+ character) is an obsolescent feature that is a concession to existing implementations.
+
+
+Contents
+
+6.11.4 Character escape sequences
+
+
+1
+ Lowercase letters as escape sequences are reserved for future standardization. Other
+ characters may be used in extensions.
+
+
+Contents
+
+6.11.5 Storage-class specifiers
+
+
+1
+ The placement of a storage-class specifier other than at the beginning of the declaration
+ specifiers in a declaration is an obsolescent feature.
+
+
+Contents
+
+6.11.6 Function declarators
+
+
+1
+ The use of function declarators with empty parentheses (not prototype-format parameter
+ type declarators) is an obsolescent feature.
+
+
+Contents
+
+6.11.7 Function definitions
+
+
+1
+ The use of function definitions with separate parameter identifier and declaration lists
+ (not prototype-format parameter type and identifier declarators) is an obsolescent feature.
+
+
+Contents
+
+6.11.8 Pragma directives
+
+
+1
+ Pragmas whose first preprocessing token is STDC are reserved for future standardization.
+
+
+Contents
+
+6.11.9 Predefined macro names
+
+
+1
+ Macro names beginning with __STDC_ are reserved for future standardization.
+
+
+Contents
