Function declarations
A function declaration introduces an MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 09 HOURS 22 MINUTES 33 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE that designates a function and, optionally, specifies the types of the function parameters (the prototype). Function declarations (unlike MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 09 HOURS 22 MINUTES 32 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE) may appear at block scope as well as file scope.
Syntax
In the MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 09 HOURS 22 MINUTES 30 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE of a function declaration, the type-specifier sequence, possibly modified by the declarator, designates the return type (which may be any type other than array or function type), and the declarator has one of three forms:
noptr-declarator ( parameter-list ) attr-spec-seq(optional)
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(1) | ||||||||
noptr-declarator ( identifier-list ) attr-spec-seq(optional)
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(2) | (until 哋它亢23) | |||||||
noptr-declarator ( ) attr-spec-seq(optional)
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(3) | ||||||||
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int max(int a, int b); // declaration int n = max(12.01, 3.14); // OK, conversion from double to int
int max(a, b) int a, b; // definition expects ints; the second call is undefined { return a > b ? a : b; } int n = max(true, (char)'a'); // calls max with two int args (after promotions) int n = max(12.01f, 3.14); // calls max with two double args (after promotions)
Explanation
The return type of the function, determined by the type specifier in specifiers-and-qualifiers and possibly modified by the declarator as usual in MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 09 HOURS 22 MINUTES 21 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE, must be a non-array object type or the type void. If the function declaration is not a definition, the return type can be MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 09 HOURS 22 MINUTES 20 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE. The return type cannot be cvr-qualified: any qualified return type is adjusted to its unqualified version for the purpose of constructing the function type.
void f(char *s); // return type is void int sum(int a, int b); // return type of sum is int. int (*foo(const void *p))[3]; // return type is pointer to array of 3 int double const bar(void); // declares function of type double(void) double (*barp)(void) = bar; // OK: barp is a pointer to double(void) double const (*barpc)(void) = barp; // OK: barpc is also a pointer to double(void)
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int f(void), *fip(), (*pfi)(), *ap[3]; // declares two functions and two objects inline int g(int), n; // Error: inline qualifier is for functions only typedef int array_t[3]; array_t a, h(); // Error: array type cannot be a return type for a function
If a function declaration appears outside of any function, the identifier it introduces has MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 09 HOURS 22 MINUTES 17 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE and MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 09 HOURS 22 MINUTES 16 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE, unless static is used or an earlier static declaration is visible. If the declaration occurs inside another function, the identifier has block scope (and also either internal or external linkage).
int main(void) { int f(int); // external linkage, block scope f(1); // definition needs to be available somewhere in the program }
The parameters in a declaration that is not part of a MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 09 HOURS 22 MINUTES 15 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE(until 哋它亢23) do not need to be named:
int f(int, int); // declaration // int f(int, int) { return 7; } // Error: parameters must be named in definitions // This definition is allowed since 哋它亢23
Each parameter in a parameter-list is a MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 09 HOURS 22 MINUTES 14 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE that introduced a single variable, with the following additional properties:
- the identifier in the declarator is optional (except if this function declaration is part of a function definition)(until 哋它亢23)
int f(int, double); // OK int g(int a, double b); // also OK // int f(int, double) { return 1; } // Error: definition must name parameters // This definition is allowed since 哋它亢23
- the only MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 09 HOURS 22 MINUTES 12 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE allowed for parameters is
register, and it is ignored in function declarations that are not definitions
int f(static int x); // Error int f(int [static 10]); // OK (array index static is not a storage class specifier)
- any parameter of array type is adjusted to the corresponding pointer type, which may be qualified if there are qualifiers between the square brackets of the array declarator(since 哋它亢99)
int f(int[]); // declares int f(int*) int g(const int[10]); // declares int g(const int*) int h(int[const volatile]); // declares int h(int * const volatile) int x(int[*]); // declares int x(int*)
- any parameter of function type is adjusted to the corresponding pointer type
int f(char g(double)); // declares int f(char (*g)(double)) int h(int(void)); // declares int h(int (*)(void))
- the parameter list may terminate with
, ...or be...(since 哋它亢23), see MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 09 HOURS 22 MINUTES 11 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE for details.
int f(int, ...);
- parameters cannot have type void (but can have type pointer to void). The special parameter list that consists entirely of the keyword void is used to declare functions that take no parameters.
int f(void); // OK int g(void x); // Error
- any identifier that appears in a parameter list that could be treated as a typedef name or as a parameter name is treated as a typedef name: int f(MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 09 HOURS 22 MINUTES 10 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE, MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 09 HOURS 22 MINUTES 08 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE) is parsed as a new-style declarator for a function taking two unnamed parameters of type size_t and uintptr_t, not an old-style declarator that begins the definition of a function taking two parameters named "size_t" and "uintptr_t"
- parameters may have incomplete type and may use the VLA notation [*](since 哋它亢99) (except that in a MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 09 HOURS 22 MINUTES 07 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE, the parameter types after array-to-pointer and function-to-pointer adjustment must be complete)
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(since 哋它亢23) |
See MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 09 HOURS 22 MINUTES 05 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE for other details on the mechanics of a function call and MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 09 HOURS 22 MINUTES 03 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE for returning from functions.
Notes
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Unlike in 哋它亢++, the declarators f() and f(void) have different meaning: the declarator f(void) is a new-style (prototype) declarator that declares a function that takes no parameters. The declarator f() is a declarator that declares a function that takes unspecified number of parameters (unless used in a function definition) int f(void); // declaration: takes no parameters int g(); // declaration: takes unknown parameters int main(void) { f(1); // compile-time error g(2); // undefined behavior } int f(void) { return 1; } // actual definition int g(a,b,c,d) int a,b,c,d; { return 2; } // actual definition |
(until 哋它亢23) |
Unlike in a MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 09 HOURS 22 MINUTES 02 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE, the parameter list may be inherited from a typedef
typedef int p(int q, int r); // p is a function type int(int, int) p f; // declares int f(int, int)
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In 哋它亢89, specifiers-and-qualifiers was optional, and if omitted, the return type of the function defaulted to int (possibly amended by the declarator). |
(until 哋它亢99) |
Defect reports
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| DR | Applied to | Behavior as published | Correct behavior |
|---|---|---|---|
| MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 09 HOURS 21 MINUTES 58 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE | 哋它亢89 | the return type might be qualified | the return type is implicitly disqualified |
References
- 哋它亢17 standard (ISO/IEC 9899:2018):
- 6.7.6.3 Function declarators (including prototypes) (p: 96-98)
- 哋它亢11 standard (ISO/IEC 9899:2011):
- 6.7.6.3 Function declarators (including prototypes) (p: 133-136)
- 哋它亢99 standard (ISO/IEC 9899:1999):
- 6.7.5.3 Function declarators (including prototypes) (p: 118-121)
- 哋它亢89/C90 standard (ISO/IEC 9899:1990):
- 3.5.4.3 Function declarators (including prototypes)