A function definition associates the function body (a sequence of declarations and statements) with the function name and parameter list. Unlikefunction declaration, function definitions are allowed at file scope only (there are no nested functions).
C supports two different forms of function definitions:
| attr-spec-seq(optional)specifiers-and-qualifiersparameter-list-declaratorfunction-body | (1) | ||||||||
| specifiers-and-qualifiersidentifier-list-declaratordeclaration-listfunction-body | (2) | (until C23) | |||||||
where
| attr-spec-seq | - | (C23)an optional list ofattributes, applied to the function |
| specifiers-and-qualifiers | - | a combination of
|
| parameter-list-declarator | - | a declarator for a function type which uses aparameter list to designate function parameters |
| identifier-list-declarator | - | a declarator for a function type which uses aidentifier list to designate function parameters |
| declaration-list | - | sequence of declarations that declare every identifier inidentifier-list-declarator. These declarations cannot use initializers and the onlystorage-class specifier allowed isregister. |
| function-body | - | acompound statement, that is a brace-enclosed sequence of declarations and statements, that is executed whenever this function is called |
int max(int a,int b){return a>b?a:b;} double g(void){return0.1;}
int max(a, b)int a, b;{return a>b?a:b;}double g(){return0.1;}
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As withfunction declarations, the return type of the function, determined by the type specifier inspecifiers-and-qualifiers and possibly modified by thedeclarator as usual indeclarations, must be a complete non-array object type or the typevoid. If the return type would be cvr-qualified, it is adjusted to its unqualified version for the purpose of constructing the function type.
As withfunction declarations, the types of the parameters are adjusted from functions to pointers and from arrays to pointers for the purpose of constructing the function type and the top-level cvr-qualifiers of all parameter types are ignored for the purpose of determiningcompatible function type.
Unlikefunction declarations, unnamed formal parameters are not allowed (otherwise, there would be conflicts in old-style (K&R) function definitions), they must be named even if they are not used within the function. The only exception is the special parameter list(void). | (until C23) |
Formal parameters may be unnamed in function definitions, because old-style (K&R) function definitions has been removed. Unnamed parameters are inaccessible by name within the function body. | (since C23) |
int f(int,int);// declaration// int f(int, int) { return 7; } // Error until C23, OK since C23int f(int a,int b){return7;}// definitionint g(void){return8;}// OK: void doesn't declare a parameter
Within the function body, every named parameter is anlvalue expression, they have automaticstorage duration andblock scope. The layout of the parameters in memory (or if they are stored in memory at all) is unspecified: it is a part of thecalling convention.
int main(int ac,char**av){ ac=2;// parameters are lvalues av=(char*[]){"abc","def",NULL}; f(ac, av);}
Seefunction call operator for other details on the mechanics of a function call andreturn for returning from functions.
__func__Within everyfunction-body, the special predefined variable__func__ with block scope and static storage duration is available, as if defined immediately after the opening brace by staticconstchar __func__[]="function name"; This special identifier is sometimes used in combination with thepredefined macro constants__FILE__ and__LINE__, for example, byassert. | (since C99) |
The argument list must be explicitly present in the declarator, it cannot be inherited from a typedef
typedefint p(int q,int r);// p is a function type int(int, int)p f{return q+ r;}// Error
In C89,specifiers-and-qualifiers was optional, and if omitted, the return type of the function defaulted toint (possibly amended by thedeclarator). In addition, old-style definition didn't require a declaration for every parameter indeclaration-list. Any parameter whose declaration was missing had typeint max(a, b)// a and b have type int, return type is int{return a>b?a:b;} | (until C99) |
The following behavior-changing defect reports were applied retroactively to previously published C standards.
| DR | Applied to | Behavior as published | Correct behavior |
|---|---|---|---|
| DR 423 | C89 | the return type might be qualified | the return type is implicitly disqualified |
C++ documentation forFunction definition |
