Function definitions

A function definition associates the function body (a sequence of declarations and statements) with the function name and parameter list. Unlike function declaration , function definitions are allowed at file scope only (there are no nested functions).

C supports two different forms of function definitions:

where

int max(int a, int b)
{
    return a>b?a:b;
}
double g(void)
{
    return 0.1;
}

int max(a, b)
int a, b;
{
    return a>b?a:b;
}
double g()
{
    return 0.1;
}

Explanation

As with function declarations , the return type of the function, determined by the type specifier in specifiers-and-qualifiers and possibly modified by the declarator as usual in declarations , must be a complete non-array object type or the type void . If the return type would be cvr-qualified, it is adjusted to its unqualified version for the purpose of constructing the function type.

void f(char *s) { puts(s); } // return type is void
int sum(int a, int b) { return a+b; } // return type is int
int (*foo(const void *p))[3] { // return type is pointer to array of 3 int
    return malloc(sizeof(int[3]));
}

As with function 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 determining compatible function type .

int f(int, int); // declaration
// int f(int, int) { return 7; } // Error until C23, OK since C23
int f(int a, int b) { return 7; } // definition
int g(void) { return 8; } // OK: void doesn't declare a parameter

Within the function body, every named parameter is an lvalue expression, they have automatic storage duration and block scope . The layout of the parameters in memory (or if they are stored in memory at all) is unspecified: it is a part of the calling convention .

int main(int ac, char **av)
{
    ac = 2; // parameters are lvalues
    av = (char *[]){"abc", "def", NULL};
    f(ac, av);
}

See function call operator for other details on the mechanics of a function call and return for returning from functions.

static const char __func__[] = "function name";

Notes

The argument list must be explicitly present in the declarator, it cannot be inherited from a typedef

typedef int p(int q, int r); // p is a function type int(int, int)
p f { return q + r; } // Error

max(a, b) // a and b have type int, return type is int
{
    return a>b?a:b;
}

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C standards.

References

See also