Function definitions

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C language
 
Functions
 

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:

specifiers-and-qualifiers parameter-list-declarator function-body (1)
specifiers-and-qualifiers identifier-list-declarator declaration-list function-body (2)

where

specifiers-and-qualifiers - a combination of
parameter-list-declarator - a declarator for a function type which uses a parameter list to designate function parameters
identifier-list-declarator - a declarator for a function type which uses a identifier list to designate function parameters
declaration-list - sequence of declarations that declare every identifier in identifier-list-declarator. These declarations cannot use initializers and the only storage-class specifier allowed is register.
function-body - a compound statement, that is a brace-enclosed sequence of declarations and statements, that is executed whenever this function is called
1) New-style (C89) function definition. This definition both introduces the function itself and serves as a function prototype for any future function call expressions, forcing conversions from argument expressions to the declared parameter types. 
int max(int a, int b)
{
    return a>b?a:b;
}
 
double g(void)
{
    return 0.1;
}
2) Old-style (K&R) function definition. This definition does not behave as a prototype and any future function call expressions will perform default argument promotions. 
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. 

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, if the return type would be cvr-qualified, it is adjusted to its unqualified version for the purpose of constructing the function type.

(since C17)

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.

Unlike function declarations, unnamed formal parameters are not allowed, they must be named even if they are not used within the function. The only exception is the special parameter list (void) 

int f(int, int); // declaration
// int f(int, int) { return 7; } // Error
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 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.

__func__

Within every function-body, the special predefined variable __func__ with block scope and static storage duration is available, as if defined immediately after the opening brace by 

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

This special identifier is sometimes used in combination with the predefined macro constants __FILE__ and __LINE__, for example, by assert.

(since C99)

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

In C89, specifiers-and-qualifiers was optional, and if omitted, the return type of the function defaulted to int (possibly amended by the declarator).

In addition, old-style definition didn't require a declaration for every parameter in declaration-list. Any parameter whose declaration was missing had type int 

max(a, b) // a and b have type int, return type is int
{
    return a>b?a:b;
}
 (until C99)

References

  • C11 standard (ISO/IEC 9899:2011):
  • 6.9.1 Function definitions (p: 156-158)
  • C99 standard (ISO/IEC 9899:1999):
  • 6.9.1 Function definitions (p: 141-143)
  • C89/C90 standard (ISO/IEC 9899:1990):
  • 3.7.1 Function definitions

See also