Functions

A function is a C language construct that associates a compound statement (the function body) with an identifier (the function name). Every C program begins execution from the main function, which either terminates, or invokes other, user-defined or library functions.

// function definition.
// defines a function with the name "sum" and with the body "{ return x+y; }"
int sum(int x, int y) 
{
    return x + y;
}

Functions may accept zero or more parameters, which are initialized from the arguments of a function call operator, and may return a value to its caller by means of the return statement.

int n = sum(1, 2); // parameters x and y are initialized with the arguments 1 and 2

The body of a function is provided in a function definition. Each function that is actually called must be defined only once in a program, unless the function is inline (since C99).

There are no nested functions (except where allowed through non-standard compiler extensions): each function definition must appear at file scope, and functions have no access to the local variables from the caller:

int main(void) // the main function definition
{
    int sum(int, int); // function declaration (may appear at any scope)
    int x = 1;  // local variable in main
    sum(1, 2); // function call
 
//    int sum(int a, int b) // error: no nested functions
//    {
//        return  a + b; 
//    }
}
int sum(int a, int b) // function definition
{
//    return x + a + b; //  error: main's x is not accessible within sum
    return a + b;
}

References

• C17 standard (ISO/IEC 9899:2018):
• 6.7.6.3 Function declarators (including prototypes) (p: 96-98)
• 6.9.1 Function definitions (p: 113-115)
• C11 standard (ISO/IEC 9899:2011):
• 6.7.6.3 Function declarators (including prototypes) (p: 133-136)
• 6.9.1 Function definitions (p: 156-158)
• C99 standard (ISO/IEC 9899:1999):
• 6.7.5.3 Function declarators (including prototypes) (p: 118-121)
• 6.9.1 Function definitions (p: 141-143)
• C89/C90 standard (ISO/IEC 9899:1990):
• 3.5.4.3 Function declarators (including prototypes)
• 3.7.1 Function definitions

See also