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 non-inline (since C99) function that is used in an expression (unless unevaluated) must be defined only once in a program.
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; }