# Functions

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< c‎ | language

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 must be defined only once in a program, unless the function is inline.

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

- 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

C++ documentation for Declaring functions

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