A function prototype declares a function’s name, return type, and parameter types before the function is called. It allows the compiler to check function calls against the declared interface.

#include <stdio.h>

int multiply(int a, int b);

int main(void)
{
    int result = multiply(4, 5);

    printf("Result = %d\n", result);

    return 0;
}

int multiply(int a, int b)
{
    return a * b;
}

Example Output #

Result = 20

Explanation #

The declaration:

int multiply(int a, int b);

is the function prototype. It tells the compiler that multiply returns an int and accepts two int parameters.

The prototype ends with a semicolon because it declares the function without defining its body.

The function is called before its definition appears:

int result = multiply(4, 5);

The prototype makes the function’s interface known at this point.

The function definition appears later:

int multiply(int a, int b)
{
    return a * b;
}

Function Prototype Syntax #

return_type function_name(parameter_types);

Parameter names may be included or omitted in a prototype:

int add(int, int);
int add(int first, int second);

Both declarations specify the same function interface.

In a function prototype, (void) explicitly means that the function accepts no parameters. An empty parameter list () has a different meaning in a declaration in C: it does not specify the parameter types. For a fully specified function interface, use (void) when no parameters are accepted.