require_prototypes

Controls whether or not the compiler should expect function prototypes.

#pragma require_prototypes on | off | reset
  

This pragma only affects non-static functions.

If you enable this pragma, the compiler generates an error message if you use a function that does not have a preceding prototype. Use this pragma to prevent error messages caused by referring to a function before you define it. For example, without a function prototype, you might pass data of the wrong type. As a result, your code might not work as you expect even though it compiles without error.

In Unnoticed Type-mismatch, function main() calls PrintNum() with an integer argument even though PrintNum() takes an argument of type float .

#include <stdio.h>

void main(void)
{
  PrintNum(1);  /* PrintNum() tries to interpret the 
                   integer as a float. Prints 0.000000. */
}

void PrintNum(float x)
{
    printf("%f\n", x);
}

When you run this program, you could get this result:

0.000000

Although the compiler does not complain about the type mismatch, the function does not give the result you intended. Since PrintNum() does not have a prototype, the compiler does not know to generate instructions to convert the integer to a floating-point number before calling PrintNum() . Consequently, the function interprets the bits it received as a floating-point number and prints nonsense.

A prototype for PrintNum() , as in Using a Prototype to Avoid Type-mismatch, gives the compiler sufficient information about the function to generate instructions to properly convert its argument to a floating-point number. The function prints what you expected.

#include <stdio.h>

void PrintNum(float x); /* Function prototype. */

void main(void)
{
    PrintNum(1);        /*  Compiler converts int to float.
}                           Prints 1.000000. */

void PrintNum(float x)
{
    printf("%f\n", x);
}

In other situations where automatic conversion is not possible, the compiler generates an error message if an argument does not match the data type required by a function prototype. Such a mismatched data type error is easier to locate at compile time than at runtime.