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The Basics of C Programming

        Tech | Programming

Functions: Function Prototypes

It is now considered good form to use function prototypes for all functions in your program. A prototype declares the function name, its parameters, and its return type to the rest of the program prior to the function's actual declaration. To understand why function prototypes are useful, enter the following code and run it:

#include <stdio.h>

void main()
{
    printf("%d\n",add(3));
}

int add(int i, int j)
{
    return i+j;
}

This code compiles on many compilers without giving you a warning, even though add expects two parameters but receives only one. It works because many C compilers do not check for parameter matching either in type or count. You can waste an enormous amount of time debugging code in which you are simply passing one too many or too few parameters by mistake. The above code compiles properly, but it produces the wrong answer.

To solve this problem, C lets you place function prototypes at the beginning of (actually, anywhere in) a program. If you do so, C checks the types and counts of all parameter lists. Try compiling the following:

#include <stdio.h>

int add (int,int); /* function prototype for add */

void main()
{
    printf("%d\n",add(3));
}

int add(int i, int j)
{
    return i+j;
}

The prototype causes the compiler to flag an error on the printf statement.

Place one prototype for each function at the beginning of your program. They can save you a great deal of debugging time, and they also solve the problem you get when you compile with functions that you use before they are declared. For example, the following code will not compile:

#include <stdio.h>

void main()
{
    printf("%d\n",add(3));
}

float add(int i, int j)
{
    return i+j;
}

Why, you might ask, will it compile when add returns an int but not when it returns a float? Because older C compilers default to an int return value. Using a prototype will solve this problem. "Old style" (non-ANSI) compilers allow prototypes, but the parameter list for the prototype must be empty. Old style compilers do no error checking on parameter lists.


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