including a header file in the .c file that contains its definitions

Question

I have the following files:

main.c

#include <stdio.h>
#include "test.h"

int main(){
    printf("%d", testFunction());
    return 0;
}

test.h

int testFunction();

test.c

int testFunction(){
    return 1;
}

should i include test.h in test.c?

test.c

#include "test.h" //is this necessary?

int testFunction(){
    return 1;
}

If I run the main, there are no errors in either case.

Not necessary in your specific case, but usually is done because headers might contain some declarations needed for the implementation. Don't forget include guards BTW — Eugene Sh., May 18 '21 at 20:11
https://softwareengineering.stackexchange.com/questions/286490/what-is-the-difference-between-function-and-functionvoid/287002 Summary: use `int testFunction(void)` instead of `int testFunctio()` — William Pursell, May 18 '21 at 20:37

score 2 · Accepted Answer · answered May 18 '21 at 20:11

Q: should i include test.h in test.c?

A: Generally, yes.

It's essential with a more complex header that contains constants, struct definitions, etc. It's "good practice" in any case.

ALSO: Don't forget to add header guards to your .h file.

EXAMPLE:

#ifndef TEST_H
#define TEST_H

int testFunction();

#endif

If you prefer, most compilers also support #pragma once as an alternative.

score 0 · Answer 2 · answered May 18 '21 at 20:42

It is not necessary in your case, but may save you from trouble as your program gets more complicated. A function declaration e.g.

int f1(int x);

should be seen by the compiler before the function call e.g.

return f1(5);

so that it knows what to expect. Using a header file myheader.h

int f1(int x);

and main.c

#include "myheader.h"

int main(){
    return f1(5);
}

int f1(int x){
    return x*x;
}

ensures this is happening, try removing the header file in this example and you get into trouble...

2 Answers2