Best practice for using includes in C

Question

I am learning C and I am unsure where to include files. Basically I can do this in .c or in .h files:

Option 1

test.h

int my_func(char **var);

test.c

#include <stdio.h>
#include "test.h"
int my_func(char **var) {printf("%s\n", "foo");}
int main() {...}

Option 2

test.h

#include <stdio.h>
int my_func(char **var);

test.c

#include "test.h"
int my_func(char **var) {printf("%s\n", "foo");}
int main() {...}

With option 2 I would only need to include test.h in whatever .c file I need the library. Most of the examples I see use option 1.

Are there some general rules when to do what or is this a question of personal preferences?

Answer 1

Don't use includes, you don't need.

I'd choose something like "Option 1". Why "something like" ? Because I'd create a separate file for the main and I'd keep all declaraions inside the .h and all definitions inside the corresponding .c.

Of course, both options are valid.

If you want to include only one header in your main, you can just create a header file, containing only includes - that's a common practice. This way, you can include only one header, instead of several.

Answer 2

I tend to prefer Option 1, as cyclic dependencies will come and bite you very quickly in option 2, and reducing the input size is the best way to guarantee faster compile times. Option 2 tends towards including everything everywhere, whether you really need it or not.

That said, it might be best to experiment a little with what works for structuring your projects. Hard and fast rules tend to not apply universally to these kinds of questions.