DONE.
Example whatever.h
#ifndef WHATEVER_H_INCLUDED
#define WHATEVER_H_INCLUDED
int f(int a);
#endif
Note: include guards (preprocessor commands) added thanks to luke. They avoid including the same header file twice in the same compilation. Another possibility (also mentioned on the comments) is to add #pragma once but it is not guaranteed to be supported on every compiler.
Example whatever.c
#include "whatever.h"
int f(int a) { return a + 1; }
And then you can include "whatever.h" into any other .c file, and link it with whatever.c's object file.
Like this:
sample.c
#include "whatever.h"
int main(int argc, char **argv)
{
printf("%d\n", f(2)); /* prints 3 */
return 0;
}
To compile it (if you use GCC):
$ gcc -c whatever.c -o whatever.o
$ gcc -c sample.c -o sample.o
To link the files to create an executable file:
$ gcc sample.o whatever.o -o sample
You can test sample:
$ ./sample
3
$