foo.h
#ifndef FOO_H_ /* Include guard */
#define FOO_H_
int foo(int x); /* An example function declaration */
#endif // FOO_H_foo.c
#include "foo.h" /* Include the header (not strictly necessary here) */
int foo(int x) /* Function definition */
{
return x + 5;
}main.c
#include <stdio.h>
#include "foo.h" /* Include the header here, to obtain the function declaration */
int main(void)
{
int y = foo(3); /* Use the function here */
printf("%d\n", y);
return 0;
}To compile using GCC
gcc -o my_app main.c foo.c