Headers, linkage, and separate compilation

The idea

A C program can be compiled from multiple source files, each forming a translation unit after preprocessing. A header publishes declarations that callers and implementations should both include. Keeping one shared declaration prevents subtly different parameter or result expectations from being copied into separate files. Compilation checks a translation unit; linking resolves the external definitions used across them.

An include guard prevents repeated contents within one translation unit. It does not make repeated external variable definitions across different source files harmless. Declare shared objects with extern in a header and define them in one source file when global state is truly needed. Mark a file-private function static to give it internal linkage. Prefer passing state explicitly over adding unnecessary global variables.

/* scale.h */
#ifndef SCALE_H
#define SCALE_H
int twice(int value);
#endif
/* scale.c */
#include "scale.h"
int twice(int value) { return value * 2; }
/* main.c */
#include <stdio.h>
#include "scale.h"
int main(void) { printf("%d\n", twice(7)); }

14 when the two source files are compiled and linked together.

The header shares a declaration; scale.c supplies the one external function definition.