Header files and several sources

Put declarations in a .h file and definitions in a .c file; the IDE links them into your program.

Runs in Zerethon IDE · C (GCC 14) Official C documentation ↗

main.c

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

int main(void) {
    printf("%d\n", area(3, 4));
    return 0;
}

geometry.h

#ifndef GEOMETRY_H
#define GEOMETRY_H

int area(int width, int height);

#endif

geometry.c

#include "geometry.h"

int area(int width, int height) {
    return width * height;
}

Output

Run compiles the entry together with every other .c file whose name does not start with main or test_ (those are separate programs with their own main).
Include your own headers with quotes ("geometry.h"), standard ones with angle brackets.
Only the C standard library (musl) and -lm math are linked; other libraries are not installed.

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