C Header Files and Several Files

Real programs are too big for one file. C lets you split a program into several .c files, each with a clear job, and share what they offer through header files (.h).

The pieces

A typical split for a small geometry library looks like this:

• geometry.h declares what the library offers: function prototypes, types and constants.
• geometry.c defines those functions.
• main.c includes geometry.h and uses the functions.

/* geometry.h */
#ifndef GEOMETRY_H
#define GEOMETRY_H

double circleArea(double r);
double rectangleArea(double w, double h);

#endif

/* geometry.c */
#include "geometry.h"

#define PI 3.14159265358979

double circleArea(double r) {
    return PI * r * r;
}

double rectangleArea(double w, double h) {
    return w * h;
}

/* main.c */
#include <stdio.h>
#include "geometry.h"

int main(void) {
    printf("circle: %.2f\n", circleArea(2));
    printf("rectangle: %.2f\n", rectangleArea(3, 4));
    return 0;
}

Try it in the editor opens all three files in one folder. They are compiled together: every .c file must be on the command line (or use *.c, which is what Run does), and the header is found through #include. In the terminal you can type the command yourself:

gcc -Wall main.c geometry.c -o main && ./main

circle: 12.57
rectangle: 12.00

The Run button compiles all the .c files in the folder of the open file, so it works too.

Include guards

The lines #ifndef GEOMETRY_H, #define GEOMETRY_H and #endif are an include guard. If a header is included twice, for example by two other headers, the second copy is skipped, and the compiler does not complain about things declared twice. Every header should have one. Many compilers also accept the shorter #pragma once.

What goes where

• Header: prototypes, typedef and struct definitions, #define constants, extern declarations of shared variables. No function bodies, no variable definitions.
• Source file: function definitions, and variables. Helpers that other files should not use are marked static, which keeps them private to that file.

How the build works

Each .c file is compiled on its own into an object file. The linker then joins the object files into one program and connects each call to its function. A forgotten .c file on the command line shows up as a linker error such as undefined symbol: circleArea: the prototype was there, but the body was not.

Practice exercises

1. Create the geometry files in the editor and build the program with the command above.
2. Add double triangleArea(double base, double height) to the header and the source file, and use it from main.
3. Leave geometry.c out of the command and read the linker error.
4. Add a static helper function to geometry.c and try to call it from main.c.