My C file is too big! Multifile source compilation

In this basic example, we will see how to use a header file, compile two source files into one executable, and separate a C program into its component parts using gcc.

File Structure:

  • main.c: The main source file.

  • functions.c: A source file containing additional functions.

  • functions.h: A header file containing function declarations.


// functions.h

// Function declaration
void print_hello();

#endif // FUNCTIONS_H


// functions.c
#include "functions.h"
#include <stdio.h>

// Function definition
void print_hello() {
    printf("Hello, World!\n");


// main.c
#include "functions.h"

int main() {
    // Call function from another file
    return 0;

Compilation Steps:

  1. Compile functions.c into an object file:

     gcc -c functions.c -o functions.o

    This command compiles functions.c into an object file functions.o without linking.

  2. Compile main.c into an object file:

     gcc -c main.c -o main.o

    This command compiles main.c into an object file main.o without linking.

  3. Link the object files into a single executable:

     gcc main.o functions.o -o myprogram

    This command links main.o and functions.o together to create the final executable myprogram.

After these steps, you will have an executable file named myprogram that you can run on your system. To execute the program, use:


This will output:

Hello, World!

This is only a basic example; in practice, your program's complexity will determine the number of source and header files you require, as well as whether or not you need to link with other libraries or include additional compiler options.