# 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:**

```c
// functions.h
#ifndef FUNCTIONS_H
#define FUNCTIONS_H

// Function declaration
void print_hello();

#endif // FUNCTIONS_H
```

**functions.c:**

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

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

**main.c:**

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

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

**Compilation Steps:**

1. Compile `functions.c` into an object file:
    
    ```sh
    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:
    
    ```sh
    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:
    
    ```sh
    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:

```sh
./myprogram
```

This will output:

```plaintext
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.