# Headers, Multi-File Programs & Compilation — C Programming

Source: https://www.geekswithgeeks.com/en/c/c-headers-compilation

> How #include, header guards and the linker fit together.

## #include pastes a file in

`#include <stdio.h>` (angle brackets — system headers) or `#include "myheader.h"` (quotes — your own files) literally pastes that file's text at that point before compiling.

## Header guards prevent double inclusion

If two files both `#include` the same header, the guard ensures its contents are processed only once, avoiding "redefined" errors.

```c
#ifndef MATH_UTILS_H
#define MATH_UTILS_H

int add(int a, int b);
int sub(int a, int b);

#endif // MATH_UTILS_H
```

## Compile -> object files -> link

Each `.c` file compiles separately into an `.o` (object) file; the **linker** then stitches all the `.o` files and library code together into one executable.

## Compiling multiple files

gcc handles compiling and linking together in one command when given all sources.

```bash
gcc main.c math_utils.c -o app
./app
```