# C pragma once

**[#pragma once](https://en.wikipedia.org/wiki/Pragma_once)** is a non-standard preprocessor directive that prevents multiple inclusion of a header file. It's simpler than [[header-guard|header guards]] and achieves the same goal, though it's not officially part of the C standard.

Modern compilers universally support `#pragma once`. Use it for cleaner headers, or use traditional header guards for portability.

## Example

This example shows pragma once preventing double inclusion of a header.

```c
// compile: gcc -o pragma pragma.c
// run: ./pragma
// description: #pragma once prevents header redefinition

// math_util.h
#pragma once

int add(int a, int b) { return a + b; }

// main.c
#include <stdio.h>
#include "math_util.h"
#include "math_util.h"  // second include is ignored

int main() {
    printf("2 + 3 = %d\n", add(2, 3));
    return 0;
}
```