# Header guard
**Header guards** are a pattern used in C and C++ programming languages when writing header files. Header files (.h files) are files that are included in source files (.c and .cpp files) near the top (hence the name) using the `#include <...>` directive. They are used to allow multiple inclusions of the same file but prevent multiple definition errors. 

Header guards follow the same general pattern:
```c
#ifndef __HEADER_GUARD_H__
#define __HEADER_GUARD_H__

// ...

#endif /* __HEADER_GUARD_H__ (This is just a comment) */
```

In C and C++ programming languages, `#include` is nothing but text substitution. If you include something multiple times

## Example
A prototypical example of a header guard is the following.
```c
// hello_world.h
#ifndef __HELLO_WORLD_H__
#define __HELLO_WORLD_H__

struct Hello {
    const char *text = "world";
};

#endif /* __HELLO_WORLD_H__ */
```

This allows multiple inclusion of "hello\_world.h"
```c
// main.c
#include <hello_world.h>
#include <hello_world.h>

int main() {
    Hello world;
    return 0;
}
```