# <assert.h>

**[<assert.h>](https://en.cppreference.com/w/c/header/assert)** provides the `assert` macro for runtime checking during development. It evaluates a condition and terminates the program with a diagnostic message showing the file, line, and failed condition if it's false.

Assertions are stripped entirely when compiled with `-DNDEBUG`, so they cost nothing in release builds. Never put side effects in assertions—they won't execute in production.

## Example

This example uses assertions to check preconditions, which can be completely removed in release builds.

```c
// compile: gcc -o assertexample assertexample.c
// run: ./assertexample
// description: assertions fire on invalid input; -DNDEBUG disables them entirely

#include <assert.h>
#include <stdio.h>

int divide(int a, int b) {
    assert(b != 0);
    return a / b;
}

int main() {
    printf("%d\n", divide(10, 2));
    printf("%d\n", divide(10, 0));  // assertion fires
    return 0;
}
```