# <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;
}
```

## Common macros

**Runtime assertions**:
- `assert(condition)`: if condition is false, print diagnostic and abort:
  - Message format: `Assertion failed: (condition), file filename, line N`
  - Writes to stderr
  - Terminates with abort()

**Disabling assertions**:
- `NDEBUG`: preprocessor symbol; define before including `<assert.h>` to disable all assertions
- Compile with `-DNDEBUG` to remove assertions
- In release builds, assertions cost nothing (zero overhead)

**Usage patterns**:
- Check **preconditions** on entry (e.g., pointer not NULL)
- Check **invariants** that must hold (e.g., array index in bounds)
- Never put **side effects** in assertions (e.g., `assert(i++ < 10)`: the increment won't happen in release)
- Use for debugging; for user error handling use exceptions or error codes