Write a C Macro That Manipulates Pointers

Question

## Problem

Write a C macro (e.g., `container_of`, or a swap/offset macro) that manipulates pointers correctly, and explain the pitfalls.

## Example
```
#define container_of(ptr, type, member) ...
```

## Constraints
- Avoid double-evaluation and precedence bugs

## What Fastly looks for
Low-level C fluency: pointer arithmetic, macro hygiene (parenthesize args), and offsetof.