Write a C Macro That Manipulates Pointers

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 to look for

Low-level C fluency: pointer arithmetic, macro hygiene (parenthesize args), and offsetof.