What is the strict aliasing rule and how do you safely reinterpret bytes?

Strict aliasing is the rule that lets the compiler assume two pointers of incompatible types never point at the same object (with a special exception for char *, which may alias anything). This assumption enables it to cache a value in a register across a write made through a different-typed pointer. If you violate it, e.g. write a float then read its bits through an int * cast, you get undefined behavior, and the optimizer may use a stale cached value or reorder accesses, miscompiling silently. The safe, portable way to reinterpret an object's bytes is memcpy(&dst, &src, sizeof dst): it's defined regardless of types, and compilers recognize the idiom and lower it to a plain move with no actual copy. In C (but not C++), reading a different member of a union than the one written is also permitted for type-punning. This matters in embedded when reinterpreting register values, serializing structs to byte buffers, or deserializing wire data.