Implementation-Defined, Unspecified, Undefined

The three categories

• Implementation-defined behavior — the standard lets each implementation pick and document a choice: sizeof(int), plain char signedness, two's-complement representation (C23 made this required, ending a 35-year debate). Writing code that depends on the choice is legal if you check the documentation and code defensively.
• Unspecified behavior — several valid choices, no documentation duty, the implementation may differ call-to-call: evaluation order of function arguments. Correct code does not depend on it.
• Undefined behavior — no requirements at all: signed overflow, out-of-bounds access, data races. Modules 2 and 4 covered why the optimizer treats UB as permission.

The trap for professionals: implementation-defined is portable if asked for. sizeof(int) == 4 is a fact about this target, not a law of C — ask via <limits.h>/<stdint.h> types (int32_t) when the width matters.

Endianness, honestly

Byte order inside a multi-byte object is implementation-defined. This sandbox (aarch64 Linux) is little-endian, and GCC exposes __BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__ for compile-time detection. But the honest portable move is the runtime probe: place 1 in an unsigned int and inspect its first byte — that works on every C implementation, no macro required. Detect, then branch; never assume.

Feature detection, two clocks

• Compile time: #if defined(__BYTE_ORDER__) && __BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__ — zero runtime cost, but tied to the compiler's macros (compiler-specific, labeled).
• Run time: the byte probe, or stdbit.h's C23 endian utilities where available. Costs a branch once, portable everywhere.

Choose compile-time for hot paths, run-time for guaranteed portability. Document the choice; that documentation is the portability layer.

The capstone contract

Module 24's checkpoint assembles the course: a portable utility core (endian detection + checked arithmetic + safe copy) plus integration of earlier modules' discipline. Every function it asks for has been built and tested in an earlier module — the capstone is composition under specification, the actual work of systems engineering.