standards - How do I byte-swap a signed number in C?

Question

I understand that casting from an unsigned type to a signed type of equal rank produces an implementation-defined value:

C99 6.3.1.3:

3. Otherwise, the new type is signed and the value cannot be represented in it; either the result is implementation-defined or an implementation-defined signal is raised.

This means I don't know how to byte-swap a signed number. For instance, suppose I am receiving two-byte, twos-complement signed values in little-endian order from a peripheral device, and processing them on a big-endian CPU. The byte-swapping primitives in the C library (like ntohs) are defined to work on unsigned values. If I convert my data to unsigned so I can byte-swap it, how do I reliably recover a signed value afterward?

Answer 1

As you say in your question the result is implementation-defined or an implementation-defined signal is raised - i.e. depends on the platform/compiler what happens.