[ncmncm]

Amusingly, unions are also defined so that operations that pun types are undefined. Gcc has private extensions that provide a way to pun types in unions, but those are not portable.

Specifically: if you have a union with members a and b, and you assign into member a, then reading from member b afterward is, by the C and C++ Standards, usually undefined. You are presumed to have some way to know that member a is live, and use that. You can then write into b, and later read that back out.

[not2b]

The C99 and C11 standards say that the behavior is unspecified (in standard-ese, "unspecified" and "undefined" are different), meaning that the standard doesn't say what you get. Of course that's true: the result is different on a big-endian and a little-endian machine. Packing the bytes of a double might generate a signalling NaN so you get a trap when you read it, or not, and the standards don't require that floating point representations match the IEEE standard, either.

[tpush]

That’s undefined only in C++ but not in C, AFAIK.