On Sat, 08 Jun 2019 10:07:45 +0200 Arne Babenhauserheide <arne_...@web.de> wrote: [snip] > Wow, I didn’t know that you could do that. > > However: "The details of that allocation are implementation-defined, and > it's undefined behavior to read from the member of the union that wasn't > most recently written." https://en.cppreference.com/w/cpp/language/union > > Can you guarantee that this works?
This is C and not C++ and the provision to which you refer does not apply. Reading from a member of a union other than the one last written to is implementation defined in C89/90, and defined in C99 (with Technical Corrigendum 3) and above, although it might include a trap representation (but wouldn't on any platform supported by guile). You might want to see in particular footnote 95 of C11 (which isn't normative but is intended to explain the provisions of §6.2.6.1 which are). gcc and clang have always supported type punning through unions. Chris
