> On 8 Jun 2019, at 11:08, Chris Vine <vine35792...@gmail.com> wrote: > > 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.
FYI, the site mentioned above also covers C (there is a link the bottom of the C++ page above): https://en.cppreference.com/w/c/language/union
