Unfortunately I can't change printf_handler()'s arguments — it's a callback routine for the external library, whose API I can't alter.
My words "non conforming" came from that PDF slideshow I pointed to. Looking at the C99 spec (Draft of May 6, 2005) though, footnote 215 does say you can take the address of a va_list and pass it to other functions, so Perl's API isn't broken as I originally thought. It's not obvious how passing the va_list by value affects the ability to then take the address of the copy, but it certainly seems to.
Hmm, could I copy the original va_list object using va_copy() and then take the address of the copy? That's not particularly efficient, but efficiency doesn't matter much in this case which is in an error path anyway...
Ok, so va_copy() isn't defined, but using __va_copy() works on both 32 and 64-bit systems, with no warnings. Of course using __va_copy() isn't going to be portable to other compilers, but I can probably live with a special case for 64-bit Linux.
