On Thu, Jun 19, 2025 at 05:38:15PM +0200, Thorsten Glaser wrote: > On Thu, 19 Jun 2025, Rich Felker wrote: > > >> + The glibc implementation of realloc() is not consistent with > >> + that, and as a consequence, it is dangerous to call > >> + realloc(p, 0) in glibc. > > > >It's not dangerous if you know what it's doing. Rather it's > >non-portable. > > Nope. > > It’s actually dangerous in all libcs. > > GCC is a repeat offender of taking things that are Undefined > Behaviour in C (and GCC 15 even defaults to C23) and optimising > in a way that breaks programs and libraries that depend on the > behaviour of the respektive system and libc, which they even > guarantee. > > This is an unperiodic reminder that GCC lacks a -std=posix2024 > and similar. > > This is also why I was a bit angry that C23 made it UB. Had > they made it unspecified (POSIX verbiage) / IB (C verbiage), > implementations could actually do things and compilers would > not be allowed to break things that rely on it, i.e. it would > merely have been unportable. But when ISO C says UB it’s not > unportable, it’s dangerous.
OK, this is a legitimate point in support of "dangerous", but it only applies with -std=c23 or similar. When targeting an older version of the language standard, the UB does not apply. It would be interesting to know if GCC and/or LLVM have any particular intent on this yet. My hope would be that they wait to react until a better consensus is achieved. Rich
