Hi! For context, the old discussion was in this thread: <https://inbox.sourceware.org/libc-alpha/nbyurzcgzgd5rdybbi4no2kw5grrc32k63svf7oq73nfcbus5r@77gry66kpqfr/>
Also for context, here's the excellent research by наб about malloc(0) and realloc(p, 0) in historic UNIX systems and their descendents: <https://nabijaczleweli.xyz/content/blogn_t/017-malloc0.html> We discussed last year about realloc(p, 0) being problematic currently in glibc. Ideally, realloc(p, n) should be consistent with malloc(n) in that: - It is equivalent to free(p) and malloc(n), regardless of the value of n, including when it is 0, and regardless of p, including when it is NULL. - Except that of course, if there's enough contiguous space, it doesn't free/malloc, and taht if malloc(n) fails, it doesn't free(p). This congruency existed in every UNIX system and their descendents, including the historic BSDs. It wasn't until new systems were written artificially by the letter of the standard, without regards to self-consistency, when this congruency was broken. I believe glibc was the first one to do that, and I don't know/remember if any other systems followed glibc. Paul Eggert and I are convinced that changing the implementation to conform to this behavior consistent with malloc(0) would be harmless. As such, we modified gnulib's realloc-posix module to conform to that. This was done in d884e6fc4a60 (2024-11-03, 2024-11-04; "realloc-posix: realloc (..., 0) now returns nonnull") After more than half a year of changing the behavior in gnulib, there have been no dramatic consequences. As expected, no fallout at all. Now, can we please make the same change in glibc? I've CCed musl and Elliott (Bionic), because I don't remember what's the behavior in their libraries. Is it already self-consistent in musl and Bionic? Or do they need a similar fix? glibc (and possibly libraries that attempt glibc compatibility) is the only libc implementation that is not self consistent, and by which ISO C has UB in realloc(p, 0). We expect that when glibc is fixed, realloc(p, 0) can be allowed again in ISO C, and can be specified to be consistent with malloc(0), thus removing a case of UB. Have a lovely day! Alex -- <https://www.alejandro-colomar.es/>
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