Date: Wed, 25 Nov 2020 18:41:14 -0500 From: Dominic Chen <d.c.ddcc@...il.com> To: Rich Felker <dalias@...c.org> Cc: musl@...ts.openwall.com Subject: Re: [PATCH] Fix crash in malloc_usable_size() if nullptr On 11/25/2020 5:53 PM, Rich Felker wrote: > Thanks. I wasn't aware of this. I did some research to see if this is > actually documented as supported, since the Linux man pages aren't > normative but just descriptive, and sometimes document things that > aren't actually contracts. It seems glibc doesn't even document the > existence of this function at all though. > > FreeBSD documents it but without any special handling of null > pointers. But Solaris documents the same behavior you described. So it > seems this is at least not entirely glibc-specific. Do you know if > there are other implementations that do the same? Unfortunately, my understanding is just that this functionality (looking up heap allocation sizes) is platform-specific and not standardized. UNIX platforms seem to provide malloc_usable_size() (although it's not in SUSv4), whereas Mac provides malloc_size(), and Windows provides _msize(). Of other platforms, only the Windows documentation explicitly mentions null checks, but it calls a platform-specific invalid parameter handler instead of returning zero. On UNIX platforms, although documentation does not consistently describe returning zero for nullptr, I believe most do actually implement it. I took a look at the source for dlmalloc, glibc, jemalloc, and scudo, and can confirm that all four do so. Thanks, Dominic
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