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Date: Wed, 25 Nov 2020 19:38:07 -0500
From: Rich Felker <>
To: Dominic Chen <>
Subject: Re: [PATCH] Fix crash in malloc_usable_size() if nullptr

On Wed, Nov 25, 2020 at 06:41:14PM -0500, Dominic Chen wrote:
> 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 for the followup. When implementing nonstandard extensions, we
generally try to follow existing practice for what applications can
"portably" expect from them. Here it sounds like accepting null is
widespread, so I think it makes sense to apply your patch.


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