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Date: Fri, 8 Aug 2014 16:15:29 +0400 (MSK)
From: Alexander Monakov <>
Subject: Replacing malloc

[changing topic, subject adjusted]

On Fri, 8 Aug 2014, Rich Felker wrote:
> The fourth issue is much bigger: replacing malloc is UB and does not
> work, especially not on musl. :-)

Whoa.  Let me ask for further clarifications.

You probably don't need me to tell you that most people expect that replacing
malloc would in fact work, with at least two use cases in mind: a tracking
wrapper around libc malloc (obtained via dlsym), or an entire custom allocator
that obtains fresh memory via mmap.  So you can LD_PRELOAD a "malloc
debugging library" or preload or even link against an alternative allocator.

Of course it's not without inherent issues.  If the alternative allocator
provides malloc/realloc/calloc/free, it's going to see an unexpected but
legitimate free when the application passes pointer obtained via
posix_memalign.  Or when the application obtains a free()-able pointer via
other libc functionality such as asprintf and the libc is linked in such a way
that internal malloc calls are not interposable.  Or on glibc a malloc wrapper
needs to handle malloc->dlsym->malloc recursion.

I hope above you didn't mean to say that anybody wishing to use malloc
wrappers or custom mmap-based malloc replacements on musl should abandon all
hope, period; but merely that it is not for production use, and attempting to
do so should be with care, for instance if gnash library uses custom malloc,
it may not return pointers to that memory to be free()'d by the main
executable (calling libc's free).  But it would be like that on any libc.  So
I have to wonder what "especially not on musl" stands for.

But so far every time I speak about a problem with musl the problem is
deeper than I initially think -- so please clarify :)


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