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Date: Tue, 28 Jan 2020 08:29:00 -0500
From: Rich Felker <>
To: Leesoo Ahn <>
Subject: Re: Memory leak issue in multi-threaded program

On Tue, Jan 28, 2020 at 02:44:07PM +0900, Leesoo Ahn wrote:
> Dear musl developers,
> Hello!, it seems that musl currently has a memory leak issue in
> multi-threaded program. It occurs in the below situation of latest
> (v1.1.24) source. Also, not only in 32-bits[1], but also 64-bits[2]
> as well.
> When a program create and run, at least, two threads or more with
> pthread APIs, VSZ of the program by ps command keeps increasing. But
> here is a weird thing that it is fine 'IF ONLY ONE' pthread is
> created and run.
> To confirm the issue in your host machine, please follow the instructions,
> 0. Clone the musl git and get inside.
> 1. Build with these options for static build, ./configure
> --prefix=$(pwd)/_build_dir --disable-shared
> 2. Download the test code[3], then build with the command,
> ../_build_dir/bin/musl-gcc ./test.c
> 3. Run this script, ./a.out &; while [ 1 ]; do { ps aux | grep
> [a].out | grep -v grep; sleep 1; } done
> You may figure out that VSZ keeps increasing.
> BUT, when I make it to try to allocate memory all the time by kernel
> mmap with this diff[4] as workaround, although it creates more
> pthreads than 2, the issue never happens.
> It would be really thankful if you guys could confirm it and find
> out the way to fix the bug.

This is a known issue described in:

and likely several times before that, though it was not realized that
people were hitting it in practice (vs it just being theoretical)
until around that time. I posted an experimental mitigation patch last

but it's not heavily tested and its impact on performance is
significant. I think it should be ok if you need an immediate fix, but
you should do some testing to make sure. If you go this route, reports
of any problems (or success) would be nice to hear about.

Further work in that direction was not done because it was already
planned that musl's malloc implementation will be replaced, and that
the replacement will solve this and other problems in much better
ways. This is work in progress and is intended for merge in the next
release cycle:

Hope this information helps.


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