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Message-ID: <13bc2483-b33f-413f-a370-588e33c6cd86@gmail.com>
Date: Wed, 12 Nov 2025 14:12:55 -0500
From: Demi Marie Obenour <demiobenour@...il.com>
To: musl@...ts.openwall.com, Fredrik Orderud <forderud@...il.com>,
Rich Felker <dalias@...c.org>
Subject: Re: Safe to load musl shared lib into glibc executable?
On 11/12/25 13:26, Fredrik Orderud wrote:
> On Wed, Nov 12, 2025 at 3:47 PM Rich Felker <dalias@...c.org> wrote:
>> If you're making a shared library, you cannot static link libc into
>> it. This is breaking whether you are using glibc or musl.
>
> Thanks a lot for quick and clear answers, Rich! I'll then continue
> with my current approach of linking to an old version of glibc when
> building the shared lib. in question.
>
>> The problem is that, even if you could embed your own libc instance
>> into your library, it will assume it has ownership of process-wide
>> singletons that the "real" host libc the rest of the program is using
>> also assumes it has ownership over. Things like the thread pointer
>> register, signal disposition, heap brk point, etc.
>>
>> In principle one could make a libc that's designed to be hosted by
>> something external to itself, with functionality limited to what it
>> could do without owning process state, but that is not what musl is.
>
> My background is mostly from Windows where it's always been simple to
> link statically to the C library when building a shared library. The
> glibc and musl limitations in this area therefore came as a surprise
> to me. Do you think it could be possible to update the musl
> documentation to more explicitly mention this limitation, so that
> this is clear for the next developer down the road?
This is one of the factors. The other is that Windows supports
two-stage symbol lookup, meaning that symbol X in your DLL can be
different than symbol X in a different DLL or the main executable.
This is a huge advantage for shipping stuff in binary form.
It would also be useful for musl to implement this, but I'm not going
to ask for it because I don't have the time to send a patch right now.
--
Sincerely,
Demi Marie Obenour (she/her/hers)
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