Re: [musl] sizeof(pthread_mutex_t) on aarch64

[Rich Felker <dalias@libc.org>]

On Wed, Aug 23, 2023 at 02:09:35AM +0000, Mike Granby wrote:
> I've been running glibc-compiled programs on Alpine and thus on musl
> with a high level of success, but I just hit the wall when working
> on a Raspberry PI running aarch64 Alpine. I tracked the issue down
> to a difference in the size of pthread_mutex_t. It appears that musl
> uses 6 ints for platforms with 32-bit longs, and 10 ints for those
> with 64-bit longs, and this seems to match glibc on all of the
> platforms I've played with to date. But on aarch64, it appears that
> glibc is using 48 bytes rather than 40 bytes that musl expects. This
> doesn't actually cause an issue in many cases as the application
> just allocates too much space, but if you're using inlining and
> std::ofstream, for example, you end up with the inline code in the
> application having a different layout for the file buffer object
> compared to that implemented on the target platform. Now, perhaps
> the answer is just, well, stop expecting to run glibc code on musl,
> but given that aarch64 seems to be an outlier in my experience to
> date, I thought I'd mention it.

I guess that's interesting to know, but not something actionable. It's
not like we can change and break ABI for the sake of glibc-ABI-compat,
nor like we'd want to make every program use even more excess memory
space for mutexes than they're already using. At some point the
decision was made, based on our existing practice at the time and
possibly loosely on glibc doing the same, to have the pthread types be
arch-independent and only depend on wordsize, and this determined the
ABI for all archs added later, without the types getting evaluated for
glibc-ABI-compat since they were no longer arch types.

FWIW I think the program would likely "work" with a glibc version of
libstdc++. However, there are a lot of other places the ABI-compat
breaks down, like mismatching stat structs, ipc structs, etc. The
future roadmap is for glibc-ABI-compat handling to be shifted out of
libc to the gcompat package, which could do some sort of shims (and
which musl ldso would assist it in shimming by letting a delegated
library interpose on modules that have libc.so.6 dep) and perhaps make
something like this work..

Rich