[musl] sizeof(pthread_mutex_t) on aarch64

[Mike Granby <mikeg@mikeg.net>]

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.

Mike Granby
Granby Consulting LLC