[libc] implement pthread condition variable support (pthread_cond_*)

[nickdesaulniers]

int pthread_cond_broadcast(pthread_cond_t ); int pthread_cond_destroy(pthread_cond_t ); int pthread_cond_init(pthread_cond_t , const pthread_condattr_t ); int pthread_cond_signal(pthread_cond_t ); int pthread_cond_timedwait(pthread_cond_t , pthread_mutex_t , const struct timespec ); int pthread_cond_wait(pthread_cond_t , pthread_mutex_t );

Specifically, FWICT libc++ depends on:

pthread_cond_broadcast
pthread_cond_destroy
pthread_cond_signal
pthread_cond_timedwait
pthread_cond_wait

(but not pthread_cond_init?? PTHREAD_COND_INITIALIZER is used instead) I don't think we implement any of these. We'll likely need to implement pthread_cond_init first to begin testing the rest.

Rust needs us to implement:

pthread_cond_broadcast
pthread_cond_wait

[llvmbot]

@llvm/issue-subscribers-libc

Author: Nick Desaulniers (nickdesaulniers)

int [pthread_cond_broadcast](https://pubs.opengroup.org/onlinepubs/7908799/xsh/pthread_cond_broadcast.html)(pthread_cond_t *); int [pthread_cond_destroy](https://pubs.opengroup.org/onlinepubs/7908799/xsh/pthread_cond_destroy.html)(pthread_cond_t *); int [pthread_cond_init](https://pubs.opengroup.org/onlinepubs/7908799/xsh/pthread_cond_init.html)(pthread_cond_t *, const pthread_condattr_t *); int [pthread_cond_signal](https://pubs.opengroup.org/onlinepubs/7908799/xsh/pthread_cond_signal.html)(pthread_cond_t *); int [pthread_cond_timedwait](https://pubs.opengroup.org/onlinepubs/7908799/xsh/pthread_cond_timedwait.html)(pthread_cond_t *, pthread_mutex_t *, const struct timespec *); int [pthread_cond_wait](https://pubs.opengroup.org/onlinepubs/7908799/xsh/pthread_cond_wait.html)(pthread_cond_t *, pthread_mutex_t *); Specifically, FWICT libc++ depends on: ``` pthread_cond_broadcast pthread_cond_destroy pthread_cond_signal pthread_cond_timedwait pthread_cond_wait ``` (but not pthread_cond_init??) I don't think we implement any of these. We'll likely need to implement pthread_cond_init first to begin testing the rest.

[elhewaty]

Hi, I am new to libc, but I have experience with LLVM. Can I work on this? are there relevant work I can look at?

[michaelrj-google]

You can absolutely try if you're interested, I will warn you it may be difficult. You can look at https://github.com/llvm/llvm-project/issues/85289 for a list of general things needed to add a new function.

For relevant work I'd recommend looking at the pthread functions that have already been implemented in LLVM-libc, they're in https://github.com/llvm/llvm-project/tree/main/libc/src/pthread

Feel free to reach out if you have more questions or want more advice on where to look.

[nickdesaulniers]

In particular, we seem to already have support for C11 threads. We have a CndVar type in libc/src/threads/linux/CndVar.h; perhaps that can be reused between C11 threads and pthreads?

[changkhothuychung]

I would like to take this if this is available, thanks!

[nickdesaulniers]

@elhewaty are you working on this? Or should I assign this to someone else?

(These may be more difficult to implement than some of our other missing functions)

[elhewaty]

I am sorry I will work on it, I was busy with working on this (https://github.com/llvm/llvm-project/pull/85592). If you think it'll be difficult for me you can suggest something that's not very hard but not trivial too.

Thanks

[changkhothuychung]

@nickdesaulniers just random curiosity. So currently, if a clang user tries to call these pthreadcond functions, it should compile, I believe. But these functions do not seem to be in libc yet, so I am curious what current implementations will be linked for those pthreadcond functions?

[nickdesaulniers]

But these functions do not seem to be in libc yet

Correct, this bug is about implementing them in llvm-libc.

So currently, if a clang user tries to call these pthreadcond* functions, it should compile, I believe.

If they're calling these and trying to use llvm libc then depending on what mode they're trying to use llvm-libc in, I'd expect 1 of 2 possible different failures:

• in fullbuild mode I'd expect a compile time diagnostic that these functions were not declared before used (I think this is an error in new C modes).
• in overlay mode then the code may compile+link, but I'd expect there'd probably be some kind of mysterious bugs due to possible ABI differences; we probably don't want users overlaying some of these pthread functions as opposed to all of them together.

[michaelrj-google]

What Nick says is correct, but also if you're just using clang and not LLVM-libc, then it will just use your system's libc.

Additionally, we don't support any of the pthread functions in overlay mode due to the problems with the LLVM-libc ABI and system libc ABI possibly not matching, so in overlay mode you'll get your system libc's version.

[nickdesaulniers]

Looking into this a bit closer, I suspect that if we can use a similar definition for our pthread_cond_t as our cnd_t, then we can probably reuse most (all?) of libc/src/threads/linux/CndVar.h. Looking at libc/test/integration/src/threads/cnd_test.cpp, it looks like to start testing any of these, we'll need pthread_cond_init, pthread_cond_signal, pthread_cond_wait, and pthread_cond_destroy all implemented together in one PR. Or perhaps just create+destroy in one PR to start.

I'm going to take this; @elhewaty I'm going to refer you to some of our perhaps easier beginner bugs: https://github.com/llvm/llvm-project/issues?q=is%3Aopen+is%3Aissue+label%3Alibc+label%3A%22good+first+issue%22

[nickdesaulniers]

It might also be useful to add support for pthread_condattr_init first. FWICT, pthread_condattr_t have no analog in C11 threads. https://docs.oracle.com/cd/E19683-01/806-6867/sync-67790/index.html has good docs relative to man pages on what that means.