Howard Hinnant wrote: [... cv.wait with recursively locked mutex ...]
This is the big question in my mind right now. I know how to make it work with a home-grown recursive mutex. But I don't know how to make it work with a native recursive mutex, combined with a native condition. I'm somewhat surprised that it doesn't just work.
It does just work. As long as you call cv.wait with a recursive mutex locked by the calling threads and its "lock count == 1". Calling it with "lock count != 1" is a bug (violation of a precondition in C++ terms).
Alexander, can we send a bug report to the pthreads guys?
http://www.opengroup.org/austin/defectform.html
Or would we just be laughed out of the room? I'm not educated enough to know what the problems are to making it work.
It is a philosophical issue, not technical. I wouldn't oppose if you would propose to add int pthread_mutex_setlockcount( // must be locked by the calling thread pthread_mutex_t * mutex, // must be positive (> 1 can be set on PTHREAD_MUTEX_RECURSIVE only) unsigned lockcount, // can be null unsigned * oldlockcount ); (or something like that, "get" aside for a moment) so that you could do [...] while (!predicate) { unsigned lockcount; pthread_mutex_setlockcount(&mutex, 1, &lockcount); pthread_cond_wait(&condvar, &mutex); pthread_mutex_setlockcount(&mutex, lockcount, 0); } [...] if/when you're ablsolutely 100% sure that what you're doing is OK.
And clearly it doesn't work today, so it's a problem whether or not it can be fixed. <sigh> That probably backs us into undefined behavior if the mutex is recursively locked,
Exactly. "may fail error" (in POSIX terms; optional "throws" that end up in std::unexpected with no return from the "failed" call). http://groups.google.com/groups?selm=3434E6BA.ABD%40zko.dec.com ---- The one omission in XSH5 recursive mutex support was a standard error code for the programming ERROR of trying to wait on a condition with a recursively locked mutex. If it's safe to release the recursive locks, the code should have done so. If it's not safe to release, then the runtime MUST NOT do so. ----
or refusing to compile at all with a recursive mutex.
No, that's not good. regards, alexander.