On Wed, Dec 02, 2009 at 02:36:49PM +0000, Sad Clouds wrote: | As far as Pthreads standards go, calling pthread_cond_signal() and | having the associated mutex unlocked is legal. | | Below is what the standard says: | | "The pthread_cond_signal() or pthread_cond_broadcast() functions | may be called by a thread whether or not it currently owns the mutex | that threads calling pthread_cond_wait() or pthread_cond_timedwait() | have associated with the condition variable during their waits; | however, if predictable scheduling behaviour is required, then that | mutex is locked by the thread calling pthread_cond_signal() or | pthread_cond_broadcast()." | | However reading NetBSD's man page, it suggests that having the mutex | unlocked might produce undefined results: | | "The same mutex must be held while calling pthread_cond_broadcast() | and pthread_cond_signal(). Neither function enforces this | requirement, but if the mutex is not held the resulting behaviour is | undefined." | | Now my question is: does anyone think that NetBSD's implementation | of pthread_cond_signal() is broken? I think quite a few people | intentionally unlock the mutex before calling pthread_cond_signal(), | so that the thread that wakes up can lock this mutex asap. and start | running, instead of blocking. AFAICT, the NetBSD manual page and/or the implementation are wrong in this respect. Based on my investigation on this topic recently, it is valid to call pthread_cond_signal() (and _broadcast()) without the mutex held, according to the POSIX standard and other implementations I've also used (Linux 2.6, Darwin). You'll get an unpredictable wakeup of the sleeping thread, and/or possibly a spurious wakeup, so you need to retest the condition after being awoken, but that should be it. I think it's explained well here, in a post quoting from Butenhof's "Programming with POSIX Threads": http://www.linuxforums.org/forum/linux-programming-scripting/135525-pthread_cond_signal-operation.html#post646525 The most recent POSIX standard (1003.1-2008 issue 7) is at: http://www.opengroup.org/onlinepubs/9699919799/functions/pthread_cond_signal.html On a related note, I haven't yet checked if NetBSD's pthread_cond_wait() returns EPERM on an unlocked mutex when used with a PTHREAD_MUTEX_ERRORCHECK mutex. Linux 2.6 does the right thing, Darwin 10.5 does not. cheers, Luke.
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