Date: Mon, 28 Feb 2011 03:09:56 +0100 From: Pieter de Goeje <pieter@degoeje.nl> To: freebsd-hackers@freebsd.org Cc: Yuri <yuri@rawbw.com> Subject: Re: Is pthread_cond_signal(3) man page correct? Message-ID: <201102280309.56631.pieter@degoeje.nl> In-Reply-To: <4D6AC17A.7020505@rawbw.com> References: <4D6ABA14.80208@rawbw.com> <4D6AC17A.7020505@rawbw.com>
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On Sunday 27 February 2011 22:26:18 Yuri wrote:
> Also I want to add that I came to this question while observing behavior
> consistent with multiple wakeup on FreeBSD-8.1. The heavily
> multi-threaded code that assumes that only one thread can be woken up by
> one pthread_cond_signal call crashes, and the only reasonable
> explanation so far is that more than one threads are actually being
> woken up.
pthread_cond_signal() can indeed wake up more than one thread. That's why you
should always wrap pthread_cond_wait() in a loop. For example a blocking
queue could be implemented like this (pseudo code):
take() {
pthread_mutex_lock(mutex);
while(queue->empty()) {
pthread_cond_wait(cond, mutex);
}
item = queue->get();
pthread_mutex_unlock(mutex);
return item;
}
put(item) {
pthread_mutex_lock(mutex);
queue->put(item);
pthread_cond_signal(cond);
pthread_mutex_unlock(mutex);
}
pthread_cond_signal() itself never blocks.
Hope this helps.
--
Pieter de Goeje
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