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From: Pieter de Goeje <pieter@degoeje.nl>
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Date: Mon, 28 Feb 2011 03:09:56 +0100
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Cc: Yuri <yuri@rawbw.com>
Subject: Re: Is pthread_cond_signal(3) man page correct?
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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