Subj : pthread_cond_signal() semantics
To   : comp.programming.threads
From : William Hoggarth
Date : Sat Jan 08 2005 11:32 pm

I have a thread with similar to the following:

  while(1) {
    get_input();

    pthread_mutex_lock(&mutex);
    copy_input_to_buffer();
    pthread_cond_signal(&cond);
    pthread_mutex_unlock(&mutex);
  }

The thread seems to reaquire the mutex lock before the thread I'm signalling
wakes up. Putting sched_yield() at the end of the loop results in the
behaviour I'm expecting: the thread waiting on the condition variable wakes
up, aquiring the mutex.

Does this mean that pthread_cond_signal() allows other threads to lock the
mutex before the waiting thread wakes up with the lock on the mutex?

Does pthread_cond_signal() just make the signalled thread runnable, with the
guarantee of the mutex lock when it runs?

What are the exact semantics of pthread_cond_signal()?

Thanks for the help,

William Hoggarth

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