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POPOSIX C Threads. pthread_cond_t example. Doesn't work as expected
 

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  1. POPOSIX C Threads. pthread_cond_t example. Doesn't work as expected
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    <p>I wrote a wrote a program and it doesn't work as I expect it to. I have two threads: <code>thread</code> triggers <code>func</code> and <code>anotherThread</code> triggers <code>anotherFunc</code>. What I wanted to do is when <code>cont</code> reaches value <code>10</code> in <code>func</code>, <code>anotherThread</code> to be triggered using <code>pthread_cond_wait</code> and <code>pthread_cond_signal</code>. The strange thing is everything works fine if i uncomment the <code>sleep(1)</code> line. I'm new to threads and I was following the tutorial <a href="https://computing.llnl.gov/tutorials/pthreads/#ConditionVariables" rel="noreferrer">here</a> and if I comment the <code>sleep</code> line in their example it breaks as well. </p> <p>My question is how can I make this work without any <code>sleep()</code> calls? And what happens if in my code both <code>func</code> reaches <code>pthread_mutex_lock</code> after <code>anotherFunc</code>? How can I control these things? This is my code:</p> <pre><code>#include &lt;iostream&gt; #include &lt;stdlib.h&gt; #include &lt;stdio.h&gt; #include &lt;pthread.h&gt; pthread_mutex_t myMutex; pthread_cond_t cond; pthread_attr_t attr; int cont; void *func(void*) { printf("func\n"); for(int i = 0; i &lt; 20; i++) { pthread_mutex_lock(&amp;myMutex); cont++; printf("%d\n", cont); if(cont == 10) { printf("signal:\n"); pthread_cond_signal(&amp;cond); // sleep(1); } pthread_mutex_unlock(&amp;myMutex); } printf("Done func\n"); pthread_exit(NULL); } void *anotherFunc(void*) { printf("anotherFunc\n"); pthread_mutex_lock(&amp;myMutex); printf("waiting...\n"); pthread_cond_wait(&amp;cond, &amp;myMutex); cont += 10; printf("slot\n"); pthread_mutex_unlock(&amp;myMutex); printf("mutex unlocked anotherFunc\n"); printf("Done anotherFunc\n"); pthread_exit(NULL); } int main(int argc, char *argv[]) { pthread_t thread; pthread_t anotherThread; pthread_attr_init(&amp;attr); pthread_mutex_init(&amp;myMutex, NULL); pthread_attr_setdetachstate(&amp;attr, PTHREAD_CREATE_JOINABLE); pthread_cond_init(&amp;cond, NULL); pthread_create(&amp;anotherThread, &amp;attr, anotherFunc, NULL); pthread_create(&amp;thread, &amp;attr, func, NULL); pthread_join(thread, NULL); pthread_join(anotherThread, NULL); printf("Done MAIN()"); pthread_mutex_destroy(&amp;myMutex); pthread_cond_destroy(&amp;cond); pthread_attr_destroy(&amp;attr); pthread_exit(NULL); return 0; } </code></pre> <p>Sorry for the long post but I'm new to threads and I'm willing to learn. Also do you know some good references or courses/tutorials on threads and networking on Linux? I want to learn create an chat client and I heard that I have to know threads and networking for that. Problem is I don't know pretty good if what I learn is ok since I don't know what I have to know.</p> <p>Thank you so much :)</p>
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    1. USPavan Manjunath
    1. USJacob Krieg
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      1. POPOSIX C Threads. pthread_cond_t example. Doesn't work as expected
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      1. POPOSIX C Threads. pthread_cond_t example. Doesn't work as expected
      1. USKaroly Horvath
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    1. COOther remarks. I addition to not having to use a `pthread_attr_t`, you can use static initializers for the condition and mutex to simplify the code. Furthermore, your thread functions can just `return NULL;` or `return 0;` instead of calling `pthread_exit`. And your main thread can just `return 0;` from main without calling `pthread_exit`. The other threads are not running at that point since they were joined. return from main forces a process exit, but `pthread_exit` in the primary thread does not force a process exit, which is useful for keeping other threads running.
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      1. POPOSIX C Threads. pthread_cond_t example. Doesn't work as expected
      1. USKaz
    2. COAlso, there is no need to clean up static mutexes and conditions. Just use `pthread_mutex_t myMutex = PTHREAD_MUTEX_INITIALIZER;`. The complex initializations are needed for situations where you want to set unusual attributes. E.g. make a process-shared robust mutex.
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