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POPOSIX semaphores/threads not working correctly?
 

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  1. POPOSIX semaphores/threads not working correctly?
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    <p>I have the following code:</p> <pre><code>#include&lt;stdio.h&gt; #include&lt;semaphore.h&gt; #include&lt;pthread.h&gt; sem_t semr; void* func(void* i) { sem_wait(&amp;semr); printf("\nInstance %d running",*(int*)i); //sem_post(&amp;semr); return NULL; } int main(void) { sem_init(&amp;semr,0,1); void* (*fp)(void*); int s1,s2,s3,val=0; pthread_t t1,t2,t3; fp=&amp;func; val=1; s1=pthread_create(&amp;t1,NULL,fp,(void*)&amp;val); val=2; s2=pthread_create(&amp;t2,NULL,fp,(void*)&amp;val); val=3; s3=pthread_create(&amp;t3,NULL,fp,(void*)&amp;val); pthread_join(t1,NULL); pthread_join(t2,NULL); pthread_join(t3,NULL); return 0; } </code></pre> <h2>This is my understanding of what happens:</h2> <p>The first thread(<code>t1</code>) executes successfully. The subsequent threads(<code>t2</code> and <code>t3</code>) though, are blocked, since I never <code>sem_post</code> the semaphore. The <code>pthread_join</code>s will make <code>main()</code> wait for all 3 threads to terminate.</p> <h2>This is what happens:</h2> <p>Neither thread will output <em>anything</em>. Not even <code>t1</code>s output(see question 1 below)</p> <h2>However,</h2> <p>removing all <code>pthread_join</code>s has a better effect in terms of what I expect: <code>t1</code> executes successfully and the command prompt is returned.</p> <p>My questions:</p> <ol> <li><p>According to the sample code on <a href="http://www.yolinux.com/TUTORIALS/LinuxTutorialPosixThreads.html" rel="nofollow">this page</a>, <code>main()</code> should wait for <code>t2</code> and <code>t3</code> to terminate (in addition to successfully executing <code>t1</code> <em>and</em> outputting <em>something</em>). Am I using <code>pthread_join</code> incorrectly here? What's happening?</p></li> <li><p>Why happens to the blocked threads(<code>t2</code> and <code>t3</code>)? Are the threads forced to terminate due to <code>main()</code> returning?</p></li> </ol>
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    1. USKedar Paranjape
    1. USKedar Paranjape
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      1. POPOSIX semaphores/threads not working correctly?
      1. USKedar Paranjape
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    1. COYour program (once you've fixed `var` not being undeclared) does not do what you say it does. It **does** output something (a newline).
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      1. POPOSIX semaphores/threads not working correctly?
      1. USMat
    2. COThis code won't compile. `val` is never declared. Please post a complete example that exhibits your problems.
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      1. POPOSIX semaphores/threads not working correctly?
      1. USCB Bailey
    3. CO@black_station : I edited you code - added \n ..now copy this code and execute --will work fine.
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      1. POPOSIX semaphores/threads not working correctly?
      1. USGrijesh Chauhan
 

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