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POProblem with semaphores and sem_wait()
 

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  1. POProblem with semaphores and sem_wait()
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    <p>I have a queue structure that is being used by several pthreads. The threads are supposed to dequeue from the queue if it's not empty and then do their business.</p> <p>I initially had this set up as a while loop where the threads checked whether the queue was empty using a mutex_lock. Unfortunately this slowed my program down to a crawl.</p> <p>I tried to implement a semaphore as the "count" variable of my queue, but unfortunately I'm running into a segfault when I try and call sem_wait(). I've found the gdb and semaphore.h don't play well together, so I'm really at a loss. I may be making a novice mistake, so any help or suggestions would be appreciated.</p> <p>Queue structure:</p> <pre><code>typedef struct { int q[QUEUESIZE+1]; int first; int last; sem_t count; } queue; </code></pre> <p>Here is the initialization of it:</p> <pre><code>queue *CreateQueue(void) { queue *q; q = (queue*)malloc(sizeof(queue)); if (q == NULL) return NULL; q-&gt;first = 0; q-&gt;last = 0; sem_init(&amp;(q-&gt;count),0, 0); } </code></pre> <p>And I make sure that I call:</p> <pre><code> queue *q; q = CreateQueue(); </code></pre> <p>before any threads are created.</p> <p>Here is the call that seg faults</p> <pre><code>void *ThreadWait(void *t) { while(1) { sem_wait(&amp;(q-&gt;count)); //THIS SEGFAULTS ThreadFun(); //this is the function the thread would go to to do all the work } } </code></pre> <p>I'm hoping this is just a simple mistake on my part that I can't see right now.</p> <p>Thanks in advance.</p> <p>EDIT: to add some clarifying code</p>
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    1. COCan you post a bit more complete code. It's not clear if you init and wait on the same sem_t. Also do you have a core file? Stack traces?
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      1. USNikolai Fetissov
    2. CO@Nikolai Thanks, I added more code, don't have a core file or stack trace. Is there a way to grab a stack trace without gdb?
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    3. COAnother ambiguity in your sample code is the variable `q` in `ThreadWait()`. Is this a global variable? Could the problem be a memory over write? Also, are you using fork() or pthreads?
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