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copied!<p>You have two problems. The first is that you aren't using <code>while()</code> loops correctly - for example, here:</p> <pre><code>//wait for turn while( pthread_cond_wait(&condA, &mutex) != 0 ) i++; </code></pre> <p>The body of the <code>while</code> loop is the statement <code>i++</code> - this will execute <code>pthread_cond_wait()</code> and <code>i++</code> until the <code>pthread_cond_wait()</code> returns an error, so this is essentially an endless loop.</p> <p>The second is that you can't use a pthreads condition variable on its own - it needs to be paired with some actual shared state (at its simplest, this shared state might just be a flag variable protected by a mutex). The <code>pthread_cond_wait()</code> function is used to wait for the shared state to reach a certain value, and the <code>pthread_cond_signal()</code> function is used when a thread has altered the shared state. Reworking your example to use such a variable:</p> <pre><code>//global variables /* STATE_A = THREAD A runs next, STATE_B = THREAD B runs next */ enum { STATE_A, STATE_B } state = STATE_A; pthread_cond_t condA = PTHREAD_COND_INITIALIZER; pthread_cond_t condB = PTHREAD_COND_INITIALIZER; pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER; void *threadA() { int i = 0, rValue, loopNum; while(i<5) { /* Wait for state A */ pthread_mutex_lock(&mutex); while (state != STATE_A) pthread_cond_wait(&condA, &mutex); pthread_mutex_unlock(&mutex); //do stuff for(loopNum = 1; loopNum <= 5; loopNum++) printf("Hello %d\n", loopNum); /* Set state to B and wake up thread B */ pthread_mutex_lock(&mutex); state = STATE_B; pthread_cond_signal(&condB); pthread_mutex_unlock(&mutex); i++; } return 0; } void *threadB() { int n = 0, rValue; while(n<5) { /* Wait for state B */ pthread_mutex_lock(&mutex); while (state != STATE_B) pthread_cond_wait(&condB, &mutex); pthread_mutex_unlock(&mutex); //do stuff printf("Goodbye\n"); /* Set state to A and wake up thread A */ pthread_mutex_lock(&mutex); state = STATE_A; pthread_cond_signal(&condA); pthread_mutex_unlock(&mutex); n++; } return 0; } </code></pre> <p>Note that the use of two condition variables <code>condA</code> and <code>condB</code> is unnecessary here - the code would be just as correct if only one condition variable was used instead.</p>

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