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POSimple Generator-monitor program with condition variables
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copied!<p>I created a simple program that uses condition variables to create synchronization between two threads. I'm getting a strange output that I cannot seem to find the solution to. </p> <p><strong>What the program does is</strong>, in the generator thread, produces 1000 random integers and checks to see if they are perfect squares. If the number is a perfect square then it signals the monitor thread which prints the square root of the number. </p> <p><strong>The problem</strong> I am having is most likely some sort of race condition, because the monitor simply isn't printing out the square root when the generator signals. </p> <p><strong>The strange part</strong> is that when I debug in gdb b stepping through each time the variable <code>is_square</code> changes, the problem is nonexistent. </p> <p>Any insight would appreciated. I feel it has to do with my mutex or placement of conditions.</p> <pre><code>#include <stdio.h> #include <stdlib.h> #include <math.h> #include <pthread.h> #include <time.h> int square_root; int is_square = 0; int done = 0; int count = 0; //used to count how many perfect squares generator_func finds int count1 = 0; //used to compare how many responses monitor makes to signal pthread_mutex_t mutex; pthread_cond_t mon; void* generator_func(void* args){ srand(time(NULL)); int i, temp, sq; for(i = 0; i<1000; i++){ temp = rand() % 10000; sq = sqrt((double)temp); if((pow(sq,2)) == temp){ pthread_mutex_lock(&mutex); count++; square_root = sq; is_square = 1; fprintf(stderr, "Square root of %d is", temp); pthread_cond_signal(&mon); pthread_mutex_unlock(&mutex); } } pthread_mutex_lock(&mutex); done = 1; is_square = -1; pthread_cond_signal(&mon); pthread_mutex_unlock(&mutex); } main(){ pthread_t generator; //declare thread pthread_mutex_init(&mutex, NULL); //initialize mutex pthread_cond_init(&mon, NULL); //initialize condition variable pthread_create(&generator, NULL, generator_func, NULL); //create thread //monitor while(done != 1){ pthread_mutex_lock(&mutex); while(is_square == 0){ pthread_cond_wait(&mon, &mutex); } if(is_square == 1 && done != 1){ count1++; fprintf(stderr, " %d\n", square_root); is_square = 0; } pthread_mutex_unlock(&mutex); } pthread_join(generator, NULL); printf("%d %d\n", count, count1); //shows inconsistency between generator and monitor pthread_mutex_destroy(&mutex); pthread_cond_destroy(&mon); } </code></pre>

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