threads and semaphores

[jas_atwal]

I'm using the following code to demonstarte threads and semaphores. it is works well all the way but the program hangs at the point where the threads join. They never join and program hangs there indefinatly. Anybody please, what am I doing wrong??!!

Main Code:

void* thread_function(void* arg);
sem_t bin_sem;

int main() {
 int res;
 void* thread_return;
 pthread_t a_thread;

 res = sem_init(&bin_sem, 0, 0);
 if(res != 0) {
        printf("Failed to initialize semaphore. Exiting..\n");
        exit(EXIT_FAILURE);
 }

 res = pthread_create(&a_thread, NULL, thread_function, NULL);
 if(res != 0) {
        printf("Failed to create a thread. Exiting..\n");
        exit(EXIT_FAILURE);
 }

 int i = 5;

 for(i = 5; i > 0; i--){
 printf("This is parent!!\n");
 sem_post(&bin_sem);
 sleep(1);
 }

 printf("Waiting for thread to end..\n");
 res = pthread_join(a_thread, &thread_return);
 if(res != 0){
        printf("Failed to join thread. Exiting . . \n");
        exit(EXIT_FAILURE);
 }

 printf("Thread joined\n");
 sem_destroy(&bin_sem);
 exit(EXIT_SUCCESS);
}

Thread Code:

void* thread_function(void* arg) {
  sem_wait(&bin_sem);

  int i;
  for( i=5; i > 0; i--){
        printf("This is child!!\n");
        sleep(1);
        sem_wait(&bin_sem);
  }
  pthread_exit(NULL);
}

[Cactus_Hugger]

I'm not an expert on semaphores, but it seems your thread makes 6 calls to sem_wait, while the parent only calls sem_post 5 times, and so your thread is waiting on a final sem_post that will never come, and hangs, which in turn hangs your thread join.

If you put a printf() outside of the for() loop in the thread, my guess is you'll never see it. (But you'll see 5 "This is child!!")