Question

I am in a soup. The idea may be bad but i do need a solution.

• I have two condition variable, say A and B.

• Threads 1, 2 and 3 are waiting on A. Thread 4 is waiting on B.

• B will be pthread_cond-signal() by thread 2, that is thread 4 will be
  signaled to wake up by thread 2.

Now, I have another thread 5 which pthread_cond_broadcasts() on condition variable A. I need all threads 1, 2 and 3 to wake up before thread 4 wakes up. That is say if thread 2 wakes up and signals on B thread 4 may wake up before thread 3 does, which is not what i want.

Any pointers will be highly appreciated.

thanks

Answer 1

Use a semaphore: have each of threads 1-3 post the semaphore, and have thread 4 wait on the semaphore 3 times instead of on a condition variable.

You’ll want to use sem_init(3) or sem_open(3) to create the semaphore, sem_post(3) to post the semaphore, sem_wait(3) to wait on the semaphore, and then either sem_destroy(3) (if created with sem_init) or sem_close(3) and sem_unlink(3) (if created with sem_open) to destroy the semaphore.