Question

I have a group of pthreads which all need to complete a task, then block until the last of them completes the task, then have all of them move onto the next task. This happens for many tasks.

I was thinking of having a counter, which starts equal to the number of threads at the beginning of each task and, as each thread finishes a task, it decrements it safely with a mutex until the last thread makes it zero. At the end of the task, all threads except the last one to complete would call pthread_cond_wait, and the last thread would call pthread_cond_broadcast to tell the other threads to move onto the next task. Then the last thread would not wait, and would instead continue onto this task too.

However, I am left with one problem. There is no guarantee that the second to last thread to decrement the counter will call pthread_cond_wait before the last thread calls pthread_cond_broadcast.

Is there any way in C and POSIX to handle this safely?

Answer 1

The tool you’re looking for is barriers.

Initialize the barrier with the number of threads that will be waiting to reach it:

pthread_barrier_init(&barrier, NULL, N);

Then, each thread calls:

pthread_barrier_wait(&barrier);

The wait blocks until N threads are waiting on it, then all N wake up (conceptually) before any of them return from the wait call.