How is pthread_join implemented?

Question

I'm a little new to threading, so you'll have to forgive the naiveté of this question.

How is pthread_join implemented and how does it effect thread scheduling?

I always pictured pthread_join implemented with a while loop, simply causing the calling thread to yield until the target thread completes. Like this (very approximate pseudocode):

atomic bool done;

thread_run {

    do_stuff();
    done = true;

}

thread_join {

    while(!done) {
        thread_yield();
    //  basically, make the thread that calls "join" on
    //  our thread yield until our thread completes
    }
}

Is this an accurate depiction, or am I vastly oversimplifying the process?

Cheers!

Answer 1

Yes that's the general idea. For gory details of a particular implementation take a look at glibc.

Answer 2

pthread_join is probably internally implemented as a wait for a semaphore which is triggered when the thread exits, either when it calls pthread_exit or when its main function exits.

In any case, the source code for glibc is available, try google code search (I saw some informative stuff in there)