Re: Threading question (avoiding deadlock)

[Paul Davis <pbd Op Net>]

> I was hoping somebody could point me in the right direction here.

your periodic update thread should check a variable that tells it to
die. add a lock and a condition variable, and use it like this:

void
kill_periodic_thread ()
{
        pthread_mutex_lock(&lock_protecting_kill_flag);
        kill_flag = TRUE;
        pthread_cond_wait (&periodic_is_dead, &lock_protecting_kill_flag);
        /* at this point we know that barring a segfault,
           the periodic thread is going to exit.
         */
        pthread_mutex_unlock(&lock_protecting_kill_flag);
        pthread_join(); 
}

void *
periodic_thread (void *arg)
{
        while (!kill_flag) {
            .... do stuff ....
        }                          

        if (kill_flag) {
            ... clean up ...
            pthread_mutex_lock (&lock_protecting_kill_flag);
            pthread_cond_signal (&periodic_is_dead);
            pthread_mutex_unlock (&lock_protecting_kill_flag);
            pthread_exit ();
        }           
        ...
}

note: you don't really to need to lock before checking kill_flag on
most architectures, but you might prefer:

        pthread_mutex_lock (&lock_protecting_kill_flag);
        if (kill_flag) {
            ... clean up ...
            pthread_cond_signal (&periodic_is_dead);
            pthread_mutex_unlock (&lock_protecting_kill_flag);
            pthread_exit ();
        }           
        pthread_mutex_unlock (&lock_protecting_kill_flag);


if you want to be more formal. you might also be able to get away
without the condition variable, and just use:

        kill_flag = TRUE;
        pthread_join (...);

although this becomes harder to debug under some circumstances.

--p