c - How can I wait for any/all pthreads to complete?

Question

I just want my main thread to wait for any and all my (p)threads to complete before exiting.

The threads come and go a lot for different reasons, and I really don't want to keep track of all of them - I just want to know when they're all gone.

wait() does this for child processes, returning ECHILD when there are no children left, however wait does not (appear to work with) (p)threads.

I really don't want to go through the trouble of keeping a list of every single outstanding thread (as they come and go), then having to call pthread_join on each.

As there a quick-and-dirty way to do this?

Answer 1

Do you want your main thread to do anything in particular after all the threads have completed?

If not, you can have your main thread simply call pthread_exit() instead of returning (or calling exit()).

If main() returns it implicitly calls (or behaves as if it called) exit(), which will terminate the process. However, if main() calls pthread_exit() instead of returning, that implicit call to exit() doesn't occur and the process won't immediately end - it'll end when all threads have terminated.

• http://pubs.opengroup.org/onlinepubs/007908799/xsh/pthread_exit.html

Can't get too much quick-n-dirtier.

Here's a small example program that will let you see the difference. Pass -DUSE_PTHREAD_EXIT to the compiler to see the process wait for all threads to finish. Compile without that macro defined to see the process stop threads in their tracks.

#include <stdio.h>
#include <stdlib.h>
#include <pthread.h>
#include <time.h>

static
void sleep(int ms)
{
    struct timespec waittime;

    waittime.tv_sec = (ms / 1000);
    ms = ms % 1000;
    waittime.tv_nsec = ms * 1000 * 1000;

    nanosleep( &waittime, NULL);
}

void* threadfunc( void* c)
{
    int id = (int) c;
    int i = 0;

    for (i = 0 ; i < 12; ++i) {
        printf( "thread %d, iteration %d
", id, i);
        sleep(10);
    }

    return 0;
}


int main()
{
    int i = 4;

    for (; i; --i) {
        pthread_t* tcb = malloc( sizeof(*tcb));

        pthread_create( tcb, NULL, threadfunc, (void*) i);
    }

    sleep(40);

#ifdef USE_PTHREAD_EXIT
    pthread_exit(0);
#endif

    return 0;
}