c - When to use pthread_exit() and when to use pthread_join() in Linux?

Question

I am new to pthreads, and I am trying to understand it. I saw some examples like the following.

I could see that the main() is blocked by the API pthread_exit(), and I have seen examples where the main function is blocked by the API pthread_join(). I am not able to understand when to use what?

I am referring to the following site - https://computing.llnl.gov/tutorials/pthreads/. I am not able to get the concept of when to use pthread_join() and when to use pthread_exit().

Can somebody please explain? Also, a good tutorial link for pthreads will be appreciated.

#include <pthread.h>
#include <stdio.h>
#define NUM_THREADS     5

void *PrintHello(void *threadid)
{
   long tid;
   tid = (long)threadid;
   printf("Hello World! It's me, thread #%ld!
", tid);
   pthread_exit(NULL);
}

int main (int argc, char *argv[])
{
   pthread_t threads[NUM_THREADS];
   int rc;
   long t;
   for(t=0; t<NUM_THREADS; t++){
      printf("In main: creating thread %ld
", t);
      rc = pthread_create(&threads[t], NULL, PrintHello, (void *)t);
      if (rc){
         printf("ERROR; return code from pthread_create() is %d
", rc);
         exit(-1);
      }
   }

   /* Last thing that main() should do */
   pthread_exit(NULL);

Realized one more thing i.e.

pthread_cancel(thread);
pthread_join(thread, NULL);

Sometimes, you want to cancel the thread while it is executing. You could do this using pthread_cancel(thread);. However, remember that you need to enable pthread cancel support. Also, a clean up code upon cancellation.

thread_cleanup_push(my_thread_cleanup_handler, resources);
pthread_setcancelstate(PTHREAD_CANCEL_ENABLE, 0);

static void my_thread_cleanup_handler(void *arg)
{
  // free
  // close, fclose
}

Answer 1

As explained in the openpub documentations,

pthread_exit() will exit the thread that calls it.

In your case since the main calls it, main thread will terminate whereas your spawned threads will continue to execute. This is mostly used in cases where the main thread is only required to spawn threads and leave the threads to do their job

pthread_join will suspend execution of the thread that has called it unless the target thread terminates

This is useful in cases when you want to wait for thread/s to terminate before further processing in main thread.