Question 38554 in Ubuntu Question posted in
The official Ubuntu documentation can be found here.

Thread creation in C – Ubuntu

Can somebody please explain why the following loop for thread creation fails without the sleep function call?

   for(t=0; t<NUM_THREADS; t++) {
      printf("Main: creating thread %dn", t);
      rc = pthread_create(&thread[t], NULL, BusyWork, (void *)&t); 
      sleep(1);
      if (rc) {
         perror("pthread_create");
         exit(EXIT_FAILURE);
         }
      }

If sleep is not inserted then thread function seems to take as an argument
an arbitrary integer between 0 ad NUM_THREADS.

I’m running this on an Ubuntu machine.

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2

Answers


  1. 0

    You’re passing the same pointer to each thread, but still expecting each to see different values, which is not how things work.

    I don’t think it’s a good idea to let the main program end without doing anything to collect/join the threads back together, either.

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  2. 0

    Because you are passing t as a pointer, then change t after creating the thread. So each thread refers to the same variable. Which also is a great candidate for race condition bugs. Simply don’t do that.

    Instead create a hard copy per thread:

    type* tmp = malloc(sizeof(t));
memcpy(tmp, &t, sizeof(t));
pthread_create(&thread[t], NULL, BusyWork, tmp);
...

void* BusyWork (void* arg)
{
  ...
  free(arg);
}
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