What is the Purpose of argument in pthread_exit?

Question

basically, I am trying to understand the real purpose of pthread_exit. As you can see, there are multiple pthread_exit code I have tried. And Following are the result i observe :

- Exit 1: 42
- Exit 2: 42
- Exit 3: thread failed
- Exit 4: error
- Exit 5: 42
- Without a pthread_exit statement: 42

Anyways the value passed(10) to pthread_exit is ignored(Exit 2) and prints the value which we modified via the pointer(42). So what is the real purpose of argument of pthread_exit here? confusing.

int a;
void *myThread(void *result)
{
       int a = 5;
        *((int*)result) = 42;
        pthread_exit(result);      // Exit 1
      //pthread_exit((void *)10);  // Exit 2
      //pthread_exit(0);           // Exit 3
      //pthread_exit();            // Exit 4
      //pthread_exit((void *)&a);  // Exit 5
}
int main()
{
   pthread_t tid;
   void *status = 0;
   int result;

   pthread_create(&tid, NULL, myThread, &result);
   pthread_join(tid, &status);

   if (status != 0 ) {
       printf("%d\n",result);
   } else {
       printf("thread failed\n");
   }
   return 0;
}

Answer 1

pthread_exit() takes the pointer value you pass it and arranges for the pointer value to be returned into the void * variable whose address is passed to pthread_join().

In your case, this means that the value passed to pthread_exit() will end up in the status variable in main(). You never print the contents of status - all you do is test it against NULL in the if () condition. The value you are printing is the value stored in result, which is not modified by the pthread_exit() or pthread_join(), so of course it will always be the same.

In your my_thread() function, the pointer result is always going to be the address of the result variable in main(), so these are the cases you will see:

pthread_exit(result);      // Exit 1

In main(), status will end up equal to (void *)&result, which is necessarily non-NULL, so the test succeeds. It then prints the value of result, which was set to 42 by the first line of my_thread().

pthread_exit((void *)10);  // Exit 2

In main(), status will end up equal to (void *)10. On any common implementation of C, this value will compare unequal to NULL, so the test succeeds. It then prints the value of result, which was set to 42 by the first line of my_thread().

pthread_exit(0);           // Exit 3

In main(), status will end up equal to (void *)0. This necessarily compares equal to NULL, so the test fails. It then prints "thread failed".

If you print the value of the status pointer with:

printf("status = %p\n", status);

before the if(), you will be able to see how the value passed to pthread_exit() is returned.