Whats the difference between pthread_join and pthread_mutex_lock?

Question

The following code is taken from this site and it shows how to use mutexes. It implements both pthread_join and pthread_mutex_lock:

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

void *functionC();
pthread_mutex_t mutex1 = PTHREAD_MUTEX_INITIALIZER;
int  counter = 0;

main()
{
   int rc1, rc2;
   pthread_t thread1, thread2;

   /* Create independent threads each of which will execute functionC */

   if( (rc1=pthread_create( &thread1, NULL, &functionC, NULL)) )
   {
      printf("Thread creation failed: %d\n", rc1);
   }

   if( (rc2=pthread_create( &thread2, NULL, &functionC, NULL)) )
   {
      printf("Thread creation failed: %d\n", rc2);
   }

   /* Wait till threads are complete before main continues. Unless we  */
   /* wait we run the risk of executing an exit which will terminate   */
   /* the process and all threads before the threads have completed.   */

   pthread_join( thread1, NULL);
   pthread_join( thread2, NULL); 

   exit(EXIT_SUCCESS);
}

void *functionC()
{
   pthread_mutex_lock( &mutex1 );
   counter++;
   printf("Counter value: %d\n",counter);
   pthread_mutex_unlock( &mutex1 );
}

I ran the code as given above as it is and it produced following result:

Counter value: 1

Counter value: 2

But in the second run i removed "pthread_mutex_lock( &mutex1 );" and "pthread_mutex_unlock( &mutex1 );" . I compiled and ran the code, it again produced the same result.

Now the thing that confuses me is why mutex lock is used in above code when same thing can be done without it (using pthread_join)? If pthread_join prevents another thread from running untill the first one has finished then i think it would already prevent the other thread from accessing the counter value. Whats the purpose of pthread_mutex_lock?

Answer 1

The join prevents the starting thread from running (and thus terminating the process) until thread1 and thread2 finish. It doesn't provide any synchronization between thread1 and thread2. The mutex prevents thread1 from reading the counter while thread2 is modifying it, or vice versa.

Without the mutex, the most obvious thing that could go wrong is that thread1 and thread2 run in perfect synch. They each read zero from the counter, each add one to it, and each output "Counter value: 1".