Question

I am confused as to mutex works on code segment or variables in a code segment.

In the example below, mutex will prevent two threads trying to access mysum at the same time either from func1 or func2 or only the code-segment between mutex lock and unlock is protected

   .
   .
     pthread_mutex_t myMutex;
     int mysum;

  void func1(){
  .
  .
  .

  pthread_mutex_lock (&myMutex);
     mysum--;
     printf("Thread %ld did mysum=%d\n",id,mysum);
  pthread_mutex_unlock (&myMutex);
  .
  .
  }

  void func2(){
  .
  .
  .
  mysum++;

  pthread_mutex_lock (&myMutex);
     mysum++;
     printf("Thread %ld did mysum=%d\n",id,mysum);
  pthread_mutex_unlock (&myMutex);
  .
  .
  }

  int main (int argc, char *argv[])
  {
    pthread_mutex_init(&myMutex, NULL);
  .
  .

    pthread_create(&callThd1, &attr, func1, NULL); 

    pthread_create(&callThd2, &attr, func2, NULL); 
      pthread_create(&callThd3, &attr, func1, NULL); 

    pthread_create(&callThd4, &attr, func2, NULL); 
    pthread_create(&callThd5, &attr, func1, NULL); 

    pthread_create(&callThd6, &attr, func2, NULL); 
  .
  .


    pthread_mutex_destroy(&myMutex);
  .
  .

  }

Answer 1

The mutex only provides mutual exclusion between the code sections that lie between pthread_mutex_lock() and pthread_mutex_unlock() (these are known as critical sections).

There’s no direct link between the mutex and the shared data it’s protecting – it is up to you to ensure that you only access the shared data with the appropriate mutex locked.