Synchronized access to data in shared memory between two processes

Question

I have two processes that have data in shared memory. This data is going to be updated by both of these process. I was looking for some locking mechanism between two processes. With threads it was easy to have a shared mutex lock. In my case, I tried to save the mutex variable in the shared memory, which then will be used by both processes for locking. This didn't work though. How do I share a mutex between two processes. Some say mutexes cannot be shared, use semaphores. Why mutex cannot be shared but semaphores can be?

Answer 1

It is possible, you have to use the flag PTHREAD_PROCESS_SHARED:

pthread_mutexattr_t mattr;
pthread_mutexattr_init(&mattr);
pthread_mutexattr_setpshared(&mattr, PTHREAD_PROCESS_SHARED);


// Init the shared mem barrier
if ( (rv = pthread_mutex_init(&nshared, &mattr)) != 0 ) {
      fprintf(stderr, "Failed to initiliaze the shared mutex.\n");
      return rv;
}

Where the variable nshared is mapped in shared memory.

Take a look at this documentation. Also, keep in mind that the default value for the mutex is to not share it among processes.

also, take a look at these posts post1 post2

Bonus code to chek the status of the mutex:

void showPshared(pthread_mutexattr_t *mta) {
  int           rc;
  int           pshared;

  printf("Check pshared attribute\n");
  rc = pthread_mutexattr_getpshared(mta, &pshared);

  printf("The pshared attributed is: ");
  switch (pshared) {
  case PTHREAD_PROCESS_PRIVATE:
    printf("PTHREAD_PROCESS_PRIVATE\n");
    break;
  case PTHREAD_PROCESS_SHARED:
    printf("PTHREAD_PROCESS_SHARED\n");
    break;
  default :
    printf("! pshared Error !\n");
    exit(1);
  }
  return;
}

I don't remember were I took this piece of code ... found it! here is the source of hal knowledge.