Creating a mutex class using Allegro mutexes

Question

I'm trying to create a custom mutex class based on the Allegro mutex (cross-platform). I am not using C++11. I'm trying to do it like this, but it seems to be a bit superfluous:

template <class T>
class Mutex
  {

  private:
  ALLEGRO_MUTEX *mutex = NULL;
  ALLEGRO_THREAD *owner = NULL;
  T *data = NULL;

  public:
  Mutex ();
  Mutex (T* data);
  ~Mutex ();
  bool lock (ALLEGRO_THREAD* thread);
  bool unlock (ALLEGRO_THREAD* thread);
  bool trylock (); //Returns true if you can lock the thread
  bool set (T* data, ALLEGRO_THREAD* thread);
  bool get (T** data, ALLEGRO_THREAD* thread); //Pass reference of pointer

  }

The constructors will call al_create_mutex() and, in the case of the second one, set the data pointer. The destructor calls al_destroy_mutex().

The lock method will check to see if owner is NULL (indicating the mutex is unlocked and no thread owns it). If it is NULL, then it will set owner = thread and lock the mutex. The unlock method unlocks the mutex if thread == owner and sets owner = NULL.

The trylock method simply checks to see if owner == NULL, indicating that the mutex is not locked by any thread.

The set and get methods will set and return the stored pointer, only if the calling thread is the owner of the locked mutex.

I have a feeling that i'm going a bit overboard with the whole passing the thread every time you want to call a method, but i don't see any other way to make this class thread-safe, since Allegro doesn't have a al_get_current_thread() function or something like that. What should i do with this class?

You have initializers for non-static class members - you are in fact using C++11. ;) — Casey, Jun 20 '13 at 18:45

score 1 · Accepted Answer · answered Jun 20 '13 at 19:50

I strongly suggest that you create a class that only wraps the Allegro mutex functionality, and does nothing else. Then make a separate template class that uses a mutex to provide exclusive access to a datum.

Your accesses to Mutex::owner require synchronization to avoid data races. If you really want to provide recursive and/or trylock behavior, do that in a separate class or classes that use your basic mutex in their implementation. (Hint: use one mutex to protect the owner field and another to provide the actual mutual exclusion to clients).

I suspect that a basic RAII wrapper without the ALLEGRO_THREAD arguments is all you will really end up needing.

Creating a mutex class using Allegro mutexes

1 Answers1