Mutex

The Mutex struct is an opaque data structure to represent a mutex (mutual exclusion).

It can be used to protect data against shared access.

Take for example the following function:

  int
  give_me_next_number (void)
  {
    static int current_number = 0;

    // now do a very complicated calculation to calculate the new
    // number, this might for example be a random number generator
    current_number = calc_next_number (current_number);

    return current_number;
  }

It is easy to see that this won't work in a multi-threaded application. There current_number must be protected against shared access. A Mutex can be used as a solution to this problem:

  int
  give_me_next_number (void)
  {
    static GMutex mutex;
    static int current_number = 0;
    int ret_val;

    g_mutex_lock (&mutex);
    ret_val = current_number = calc_next_number (current_number);
    g_mutex_unlock (&mutex);

    return ret_val;
  }

Notice that the Mutex is not initialised to any particular value. Its placement in static storage ensures that it will be initialised to all-zeros, which is appropriate.

If a Mutex is placed in other contexts (eg: embedded in a struct) then it must be explicitly initialised using Mutex.

A Mutex should only be accessed via g_mutex_ functions.