RWLock

The GRWLock struct is an opaque data structure to represent a reader-writer lock.

It is similar to a Mutex in that it allows multiple threads to coordinate access to a shared resource.

The difference to a mutex is that a reader-writer lock discriminates between read-only ('reader') and full ('writer') access. While only one thread at a time is allowed write access (by holding the 'writer' lock via writer_lock), multiple threads can gain simultaneous read-only access (by holding the 'reader' lock via reader_lock).

It is unspecified whether readers or writers have priority in acquiring the lock when a reader already holds the lock and a writer is queued to acquire it.

Here is an example for an array with access functions:

  GRWLock lock;
  GPtrArray *array;

  gpointer
  my_array_get (guint index)
  {
    gpointer retval = NULL;

    if (!array)
      return NULL;

    g_rw_lock_reader_lock (&lock);
    if (index < array->len)
      retval = g_ptr_array_index (array, index);
    g_rw_lock_reader_unlock (&lock);

    return retval;
  }

  void
  my_array_set (guint index, gpointer data)
  {
    g_rw_lock_writer_lock (&lock);

    if (!array)
      array = g_ptr_array_new ();

    if (index >= array->len)
      g_ptr_array_set_size (array, index+1);
    g_ptr_array_index (array, index) = data;

    g_rw_lock_writer_unlock (&lock);
  }

This example shows an array which can be accessed by many readers (the my_array_get function) simultaneously, whereas the writers (the my_array_set function) will only be allowed one at a time and only if no readers currently access the array. This is because of the potentially dangerous resizing of the array. Using these functions is fully multi-thread safe now.

If a RWLock is allocated in static storage then it can be used without initialisation. Otherwise, you should call RWLock on it and g_rw_lock_clear when done.

A GRWLock should only be accessed with the g_rw_lock_ functions.

Example: RW-Locks:

public class Context : Object {
	// Lock used to protect data:
	private RWLock rwlock = RWLock ();

	// Shared Data:
	private HashTable<string, string> users = new HashTable<string, string> (str_hash, str_equal);
	private List<string> messages = new List<string> ();

	public Context () {
		users.insert ("root", "123");
		users.insert ("mean", "abc");
		users.insert ("square", "xyz");
	}

	private bool worker_func (string username, string password, string message) {
		// Login:
		rwlock.reader_lock ();
		string? stored_pw = users.lookup (username);
		rwlock.reader_unlock ();
		if (stored_pw == null || stored_pw != password) {
			print ("%s: Invalid user name or password\n", username);
			return false;
		}

		// Read all messages:
		rwlock.reader_lock ();
		messages.foreach ((str) => {
			print ("%s: %s\n", username, str);
		});
		rwlock.reader_unlock ();


		// Share a message:		
		rwlock.writer_lock ();
		messages.append (message);
		rwlock.writer_unlock ();

		return true;
	}

	public void run () {
		Thread<bool> worker1 = new Thread<bool> ("worker-1", () => {
			string username = "root";
			string password = "123";
			string message = "roots message";
			return worker_func (username, password, message);
		});

		Thread<bool> worker2 = new Thread<bool> ("worker-2", () => {
			string username = "mean";
			string password = "abc";
			string message = "abcs message";
			return worker_func (username, password, message);
		});

		Thread<bool> worker3 = new Thread<bool> ("worker-3", () => {
			string username = "square";
			string password = "xyz";
			string message = "squares message";
			return worker_func (username, password, message);
		});

		worker1.join ();
		worker2.join ();
		worker3.join ();
	}

	public static int main (string[] args) {
		if (Thread.supported () == false) {
			print ("Threads are not supported.\n");
			return 0;
		}

		// Output:
		//  ``mean: squares message``
		//  ``root: squares message``
		//  ``root: abcs message``
		// OR
		//  ``square: abcs message``
		//  ``square: roots message``
		// OR
		//  ``root: squares message``
		//  ``mean: squares message``
		// OR
		//  ``square: abcs message``
		// OR
		//  ...
		Context context = new Context ();
		context.run ();
		return 0;
	}
}

valac --target-glib 2.32 GLib.RWLock.vala