Bug 548967 - 1 bit mutex lock

Add support for a mutex lock that consumes only one bit of storage
inside of an integer on systems that support futexes.  Futex is emulated
(at a higher cost) on systems that don't have it -- but only in the
contended case.

glib/gbitlock.h

@@ -0,0 +1,43 @@
+/*
+ * Copyright © 2008 Ryan Lortie
+ * Copyright © 2010 Codethink Limited
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2 of the licence, or (at your option) any later version.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with this library; if not, write to the
+ * Free Software Foundation, Inc., 59 Temple Place - Suite 330,
+ * Boston, MA 02111-1307, USA.
+ *
+ * Author: Ryan Lortie <desrt@desrt.ca>
+ */
+
+#ifndef __G_BITLOCK_H__
+#define __G_BITLOCK_H__
+
+#include <glib/gtypes.h>
+
+#if defined(G_DISABLE_SINGLE_INCLUDES) && !defined (__GLIB_H_INSIDE__) && !defined (GLIB_COMPILATION)
+#error "Only <glib.h> can be included directly."
+#endif
+
+G_BEGIN_DECLS
+
+void      g_bit_lock                      (volatile gint *lock,
+                                           gint           lock_bit);
+gboolean  g_bit_trylock                   (volatile gint *lock,
+                                           gint           lock_bit);
+void      g_bit_unlock                    (volatile gint *lock,
+                                           gint           lock_bit);
+
+G_END_DECLS
+
+#endif /* __G_BITLOCK_H_ */