1 files changed, 48 insertions, 6 deletions
diff --git a/libs/pbd/pbd/crossthread.h b/libs/pbd/pbd/crossthread.h
index a791be5798..13ab37d38d 100644
--- a/libs/pbd/pbd/crossthread.h
+++ b/libs/pbd/pbd/crossthread.h
@@ -26,20 +26,60 @@
 
 #include <glibmm/main.h>
 
+/** A simple abstraction of a mechanism of signalling one thread from another.
+ * The signaller calls ::wakeup() to tell the signalled thread to check for
+ * work to be done. 
+ *
+ * This implementation provides both ::selectable() for use in direct
+ * poll/select-based event loops, and a Glib::IOSource via ::ios() for use
+ * in Glib main loop based situations. 
+ */
+
 class CrossThreadChannel { 
   public:
-	CrossThreadChannel(bool);
+	/** if @a non_blocking is true, the channel will not cause blocking
+	 * when used in an event loop based on poll/select or the glib main
+	 * loop.
+	 */
+	CrossThreadChannel(bool non_blocking);
 	~CrossThreadChannel();
 	
+	/** Tell the listening thread that is has work to do.
+	 */
 	void wakeup();
-	int selectable() const { return fds[0]; }
-
+	
+	/* if the listening thread cares about the precise message
+	 * it is being sent, then ::deliver() can be used to send
+	 * a single byte message rather than a simple wakeup. These
+	 * two mechanisms should not be used on the same CrossThreadChannel
+	 * because there is no way to know which byte value will be used
+	 * for ::wakeup()
+	 */
         int deliver (char msg);
+
+	/** if using ::deliver() to wakeup the listening thread, then
+	 * the listener should call ::receive() to fetch the message
+	 * type from the channel.
+	 */
         int receive (char& msg);
-        
+
+	/** empty the channel of all requests.
+	 * Typically this is done as soon as input 
+	 * is noticed on the channel, because the
+	 * handler will look at a separately managed work
+	 * queue. The actual number of queued "wakeups"
+	 * in the channel will not be important.
+	 */
 	void drain ();
 	static void drain (int fd);
 
+	/** File descriptor that can be used with poll/select to
+	 * detect when wakeup() has been called on this channel.
+	 * It be marked as readable/input-ready when this condition
+	 * is true. It has already been marked non-blocking.
+	 */
+	int selectable() const { return fds[0]; }
+
 	/* glibmm 2.22 and earlier has a terrifying bug that will
 	   cause crashes whenever a Source is removed from
 	   a MainContext (including the destruction of the MainContext),
@@ -47,15 +87,17 @@ class CrossThreadChannel {
 	   the RefPtr. I (Paul) have fixed this (https://bugzilla.gnome.org/show_bug.cgi?id=561885)
 	   but in the meantime, we need a hack to get around the issue.
 	*/
-
 	Glib::RefPtr<Glib::IOSource> ios();
 	void drop_ios ();
 
+	/** returns true if the CrossThreadChannel was
+	 * correctly constructed.
+	 */
 	bool ok() const { return fds[0] >= 0 && fds[1] >= 0; }
 
   private:
 	Glib::RefPtr<Glib::IOSource>* _ios; // lazily constructed
-	int fds[2];
+	int fds[2]; // current implementation uses a pipe/fifo
 };
 
 #endif /* __pbd__crossthread_h__ */