Re: [gtk-list] gdk_input_add

[Des Herriott <des ops netcom net uk>]

On May 08, Sascha Ziemann wrote:
> Hi,
> 
> Where can I find a description of gdk_input_add:
> 
> gint gdk_input_add	  (gint		     source,
> 			   GdkInputCondition condition,
> 			   GdkInputFunction  function,
> 			   gpointer	     data);
> 
> condition should be GDK_INPUT_READ for example but the rest is not
> clear. Is source the file descriptor? And how about function and data?
 
(See also http:/www.gtk.org/tutorial/, section 15)

Yep, source is a file descriptor.  condition can be a bitmask of one or
more of GDK_INPUT_READ, GDK_INPUT_WRITE, and GDK_INPUT_EXCEPTION.

The function gets called when the descriptor is ready to read, write
or has an exceptional condition pending, and gets called depending on 
the condition bitmask.  It has this signature:

  void handler_func(gpointer data, gint fd, GdkInputCondition cond);

data is a pointer to whatever data you passed to gdk_input_add(). 
condition is a bitmask of what conditions are true on the file descriptor, 
so if you've selected more than one condition you should do something like:

  if ((cond & GDK_INPUT_WRITE) == GDK_INPUT_WRITE) {
    /* write some data to fd */
  } else if ((cond & GDK_INPUT_READ) == GDK_INPUT_READ) {
    /* read some data from fd */
  }

> I want to play a video stream with 25 or 50 Hz. I think the easiest
> way to do that, is to write a simple timer program that can be used to
> feed Gtk's event loop via gdk_input_add. Or does anybody know a better
> way? Perhaps a way to tell gtk_main_iteration about alarm signals?

I'd avoid using alarm(), or signals in general with X-based applications.
There are too many reentrancy and general reliability issues with signals.
The most you should ever try to do with a signal handler is to set some
global flag which can be checked later as part of the event loop.

(How reentrancy-safe is Gtk?  I know some versions of Xlib are safe,
but I don't count on it)

It sounds like you may want to use a timer rather than gdk_input_add() -
gdk_input_add() is best suited for communicating with external processes
where you want to receive data as it comes in.  The file descriptor is
most likely to be a pipe or a socket.

-- 
Des Herriott
des@ops.netcom.net.uk