Re: How can I modify a keyboard event before it reaches an app?

On Saturday 10 May 2003 05:25 pm, Jim Peters wrote:
> I'm not sure this is the right place to ask, but I hope someone can
> point me to a better place to ask if there is one, or to example code
> if what I hope to achieve has already been implemented somewhere.

Maybe your question should have been over on [email protected].

> Briefly: What I want to do is map a keypress (for example AltGr+P) to
> another keypress or sequence of keypresses (for example Up-arrow) so
> that all X applications see the mapped keypress and not the original.

I probably can't help you much, but I have similar desires, so we may be 
able to compare notes.  I am particularly interested in mapping a 
keypress to a sequence of keypresses.  I have vague hopes of trying to 
modify the X server to do so, but I'm a newbie to C and to the X 
codebase.

> The reason for this is that I have the keyboard layout extensively
> customised under the Linux console, using three different custom
> modifier keys along with shift to access a huge number of shortcuts,
> common code fragments ("if ("), virtual console switching, and so on,
> and so on.  

Out of curiosity, how did you map common code fragments to shortcut keys 
in the Linux console?  Can you point me to a HOWTO or the name of a 
utility?

> Currently I have achieved the same effect in X on one machine by
> hacking the Linux kernel to remap the raw keycodes before X gets a
> hold of them, and doing further remapping within a hacked version of
> rxvt.  However, in moving to a new machine, I have decided to try and
> "do it properly" by creating an X Window Manager (based on w9wm
> currently) to handle all of this without any modifications to X or the
> kernel.

After a little bit of investigation, I decided that the right place to 
do it is in the X server.  My reasoning starts with that the X server 
handles the keyboard inputs anyway, I want them to be sent to whichever 
X client window has the focus, and I want them to be appear exactly 
like regular keystrokes.  (I found some X utilities that were a step in 
the right direction, but they sent keystrokes that were somehow 
recognizable by the X clients as faked, or from a different source 
which they could reject.  Hence, that utility didn't work for all X 
clients, notably Mozilla and KDE apps, IIRC.)

I should note that the people who've replied to me on the forum or 
XFree86 lists have stated that the X server is not the right place to 
do that, and pointed me to some other potential solutions.

> My initial attempts have not been very successful, and I am wondering
> if what I hope to achieve is in fact impossible.

Nope.  If dos and Windows can do it, Linux has a chance ;-)

> My current approach is to set a passive grab on each of my modifier
> keys, and collect the events before they reach the application.  I
> then remap these and send the replacement events to the target window
> with XSendEvent.  However, this isn't working.  I'm wondering whether
> this is simply an error on my part, or if the grab is blocking my
> events and I am trying to do something that is impossible within X.
> (For one thing, the target app appears to lose focus whilst the grab
> is active -- i.e. the cursor goes clear.)

Well, you're certainly several steps ahead of me -- I can tell just 
because you know about passive grabs and XSendEvents.  (I may be wrong, 
but I XSendEvent might be the "synthetic" event that other apps can 
recognize and choose not to receive.  (Perhaps that is even the 
default.)

> My knowledge of the internal workings of X is minimal, so any guidance
> here would be much appreciated.

Mine is less than yours, but I am trying to keep sort of a diary of my 
efforts over on WikiLearn.  See:

   * http://twiki.org/cgi-bin/view/Wikilearn/XServerModificationProject
   * http://bugs.xfree86.org/cgi-bin/bugzilla/show_bug.cgi?id=43
   * http://twiki.org/cgi-bin/view/Wikilearn/XAnnotated -- very 
preliminary

> Also, if there is some better approach I could be taking to the whole
> problem (e.g. using some X extension designed for this kind of thing),
> I would be very glad to hear about it.

My opinion is that it belongs in the X server.  That seems to be a 
minority opinion, and a patch might never make it into CVS.  (And I 
might never manage to create a patch.)  Still that is my preference.

regards,
Randy Kramer