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xlib and keyboard events

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Old 10-23-2012
N7DR N7DR is offline
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xlib and keyboard events

1. If there's some better place where xlib experts hang out, please tell me. Despite an assiduous search, I could not find an xlib reflector.

2. My actual question:
In an xterm, I want to grab and process all keyboard events in a program running inside the xterm. For example, with my program running, if I press an "x", I don't the x to be displayed, and I do want to be able to see the keypress and keyrelease events myself, in the application running inside the xterm.

I have tried quite a few things, but nothing seems to do exactly what I need.

The closest I've come is the pseudo-code sequence:

Code:
XOpenDisplay() to get the correct display
getenv("WINDOWID") to get the window ID
XGrabKeyboard( <display*>, <window>, false, GrabModeAsync, GrabModeSync, CurrentTime)
XSelectInput(<everything>)
forever: (XWindowEvent() on the window)

But that seems to have two problems:
1. it seems to grab keyboard events wherever they occur on the display, not just in the xterm;
2. keyboard events inside the xterm don't actually seem to be returned by the XWindowEvent().

This is the first time I've tried to use xlib, so it's perfectly possible (even likely) that I'm missing something basic, even though I've tried to read the pertinent documentation quite carefully.

---------- Post updated at 05:10 PM ---------- Previous update was at 04:57 PM ----------

Apparently, the call to XGrabKeyboard should be:


Code:
XGrabKeyboard(<display>, <window>, false, GrabModeAsync, GrabModeAsync, CurrentTime);

although I don't undrestand why the keyboard mode should be Async. The documentation seems to indicate that this would mean that the xterm application would also receive the keyboard activity, but that isn't what happens.

(Documentation says: If the keyboard_mode argument is GrabModeAsync, keyboard event processing continues as usual. Apparently, whatever "as usual" means, it does not seem to mean "is sent to other clients as usual".)

So I'm floundering as to <i>why</i> this works, but at least it does seem to do what I need.
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Old 10-24-2012
DGPickett DGPickett is offline Forum Advisor  
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Without getting into xterm, or X at all, one can go to sttty raw or such, or equivalent calls, and process the keystrokes uncooked as they are typed, being careful not to block in the wrong places. What you get may still be the product of the x keyboard mapping, but if you do not like what it is, setting that up is a separate task. I cannot even keep my numeric keypad happy in X!
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Old 10-24-2012
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It turns out that the change to XGrabKeyboard() is still not quite doing the right thing :-(

Now keyboard events from ALL windows are being sent to my application, even though I specifically use the correct window ID for the xterm in the call to XGrabKeyboard().

So I'm back to asking what I'm doing wrong :-(
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Old 10-25-2012
DGPickett DGPickett is offline Forum Advisor  
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If you only want the keyboard when your window is in focus, and you do not want it to echo, buffer, enhance or translate input, that echo and such is in the tty layer, not the X. You might be able to divert the keyboard for feeding the tty, but why?
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Old 10-25-2012
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After a lot more experimentation, I discovered that the trick is to use:


Code:
XGrabKey(<Display*>, AnyKey, AnyModifier, <window>, false, GrabModeAsync, GrabModeAsync);

So far, that seems to do exactly what I needed.

I do wish, though, that I had been able to find an xlib reflector or some similar resource. I felt like I was spending an awful of time flailing around just trying stuff, and a few words from an xlib expert could have pointed me in the right direction without my having to go through all that pain. The xlib documentation is pretty good, but even so there are plenty of ambiguities in the language, so much of the time I had to resort to trying combinations of plausible functions until I hit the one that worked.

DGPickett: I think you misunderstand my issue, although I tried to make it as clear as I could. If I understand you correctly, you seem to be saying that I shouldn't be involved in X at all, and in particular not an xterm. But the entire context of the problem is within an xterm, so I think that X has to be involved in the solution.
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Old 10-25-2012
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Yes, but a tty solution works a it more simply, in an xterm or not. For the input side of your requirement, I do not see any requirement for X expertise to write such an app. Keeping things as simple as possible is usually a good thing. You can even script it:
Code:
$ stty raw -echo ; cat -uvte | line ; stty -raw echo
1234^M^C^D$
           $

There are good tutorials out there for just about everything: http://tronche.com/gui/x/xlib/input/
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