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Full Discussion: xlib and keyboard events
Top Forums UNIX for Advanced & Expert Users xlib and keyboard events Post 302720329 by N7DR on Tuesday 23rd of October 2012 07:10:31 PM
Old 10-23-2012
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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Tk_Grab(3)						       Tk Library Procedures							Tk_Grab(3)

__________________________________________________________________________________________________________________________________________________

NAME
Tk_Grab, Tk_Ungrab - manipulate grab state in an application SYNOPSIS
#include <tk.h> int Tk_Grab(interp, tkwin, grabGlobal) void Tk_Ungrab(tkwin) ARGUMENTS
Tcl_Interp *interp (in) Interpreter to use for error reporting Tk_Window tkwin (in) Window on whose behalf the pointer is to be grabbed or released int grabGlobal (in) Boolean indicating whether the grab is global or application local _________________________________________________________________ DESCRIPTION
These functions are used to set or release a global or application local grab. When a grab is set on a particular window in a Tk applica- tion, mouse and keyboard events can only be received by that window and its descendants. Mouse and keyboard events for windows outside the tree rooted at tkwin will be redirected to tkwin. If the grab is global, then all mouse and keyboard events for windows outside the tree rooted at tkwin (even those intended for windows in other applications) will be redirected to tkwin. If the grab is application local, only mouse and keyboard events intended for a windows within the same application (but outside the tree rooted at tkwin) will be redi- rected. Tk_Grab sets a grab on a particular window. Tkwin specifies the window on whose behalf the pointer is to be grabbed. GrabGlobal indicates whether the grab should be global or application local; if it is non-zero, it means the grab should be global. Normally, Tk_Grab returns TCL_OK; if an error occurs and the grab cannot be set, TCL_ERROR is returned and an error message is left if interp's result. Once this call completes successfully, no window outside the tree rooted at tkwin will receive pointer- or keyboard-related events until the next call to Tk_Ungrab. If a previous grab was in effect within the application, then it is replaced with a new one. Tcl_Ungrab releases a grab on the mouse pointer and keyboard, if there is one set on the window given by tkwin. Once a grab is released, pointer and keyboard events will start being delivered to other windows again. KEYWORDS
grab, window Tk Tk_Grab(3)
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