Handle C-q with XLookupString

Question

This is a working example of X11 code that handles Ctrl-q event to quit application:

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <X11/Xlib.h>
#include <X11/Xutil.h>
#include <X11/Xos.h>

void exitOnCondition(char cond, const char *msg, int exitCode, Display *dpy, Window *w, GC *gc) {
    if(cond) {
        printf("%s\n", msg);
        if(dpy && gc) XFreeGC(dpy, *gc);
        if(dpy && w) XDestroyWindow(dpy, *w);
        if(dpy) XCloseDisplay(dpy);
        exit(exitCode);
    }
}

int main(int argc, char **argv) {
      Display *dpy = XOpenDisplay(0);
      exitOnCondition(dpy == 0, "Error: XOpenDisplay failed", -1, dpy, 0, 0);

      int blackColor = BlackPixel(dpy, DefaultScreen(dpy));
      int whiteColor = WhitePixel(dpy, DefaultScreen(dpy));

      Window w = XCreateSimpleWindow(dpy, DefaultRootWindow(dpy), 0, 0, 
                     200, 100, 0, blackColor, blackColor);

      //Tell X Server to send MapNotify events
      XSelectInput(dpy, w, StructureNotifyMask | KeyPressMask);

      //Make window appear
      XMapWindow(dpy, w);

      //Graphics Context
      GC gc = XCreateGC(dpy, w, 0, 0);

      //Set white color for drawing
      XSetForeground(dpy, gc, whiteColor);

      //Wait for the MapNotify event
      for(;;) {
        XEvent e;
        XNextEvent(dpy, &e);
        if (e.type == MapNotify) {
          break;
        }
      }

      //Draw the line
      XDrawLine(dpy, w, gc, 10, 60, 180, 20);

      //Send the "DrawLine" request to the server
      XFlush(dpy);

      char text[255];
      XEvent e;
      KeySym key;
      int numKeys = 0;
      for(;;) {
          XNextEvent(dpy, &e);
          if(e.type == KeyPress) {
              //With modifier XLookupString will return garbage(?) in text[0] and key as latin1
              if((numKeys = XLookupString(&e.xkey, text, 255, &key, 0))) {
                  printf("lookup returned:\n");
                  for(int i = 0; i < numKeys; i++) {
                      printf("text[%d]=%x\n", i, text[i]);
                  }

                  if(e.xkey.state == ControlMask && key == XK_q) {
                      exitOnCondition(1, "C-Q pressed", 0, dpy, &w, &gc);
                  }
              }
          }
      }

      XFreeGC(dpy, gc);
      XDestroyWindow(dpy,w);
      XCloseDisplay(dpy);   
}

Will this code correctly handle Ctrl-q event on any system?

Can I use e.xkey.state to check for Ctrl modifier after XLookupString for any keyboard layout even when Ctrl is rebound to CAPS Lock(or anything else)?

Why does XLookupString return one symbol text[0]==0x11 for Ctrl-q event and not text[0]==CtrlModifierCode text[1]=='q'?

This is correct code that looks for a **key** combination Ctrl+Q. The string returned by XLookupString is of no importance. Keys and strings they produce are two very different things. (There's no such character as 'CtrlModifierCode' so it cannot be a part of returned text) — n. m. could be an AI, Mar 30 '17 at 11:15
@n.m. Will ControlMask be set when user pressed capslock which was bound to ctrl? — pusheax, Mar 30 '17 at 12:42

score 0 · Accepted Answer · answered Mar 30 '17 at 10:08

XLookupString returns a sequence of ISO-8859-1 characters (at least according to the manual I have; I do not know how up-to-date it is). There is no character code in ISO-8859-1 for the "ctrl" key by itself. Strictly speaking, there isn't one for the ctrl-q combination either, but tradition dictates that ctrl + (A...Z) map to the 1...26 range.

Handle C-q with XLookupString

1 Answers1

Linked