Why does XGrabKey return a BadRequest?

Question

So, I'm working on a gtk / X11 / Linux application that does screen capture in .gif and one way to stop capturing is by pressing a key (Esc, space or end). You can also use a timeout. However, to implement a key press to complete the capture, I must be able to capture the key so that I can receive the event even if my window has no focus (it is actually invisible during capture). I believe XGrabKey is the correct X11 function for this task:

Window w = Gtk::gdk_x11_drawable_get_xid(Gtk::gtk_widget_get_window(Handle()));
KeyCode kc = XKeysymToKeycode(Gtk::gdk_display, HotKeyCode);
int r = XGrabKey(   Gtk::gdk_display,
                    kc,
                    0               /* modifiers */,
                    w               /* grab_window */,
                    TRUE            /* owner_events */,
                    GrabModeAsync   /* pointer_mode */,
                    GrabModeAsync   /* keyboard_mode */);
printf("XGrabKey(%p, 0x%x/%x)=%i\n", w, HotKeyCode, kc, r);

Where "HotKeyCode" is XK_Escape or something like:

XGrabKey(0x3e00003, 0xff1b/9)=1

XGrabKey returns '1' or BadRequest. What am I doing wrong here?

FYI, the true Xorg Xserver code in question looks here .

: :

int x_err_callback(Display *d, XErrorEvent *e)
{
    char msg[256];

    XGetErrorText(d, e->error_code, msg, sizeof(msg));

    printf("X11Error %d (%s): request %d.%d\n",
        e->error_code, msg, e->request_code,
        e->minor_code);

    return 0;
}

Gtk::GdkFilterReturn key_filter(Gtk::GdkXEvent *gdk_xevent,
                                Gtk::GdkEvent *event,
                                Gtk::gpointer data)
{
    XKeyEvent *xevent = gdk_xevent;
    if (xevent->type == KeyPress)
    {
        int key = ((XKeyEvent *)gdk_xevent)->keycode;
        int keysym = XKeycodeToKeysym(Gtk::gdk_display, key, 0);

        printf("caught keysym %i\n", keysym);

        switch (keysym)
        {
            case 1: // your_keysym
                // your key handler code
                break;
        }
    }

    return Gtk::GDK_FILTER_CONTINUE;
}


Gtk::GdkWindow *Root = Gtk::gdk_get_default_root_window();
KeyCode kc = XKeysymToKeycode(Gtk::gdk_display, HotKeyCode);

XSetErrorHandler(x_err_callback);

int r = XGrabKey(   Gtk::gdk_display,
                    kc,
                    AnyModifier /* modifiers */,
                    GDK_WINDOW_XWINDOW(Root) /* grab_window */,
                    TRUE            /* owner_events */,
                    GrabModeAsync   /* pointer_mode */,
                    GrabModeSync    /* keyboard_mode */);

Gtk::gdk_window_set_events(Root,
    (Gtk::GdkEventMask)
    (Gtk::GDK_KEY_PRESS_MASK |
    Gtk::GDK_KEY_RELEASE_MASK));
Gtk::gdk_window_add_filter(NULL, key_filter, this);

AnyModifier . '0' - . NumLock...

Answer 1

1 , BadRequest. Xlib , 1, .

, XGrabKey (GetDefaultRootWindow(Gtk::gdk_display)). Xlib:

#include <X11/Xlib.h>
#include <X11/keysym.h>
#include <stdio.h>

int main() {
    Display *d = XOpenDisplay(0);
    Window root = DefaultRootWindow(d);
    int keycode = XKeysymToKeycode(d, XK_BackSpace);

    int rv = XGrabKey(d, keycode, AnyModifier, root, 1, GrabModeAsync, GrabModeAsync);
    printf("XGrabKey returned %d\n", rv);

    XEvent evt;
    while(1) {
        XNextEvent(d, &evt);
        printf("Got event %d\n", evt.type);
    }
}

X11 GTK, gdk_window_add_filter NULL GdkFilterFunc, , :

#include <X11/Xlib.h>
#include <X11/keysym.h>
#include <gtk/gtk.h>
#include <gdk/gdkx.h>
#include <stdio.h>

GdkFilterReturn filter(GdkXEvent *xevent, GdkEvent *event, gpointer data) {
    XKeyEvent *ev = (XKeyEvent *)xevent;
    if(ev->type == 2) {
        printf("Backspace hit.\n");
    }

    return GDK_FILTER_CONTINUE;
}

int main(int argc, char *argv[]) {
    gtk_init(&argc, &argv);

    GdkScreen *scr = gdk_screen_get_default();
    GdkWindow *groot = gdk_screen_get_root_window(scr);
    gdk_window_set_events(groot, GDK_KEY_PRESS_MASK);
    gdk_window_add_filter(groot, filter, NULL);

    Display *d = gdk_x11_get_default_xdisplay();
    Window root = GDK_WINDOW_XID(groot);
    int keycode = XKeysymToKeycode(d, XK_BackSpace);
    XGrabKey(d, keycode, AnyModifier, root, 1, GrabModeAsync, GrabModeAsync);

    gtk_main();
}

- 0 , , , . "A" 0 NumLock + A. AnyModifer.