Use group 0 if current group contains no keysym

Fixes #5, perhaps fully this time.

It seems non-zero keysym groups work such that if no keysym is defined
in it, the default 0 group's keysym is assumed.

In other words, common keys that would otherwise just be duplicated in
the group's definition, such as Return, Backspace, Delete, Shift, Space,
or the numpad keys, just have no symbol mapped to them at all in the
group (XkbKeycodeToKeysym returns NoSymbol).  In such cases, it seems
the receiving program is expected to just fall back to group 0 and read
the keysym from there instead.

This is about XKeycodeToKeysym, but XkbKeycodeToKeysym which we're using
(note the different prefix) seems to function the same:
https://stackoverflow.com/questions/54483276/xkeysymtokeycode-and-keyboard-layout

This seems to be correct with the keyboard layouts I've tested.

- - -

Other Xlib users have replaced XKeycodeToKeysym with XGetKeyboardMapping
instead.  These patches to that effect imply XKeycodeToKeysym is
deprecated:

- https://lists.freedesktop.org/archives/piglit/2012-January/001795.html
- https://github.com/ArcticaProject/nx-libs/commit/c79f2d289004d419d8bb76cf8bf731980d40da5d

XkbKeycodeToKeysym is not deprecated though, and seems a much better fit
here than doing heap allocations in a loop, so I'll go with it.

1 files changed, 14 insertions, 1 deletions

diff --git a/xkbcat.c b/xkbcat.c
index c56cce0..5e28cc2 100644
--- a/xkbcat.c
+++ b/xkbcat.c
@@ -125,7 +125,20 @@ int main(int argc, char * argv[]) {
                     // regardless of what modifiers are held down.
                     KeySym s = XkbKeycodeToKeysym(
                             disp, ev->detail, group, 0 /*shift level*/);
-                    if (NoSymbol == s) continue;
+
+                    // Non-zero keysym groups are "overlays" on the base (`0`)
+                    // group.  If the current group has no keysym for this
+                    // keycode, defer to the base group instead.  (This usually
+                    // happens with common shared keys like Return, Backspace,
+                    // or numeric keypad keys.)
+                    if (NoSymbol == s) {
+                        if (group == 0) continue;
+                        else {
+                            s = XkbKeycodeToKeysym(disp, ev->detail,
+                                    0 /* base group */, 0 /*shift level*/);
+                            if (NoSymbol == s) continue;
+                        }
+                    }
                     char *str = XKeysymToString(s);
                     if (NULL == str) continue;