X11 KeySyms: Switch mode, KeySym groups and XK_ISO_Level3_Shift

Question

I'm using Xlib for a project, and I'm trying to get the KeySym relative to a pressed key.

I've got the KeyCode relative to the pressed key, keyboard modifiers and keyboard indicators. I'm referring to this page to understand how to retrieve the correct key, but it must be out of date or something, because my X doesn't behave like that page says.

I've got some questions:

What is MODE SWITCH? How can I see what key is my MODE SWITCH and how can I set it? (via Xlib functions or via a command, it's the same).
That page says that the first 4 KeySyms associated to a KeyCode (found with XGetKeyboardMapping, or shown by xmodmap -pk command) are divided into two groups. MODE SWITCH switches between groups, and within a group the first or the second KeySym is chosen according to modifiers (ie, shift key) or indicators (ie, caps lock). But on my X I can see that if shift is pressed, I switch to the third KeySym, not the second one.
On many european layouts AltGr key is bound to KeySym XS_ISO_Level3_Shift, which doesn't belong to any keyboard modifier (ie, shift, lock, control, mod1, mod2, mod3, mod4 and mod5). When such XS_ISO_Level3_Shift is pressed, the fifth KeySym associated to the KeyCode is chosen. Why? Where can I found some info about this XS_ISO_Level3_Shift and about other keysyms that are actually used, but not documented on Xlib manuals?

Then there is the XKeyboard extensions. I'm far from sure that everything which is possible with them can be described in term of the more basic xlib possibilities. Perhaps that's why you are confused if your implementation use them (xdpyinfo | grep XKEYBOARD to know if it is the case). Each time I've looked at that, I started confused and then understood. But I've never reached a state where I remembered the underlying logic of that mess. — AProgrammer
Does your system have xev? This is often a quick and simple way to resolve problems. — Joseph Quinsey

Havoc P Havoc P · Accepted Answer · 2010-11-09T17:20:19

It looks like the page you're reading predates Xkb - I'd guess that content is 10-15 years old at least.

See: http://www.xfree86.org/current/XKBproto.pdf

"xmodmap -pm" will show you the modifier keys such as mode switch.

With Xkb, the behavior of basically everything is configurable, I believe. Start looking at /usr/share/X11/xkb/ for example. I don't know a lot about it but hopefully looking at Xkb instead of old docs will get you pointed in the right direction.

X11 KeySyms: Switch mode, KeySym groups and XK_ISO_Level3_Shift

2 Answers