Grabbing events on specific keys with X11 on Linux

Question

I'm writing a program in C++ to implement the keyboard backlight feature from OS X on MacBook Pro's running a Linux distro. So far, it turns the backlight on, on boot and if no keyboard and mouse events are registered for 20 seconds, it will turn it back off, and of course turn it on yet again when an event is registered. Next thing I need the program to do, is to capture keypresses on the keyboard-backlight-up/down keys, but I'm not sure how to approach this.

I am currently using XScreenSaverQueryInfo to get the idle time of keyboard and mouse events, so a method using X11 API would be okay. I have done a lot of googling but havent found a way that I felt sure about going with. The problem I'm seeing with lots of the methods I found, is that they use keycode to identify the key, but I dont think that would be a viable solution since the program should work for any keyboard-layout available.

Any idea of a method and API I should go with? What would work the best?

Regards,

Answer 1

If you haven't done that yet, familiarize yourself with xev. Start it, give it the focus, and press the keys, to see what's happening.

Answer 2

The normal way to do this is with XGrabKey(). It uses keycodes, but you wouldn't hardcode the keycode, you'd get it with XKeysymToKeycode(). To be more correct you'd also want to redo the grab when you get a MappingNotify (XMappingEvent). (Note, MappingNotify, not MapNotify.) If there isn't a keysym for these keys - there probably isn't on old X versions, but hopefully newer X.org versions have one - then you just have to hardwire the keycode. Which won't be very robust or portable but probably works for everyone on Linux with the same hardware model.

Be prepared that key grabs are global, so if you try to XGrabKey() and something else has already grabbed that key, you'll get an X error - by default that exits the program. Another quirk of XGrabKey() is that it grabs the key with a precise modifier set. For example, to handle both with and without NumLock, you need to grab twice. See Global Hotkey with X11/Xlib

In a normal Linux setup (if you wanted to get a feature like this into upstream projects), the desktop environments don't want lots of separate apps fighting over the key grabs and getting errors. So there will be some central coordination points, such as the window manager or a special daemon might do all the keybindings and forward commands to other processes as needed. So you would probably want to look at patching the same upstream code that handles other special keys like this, if you were trying to get your feature integrated into distributions by default.

Another thing to be aware of is the Xkb API, which is a lot more complicated. There is some brain-bending way to grab keys with Xkb but I don't know of any advantage to going that route.