How do the Server Extensions work in X?

Question

Let's take an example. When I run:

xkbprint $DISPLAY keyboard.ps

what happens in the system? I would like to know how the X really work because I regularly get nasty error reports such as

Fatal Error: Cannot load geometry for /tmp/launch-NawGIk/:0

             Exiting

I know that the X11 was designed in a way that it is easy to extend. But how? How does the X11 protocol work? How do the X extensions work?

Answer 1

Whatever happens, I'm pretty certain it isn't going to be what you expect.

The syntax of xkbprint(1) is

$ xkbprint sourcefile destfile

and sourcefile should be a compiled xkb file. $DISPLAY is going to resolve to something like mymachine:0.0, which isn't a very reasonable file name.

Actually, since you're running a Mac, it's echoing exactly what it's seeing: /tmp/launch-NawGIk/:0 is the special OS/X magic to let launchd run X apps. The error message means that it's not finding a geometry or display in that file. Which is no surprise since there is no file named `/tmp/launch-NawGIk/:0

Try reading the man page, and see if you don't have more luck with a compiled .xkm file.

More generally, if you want a starting introduction to X, try this article.

Answer 2

Man xkbprint says:

The xkbprint comman generates a printable or encapsulated PostScript description of the XKB keyboard description specified by source. The source can be any compiled keymap (.xkm) file that includes a geometry description or an X display specification.

As Charlie Martin pointed out, xkbprint is misusing your $DISPLAY as a file name. Try specifying the display more precisely, to avoid ambiguity with file name. Try man xhost. Maybe passing simple ":0" will fix the problem?