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ttz

Member

From: My Box

Registered: 2013-08-01

Posts: 56

[solved] How To Obtain Atom Value From XGetWindowProperty

I've done a search through topic titles and read through a few threads, haven't found anything that specifically addresses this.

I'm trying to write some code using Xlib and C to verify if a window has a value of "_NET_WM_WINDOW_TYPE_DESKTOP"  assigned to its "_NET_WM_WINDOW_TYPE" atom.

The relevant code is:

// XDPY is already defined earlier in code as the X display
prop_desktop = XInternAtom(XDPY, "_NET_WM_WINDOW_TYPE", True);

// assume everything here is already defined
int status = XGetWindowProperty(XDPY, child, prop_desktop,
						 0L, 1L, False,
						 AnyPropertyType, &type, &format,
						 &length, &after, &data);

if (status == Success and type != None)
    printf("Atom Property Value: %s \n", (const char *) data);

According to Xlib docs, the variable <data> should contain the value of the property found by XGetWindowProperty. However, <data> does not show up as "_NET_WM_WINDOW_TYPE_DESKTOP", unlike when I call <xprop> on that specific window; the <printf> call merely returns a single character (I am guessing this is because XGetWindowProperty feeds <data> a long data type, and <printf> is just doing a direct translation).

For example, what I might see is:

Atom Property Value: n

(the character varies each time I restart X).

Using <xprop> will specifically return

_NET_WM_WINDOW_TYPE(ATOM) = _NET_WM_WINDOW_TYPE_DESKTOP

How does one get the value (the right hand side of the above equality) of a window's atom using Xlib?

Last edited by ttz (2016-02-08 17:52:47)

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Character shines in the great moments, but is polished in the little ones.

Vain

Member

Registered: 2008-10-19

Posts: 179

Website

Re: [solved] How To Obtain Atom Value From XGetWindowProperty

You already get the value.

However, an Atom is just a number. Printing it like that will return random-ish data or even crash ("data" is a pointer to an array of Atoms, not a C string). This is also why you have to call `XInternAtom(XDPY, "_NET_WM_WINDOW_TYPE", True);`: It returns the number/atom which was internally assigned to that atom name.

What you want to do is something like this:

#include <stdio.h>
#include <stdlib.h>
#include <X11/Xatom.h>
#include <X11/Xlib.h>

int
main(int argc, char **argv)
{
    Atom prop_desktop, prop_type, prop, da;
    char *an;
    Display *dpy;
    int di;
    int status;
    unsigned char *prop_ret = NULL;
    unsigned long dl;
    Window child;

    if (argc == 2)
        child = atoi(argv[1]);
    else
        exit(EXIT_FAILURE);

    dpy = XOpenDisplay(NULL);

    prop_type = XInternAtom(dpy, "_NET_WM_WINDOW_TYPE", True);
    prop_desktop = XInternAtom(dpy, "_NET_WM_WINDOW_TYPE_DESKTOP", True);

    status = XGetWindowProperty(dpy, child, prop_type, 0L, sizeof (Atom), False,
                                XA_ATOM, &da, &di, &dl, &dl, &prop_ret);

    if (status == Success && prop_ret)
    {
        prop = ((Atom *)prop_ret)[0];

        /* Debug output, not really relevant: Re-resolve atom number to
         * printable name */
        an = XGetAtomName(dpy, prop);
        fprintf(stderr, "Type found: %s\n", an);
        if (an)
            XFree(an);

        /* Compare internal atom numbers */
        if (prop == prop_desktop)
            fprintf(stderr, "It's a desktop window!\n");
    }

    exit(EXIT_SUCCESS);
}

"&da, &di, &dl, &dl" are just dummies. Note the fancy type casting of "prop_ret": We treat it as a pointer to an Atom (or a pointer to an array of Atoms) and then use the first item in that array.

This example accepts a window ID as argv[1], e.g.:

$ gcc -Wall -Wextra -o bla bla.c -lX11
$ xwininfo
... click on some window to find its ID ...
$ ./bla $(( 0x602d3c ))
Type found: _NET_WM_WINDOW_TYPE_NORMAL

Note that this is actually a list of atoms, not a single atom: https://specifications.freedesktop.org/ … 0472629520. In theory, a client could define multiple window types. Most clients don't, but, well, ...

Also note that an Atom is not a single byte. See also: `printf("%lu\n", sizeof (Atom));`.

(I always print debug messages to stderr to avoid line buffering issues.)

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Why "Open Source" misses the point of Free Software

ttz

Member

From: My Box

Registered: 2013-08-01

Posts: 56

Re: [solved] How To Obtain Atom Value From XGetWindowProperty

Thanks vain; I think the crux of my issue was that I could not understand how to properly compare what the XGetWindowProperty was returning to <prop_ret>. The documentation I found online was not clear (to me at least) what type <prop_ret> was:

https://tronche.com/gui/x/xlib/window-i … perty.html

I will give this a try and see if it solves my issue

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Character shines in the great moments, but is polished in the little ones.

ttz

Member

From: My Box

Registered: 2013-08-01

Posts: 56

Re: [solved] How To Obtain Atom Value From XGetWindowProperty

Yes, I think this is what I was looking for. Thank you for taking the time to provide such a detailed answer I learned more from this one example than all of the Xlib documentation

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Character shines in the great moments, but is polished in the little ones.