X11 exactly

Unless your X-server supports XResQueryClientIds from X-Resource v1.2 extension I know no easy way to reliably request process ID. There're other ways however.

If you just have a window in front of you and don't know its ID yet — it's easy to find it out. Just open a terminal next to the window in question, run xwininfo there and click on that window. xwininfo will show you the window-id.

So let's assume you know a window-id, e.g. 0x1600045, and want to find, what's the process owning it.

The easiest way to check who that window belongs to is to run XKillClient for it i.e.:

xkill -id 0x1600045

and see which process just died. But only if you don't mind killing it of course!

Another easy but unreliable way is to check its _NET_WM_PID and WM_CLIENT_MACHINE properties:

xprop -id 0x1600045

That's what tools like xlsclients and xrestop do.

Unfortunately this information may be incorrect not only because the process was evil and changed those, but also because it was buggy. For example after some firefox crash/restart I've seen orphaned windows (from flash plugin, I guess) with _NET_WM_PID pointing to a process, that died long time ago.

Alternative way is to run

xwininfo -root -tree

and check properties of parents of the window in question. That may also give you some hints about window origins.

But! While you may not find what process have created that window, there's still a way to find where that process have connected to X-server from. And that way is for real hackers. :)

The window-id 0x1600045 that you know with lower bits zeroed (i.e. 0x1600000) is a "client base". And all resource IDs, allocated for that client are "based" on it (0x1600001, 0x1600002, 0x1600003, etc). X-server stores information about its clients in clients[] array, and for each client its "base" is stored in clients[i]->clientAsMask variable. To find X-socket, corresponding to that client, you need to attach to X-server with gdb, walk over clients[] array, find client with that clientAsMask and print its socket descriptor, stored in ((OsCommPtr)(clients[i]->osPrivate))->fd.

There may be many X-clients connected, so in order to not check them all manually, let's use a gdb function:

define findclient
  set $ii = 0
  while ($ii < currentMaxClients)
    if (clients[$ii] != 0 && clients[$ii]->clientAsMask == $arg0 && clients[$ii]->osPrivate != 0)
      print ((OsCommPtr)(clients[$ii]->osPrivate))->fd
    end
    set $ii = $ii + 1
  end
end

When you find the socket, you can check, who's connected to it, and finally find the process.

WARNING: Do NOT attach gdb to X-server from INSIDE the X-server. gdb suspends the process it attaches to, so if you attach to it from inside X-session, you'll freeze your X-server and won't be able to interact with gdb. You must either switch to text terminal (Ctrl+Alt+F2) or connect to your machine over ssh.

Example:

1. Find the PID of your X-server:

   $ ps ax | grep X
    1237 tty1     Ssl+  11:36 /usr/bin/X :0 vt1 -nr -nolisten tcp -auth /var/run/kdm/A:0-h6syCa
   

2. Window id is 0x1600045, so client base is 0x1600000. Attach to X-server and find client socket descriptor for that client base. You'll need debug information installed for X-server (-debuginfo package for rpm-distributions or -dbg package for deb's).

   $ sudo gdb
   (gdb) define findclient
   Type commands for definition of "findclient".
   End with a line saying just "end".
   >  set $ii = 0
   >  while ($ii < currentMaxClients)
    >   if (clients[$ii] != 0 && clients[$ii]->clientAsMask == $arg0 && clients[$ii]->osPrivate != 0)
     >     print ((OsCommPtr)(clients[$ii]->osPrivate))->fd
     >     end
    >   set $ii = $ii + 1
    >   end
   >  end
   (gdb) attach 1237
   (gdb) findclient 0x1600000
   $1 = 31
   (gdb) detach
   (gdb) quit
   

3. Now you know that client is connected to a server socket 31. Use lsof to find what that socket is:

   $ sudo lsof -n | grep 1237 | grep 31
   X        1237    root   31u   unix 0xffff810008339340       8512422 socket
   

   (here "X" is the process name, "1237" is its pid, "root" is the user it's running from, "31u" is a socket descriptor)

   There you may see that the client is connected over TCP, then you can go to the machine it's connected from and check netstat -nap there to find the process. But most probably you'll see a unix socket there, as shown above, which means it's a local client.

4. To find a pair for that unix socket you can use the MvG's technique (you'll also need debug information for your kernel installed):

   $ sudo gdb -c /proc/kcore
   (gdb) print ((struct unix_sock*)0xffff810008339340)->peer
   $1 = (struct sock *) 0xffff810008339600
   (gdb) quit
   

5. Now that you know client socket, use lsof to find PID holding it:

   $ sudo lsof -n | grep 0xffff810008339600
   firefox  7725  username  146u   unix 0xffff810008339600       8512421 socket
   

That's it. The process keeping that window is "firefox" with process-id 7725

2017 Edit: There are more options now as seen at Who's got the other end of this unix socketpair?. With Linux 3.3 or above and with lsof 4.89 or above, you can replace points 3 to 5 above with:

lsof +E -a -p 1237 -d 31

to find out who's at the other end of the socket on fd 31 of the X-server process with ID 1237.

Actually, strictly speaking, X11 is the communication protocol between an X client application and an X server. The X server is the part of the system that communicates with the hardware and provides basic concepts, in particular the notion of a window on the screen, with potentially overlapping windows, and one window having the input focus. (The X server does not handle window decorations, menus, keyboard shortcuts, or anything else that is the job of the window manager.)

X is called X because it was (in part) a successor to the W window system, and X11 is version 11 of X. It took about 3 years to reach version 11 (from 1984 to 1987) and the core protocol has been stable since then.

X11 is also the base name of a software distribution that included libraries, client applications and server code using the X11 protocol. That software distribution is known as the X Window System (version 11), or X or X11 for short. This software distribution originally came from MIT, where X had been invented; it then had a sometimes tumultuous history, with an MIT-led consortium producing versions up to X11R6.3.

Nowadays, the de facto reference implementation of the X Window System is the one by the X.Org Foundation. This is a usable implementation, used by virtually all X11 installations today.

There are several X servers, most of them sharing a code base. Most users use the X.org Xorg server, which communicates with most display hardware out there. There are other servers such as Xnest, which shows an X display inside an X window on another display, and Xvfb, which is a virtual display that remains only in memory.

Xlib (which has the shared object name libX11) is just one of the libraries in the X Window System. It's a basic wrapper around the client side of the X11 protocol. Almost all X client use this library (many of them indirectly through higher-level libraries).