winfo id

A subcommand of Tk's winfo command.
http://www.purl.org/tcl/home/man/tcl8.5/TkCmd/winfo.htm

: winfo id window

Returns a hexadecimal string giving a low-level platform-specific identifier for window. On Unix platforms, this is the X window identifier. Under Windows, this is the Windows HWND. On the Macintosh the value has no meaning outside Tk.

Notice that the return value of [winfo id ...] is the id used by toplevel and wish in

    toplevel ... -use $id

and

    wish ... -use $id

Here's an example:

    label .t1 -text Above
    label .t2 -text Below
    frame .holder -container 1
    pack .t1 .holder .t2
    exec wish other_application.tcl -use [winfo id .holder] &

[Include an image here.]

Incidental remark: when CL substitutes

    exec tclkit tiny.tcl -use [winfo id .holder] &

for that last line, while running on WinNT, with tiny.tcl having content

    pack [.button -text "Push me" -command exit]

everything comes up OK. When the button is pushed, "Dr. Watson" appears. Dismissing it eventually results in a whited-out frame "hole" in the embedding Wish. Is this an error in Tclkit 8.4a3?

Is it a bug, that the results of winfo id and xwininfo differ in the window id exactly by one ?

No, it isn't, from the toplevel page:

DKF notes that, "toplevels on UNIX/X are really a collection of several windows; the window you draw on (which is what winfo id will tell you), another window for a menubar (if you've installed one) and a third one to contain the other two. If you do xwininfo -tree you should be able to find out what's really going on."

See also edit