wm protocol window ?name? ?command?
This command is used to manage window manager protocols such as WM_DELETE_WINDOW. Name is the name of an atom corresponding to a window manager protocol, such as WM_DELETE_WINDOW or WM_SAVE_YOURSELF or WM_TAKE_FOCUS. If both name and command are specified, then command is associated with the protocol specified by name. Name will be added to window 's WM_PROTOCOLS property to tell the window manager that the application has a protocol handler for name, and command will be invoked in the future whenever the window manager sends a message to the client for that protocol. In this case the command returns an empty string. If name is specified but command isn't, then the current command for name is returned, or an empty string if there is no handler defined for name. If command is specified as an empty string then the current handler for name is deleted and it is removed from the WM_PROTOCOLS property on window; an empty string is returned. Lastly, if neither name nor command is specified, the command returns a list of all the protocols for which handlers are currently defined for window.
Tk always defines a protocol handler for WM_DELETE_WINDOW, even if you haven't asked for one with wm protocol. If a WM_DELETE_WINDOW message arrives when you haven't defined a handler, then Tk handles the message by destroying the window for which it was received.
Use:
wm protocol <window> WM_DELETE_WINDOW {#}to prevent your window from closing when the [X] button is clicked.
More on this subject appears under the title "Catching window managed events".