Tk_Grab(interp, tkwin, grabGlobal)
These functions are used to set or release a global or application local grab. When a grab is set on a particular window in a Tk application, mouse and keyboard events can only be received by that window and its descendants. Mouse and keyboard events for windows outside the tree rooted at tkwin will be redirected to tkwin. If the grab is global, then all mouse and keyboard events for windows outside the tree rooted at tkwin (even those intended for windows in other applications) will be redirected to tkwin. If the grab is application local, only mouse and keyboard events intended for a windows within the same application (but outside the tree rooted at tkwin) will be redirected.
Tk_Grab sets a grab on a particular window. Tkwin specifies the window on whose behalf the pointer is to be grabbed. GrabGlobal indicates whether the grab should be global or application local; if it is non-zero, it means the grab should be global. Normally, Tk_Grab returns TCL_OK; if an error occurs and the grab cannot be set, TCL_ERROR is returned and an error message is left if interp's result. Once this call completes successfully, no window outside the tree rooted at tkwin will receive pointer- or keyboard-related events until the next call to Tk_Ungrab. If a previous grab was in effect within the application, then it is replaced with a new one.
Tcl_Ungrab releases a grab on the mouse pointer and keyboard, if there is one set on the window given by tkwin. Once a grab is released, pointer and keyboard events will start being delivered to other windows again.