gtk-window-size (window)

window -- a gtk-window widget

width -- an integer with the width, or nil
height -- an integer with the height, or nil

Obtains the current size of the window. If the window is not onscreen, it returns the size GTK will suggest to the window manager for the initial window size, but this is not reliably the same as the size the window manager will actually select. The size obtained by the gtk-window-size function is the last size received in a gdk-event-configure event, that is, GTK uses its locally stored size, rather than querying the X server for the size. As a result, if you call the gtk-window-resize function then immediately call the gtk-window-size function, the size will not have taken effect yet. After the window manager processes the resize request, GTK receives notification that the size has changed via a configure event, and the size of the window gets updated.

1. Nearly any use of this function creates a race condition, because the size of the window may change between the time that you get the size and the time that you perform some action assuming that size is the current size. To avoid race conditions, connect to the "configure-event" signal on the window and adjust your size dependent state to match the size delivered in the gdk-event-configure event.
2. The returned size does not include the size of the window manager decorations, aka the window frame or border. Those are not drawn by GTK and GTK has no reliable method of determining their size.
3. If you are getting a window size in order to position the window onscreen, there may be a better way. The preferred way is to simply set the semantic type of the window with the gtk-window-type-hint function, which allows the window manager to e.g. center dialogs. Also, if you set the transient parent of dialogs with the gtk-window-transient-for function window managers will often center the dialog over its parent window. It is much preferred to let the window manager handle these things rather than doing it yourself, because all applications will behave consistently and according to user prefs if the window manager handles it. Also, the window manager can take the size of the window decorations/border into account, while your application cannot.

In any case, if you insist on application-specified window positioning, there is still a better way than doing it yourself - the gtk-window-window-position function will frequently handle the details for you.