X11-specific commandline options

The X backend understands some additional command line arguments.

--display display.  The name of the X display to open instead of the one specified in the DISPLAY environment variable.

--screen screen_number.  The number of the screen within the default display. This overrides any screen number specified in the display name specified by by he --display command line option or the DISPLAY environment variable. If this screen cannot be opened, then GTK+ will fall back to the screen specified in the display name. This option is not useful interactively; the intended purposes is that when a program registers its command line with a session manager for later restarting, it can save the screen it is on, without having to worry if it might be restarted on a different display.

--sync.  Makes all X requests synchronously. This is a useful option for debugging, but it will slow down the performance considerably.

--gxid-host host.  The host to contact the gxid daemon on; overrides the GXID_HOST environment variable.

--gxid-port port.  The port for the connection to gxid; overrides the GXID_PORT environment variable. This option is only available if GTK+ has been configured with --gdk-target=x11.

X11-specific environment variables

The X backend can be influenced with some additional environment variables.

GXID_HOST, GXID_PORT.  The host and port to contact the gxid daemon on. gxid is only necessary on X servers which don't support using the pointer and extension devices at once, and is only built if GTK+ is configured with --with-xinput=gxi. The XFree86 and Xorg X servers don't have this restriction.

GDK_USE_XFT.  If this variable is set to 1, GTK+ will use the Pango Xft backend instead of the X backend when possible (i.e. when the X server supports the XRender extension and Pango has been built with Xft support).

Server, client, window manager

Other window systems typically put all their functionality in the application itself. With X, each application involves three different programs: the X server, the application (called a client because it's a client of the X server), and a special client called the window manager.

The X server is in charge of managing resources, processing drawing requests, and dispatching events such as keyboard and mouse events to interested applications. So client applications can ask the X server to create a window, draw a circle, or move windows around.

The window manager is in charge of rendering the frame or borders around windows; it also has final say on the size of each window, and window states such as minimized, maximized, and so forth. On Windows and MacOS the application handles most of this. On X11, if you wish to modify the window's state, or change its frame, you must ask the window manager to do so on your behalf, using an established convention.

GTK+ has functions for asking the window manager to do various things; see for example gtk_window_iconify() or gtk_window_maximize() or gtk_window_set_decorated(). Keep in mind that gtk_window_move() and window sizing are ultimately controlled by the window manager as well and most window managers will ignore certain requests from time to time, in the interests of good user interface.