A number of programs offer X modes but otherwise run in a straight
character-only mode. The easiest way for an application to determine that it
is running on an X display is to attempt to open a connection to the X
server:
	
	display = XOpenDisplay(display_name);
	if (display)
		{ do X stuff }
	else
		{ do curses or something else }

where display_name is either the string specified on the command-line
following -display, by convention, or otherwise is (char*)NULL [in which case
XOpenDisplay uses the value of $DISPLAY, if set].

This is superior to simply checking for the existence a -display command-line 
argument or checking for $DISPLAY set in the environment, neither of which is 
adequate. [5/91]

Note that there is a lengthy delay if $DISPLAY exists but is set to a machine
which is not currently running an X server.