XLIB Programming Manual, Rel. 5, Third Edition

5.11 The Default GC Versus Default Values of a GC

The server creates one GC, called the default GC, when it starts up. This GC is returned by the XDefaultGC() and the DefaultGC() macro. It contains foreground and background colors that are guaranteed contrasting (but not necessarily black and white), and it contains a default font that is guaranteed to be loaded, but is not necessarily the same font on all servers. The values in the default GC must not be changed.

The default GC can be used in simple applications. But it is not very useful since all applications should provide user customization of fonts and colors, and few can avoid the need to modify other GC components as well.

When you create a GC of your own, its default values are slightly different from the values of the default GC. Its foreground and background values are 0 and 1, respectively, so they are not necessarily black and white or contrasting. That’s why you must always set foreground and background when you create a GC. Also, the default font is implementation dependent, and it may not be loaded. Therefore, you must always load the font before attempting to draw with it.

Table 5-3 shows the default values for all members of a graphics context you create.

Table 5-3. The Default Values of a Graphics Context

Component	Value
function	GXcopy
plane_mask	all 1’s
foreground	0
background	1
line_width	0
line_style	LineSolid
cap_style	CapButt
join_style	JoinMiter
fill_style	FillSolid
fill_rule	EvenOddRule
arc_mode	ArcPieSlice
tile	Pixmap filled with ...