Under Unix, either use popen to write to the lp or lpr program, or perhaps open
a special file like /dev/lp. Under MS-DOS, write to the (nonstandard) predefined
stdio stream stdprn, or open the special files PRN or LPT1. Under some circumstances,
another (and perhaps the only) possibility is to use a window manager's screen-capture
function, and print the resulting bitmap.