21.1. Introduction

We now develop a program that can communicate with a network printer. These printers are connected to multiple computers via Ethernet and often support PostScript files as well as plaintext files. Applications generally use the Internet Printing Protocol (IPP) to communicate with these printers, although some support alternate communication protocols.

We are about to describe two programs: a print spooler daemon that sends jobs to a printer and a command to submit print jobs to the spooler daemon. Since the print spooler has to do multiple things (communicate with clients submitting jobs, communicate with the printer, read files, scan directories, etc.), this gives us a chance to use many of the functions from earlier chapters. For example, we use threads (Chapters 11 and 12) to simplify the design of the print spooler and sockets (Chapter 16) to communicate between the program used to schedule a file to be printed and the print spooler, and also between the print spooler and the network printer.