The most characteristic program-modularization technique of Unix is splitting large programs into multiple cooperating processes. This has usually been called ‘multiprocessing’ in the Unix world, but in this book we revive the older term ‘multiprogramming’ to avoid confusion with multiprocessor hardware implementations.

Multiprogramming is a particularly murky area of design, one in which there are few guidelines to good practice. Many programmers with excellent judgment about how to break up code into subroutines nevertheless wind up writing whole applications as monster single-process monoliths that founder on their own internal complexity.

The Unix style of design applies the do-one-thing-well approach at the level of cooperating programs as well as cooperating routines within a program, emphasizing small programs connected by well-defined interprocess communication or by shared files. Accordingly, the Unix operating system encourages us to break our programs into simpler subprocesses, and to concentrate on the interfaces between these subprocesses. It does this in at least three fundamental ways:

In the remainder of this chapter, we'll look at the implications of cheap process-spawning and discuss how and when to apply pipes, sockets, and other interprocess communication (IPC) methods to partition your design into cooperating processes. (In the next chapter, we'll apply the same separation-of-functions philosophy to interface design.)

While the benefit of breaking programs up into cooperating processes is a reduction in global complexity, the cost is that we have to pay more attention to the design of the protocols which are used to pass information and commands between processes. (In software systems of all kinds, bugs collect at interfaces.)