10.1. Introduction

Signals are software interrupts. Most nontrivial application programs need to deal with signals. Signals provide a way of handling asynchronous events: a user at a terminal typing the interrupt key to stop a program or the next program in a pipeline terminating prematurely.

Signals have been provided since the early versions of the UNIX System, but the signal model provided with systems such as Version 7 was not reliable. Signals could get lost, and it was difficult for a process to turn off selected signals when executing critical regions of code. Both 4.3BSD and SVR3 made changes to the signal model, adding what are called reliable signals. But the changes made by Berkeley and AT&T were incompatible. Fortunately, POSIX.1 standardized the reliable-signal routines, and that is what we describe here.

In this chapter, we start with an overview of signals and a description of what each signal is normally used for. Then we look at the problems with earlier implementations. It is often important to understand what is wrong with an implementation before seeing how to do things correctly. This chapter contains numerous examples that are not entirely correct and a discussion of the defects.