Understanding the Linux Kernel, Second Edition

Chapter 10. Signals

Signals were introduced by the first Unix systems to allow interactions between User Mode processes; the kernel also uses them to notify processes of system events. Signals have been around for 30 years with only minor changes.

The first sections of this chapter examine in detail how signals are handled by the Linux kernel, then we discuss the system calls that allow processes to exchange signals.

The Role of Signals

A signal is a very short message that may be sent to a process or a group of processes. The only information given to the process is usually a number identifying the signal; there is no room in standard signals for arguments, a message, or other accompanying information.

A set of macros whose names start with the prefix SIG is used to identify signals; we have already made a few references to them in previous chapters. For instance, the SIGCHLD macro was mentioned in Section 3.4.1. This macro, which expands into the value 17 in Linux, yields the identifier of the signal that is sent to a parent process when a child stops or terminates. The SIGSEGV macro, which expands into the value 11, was mentioned in Section 8.4 ; it yields the identifier of the signal that is sent to a process when it makes an invalid memory reference.

Signals serve two main purposes:

• To make a process aware that a specific event has occurred

• To force a process to execute a signal handler function included in its code

Of course, the two purposes are not mutually exclusive, since ...