Understanding the Linux Kernel, 3rd Edition

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 the section "The clone( ), fork( ), and vfork( ) System Calls" in Chapter 3. 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 the section "Page Fault Exception Handler" in Chapter 9; 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 cause a process to execute a signal handler function included in its code

Of course, the two purposes are not mutually exclusive, because often a process must react to some event by executing a specific routine.

Table 11-1 lists the first 31 signals handled by Linux 2.6 for the 80×86 architecture (some signal numbers, such those associated with SIGCHLD or SIGSTOP, are architecture-dependent; furthermore, some signals such as ...