Signal actions

Signals interrupt the normal flow of control in a process. Signals do not direct the execution of a process; but rather, request that the process take some action. For most signals, a process can arrange to respond in one of the following ways:

• terminate on receipt of the signal

• ignore it completely

• catch it and act on it in some way defined by the user process.

For example, an interrupt signal may be sent by pressing an appropriate key on the terminal (``delete'', ``break'' or ``rubout''). The action taken depends on the requirements of the specific program being executed. For example:

• The shell invokes most commands in such a way that they stop executing immediately (die) when an interrupt is received. For example, the pr (print) command normally dies, allowing the user to stop unwanted output.

• The shell itself ignores interrupts when reading from the terminal because the shell should continue execution even when the user stops a command like pr.

• The editor ed chooses to catch interrupts so that it can halt its current action (especially printing) without allowing itself to be terminated.

A signal is said to be ``delivered'' to a process when the process receives the signal and takes the action established for it. Signal delivery resembles the occurrence of a hardware interrupt: the signal is normally blocked from further occurrence, the current process context is saved, and a new one is built.