Signals

Although we listed them as a method of IPC in the previous section, signals are more usually employed in a wide range of other contexts, and so deserve a longer discussion.

Signals are often described as “software interrupts.” The arrival of a signal informs a process that some event or exceptional condition has occurred. There are various types of signals, each of which identifies a different event or condition. Each signal type is identified by a different integer, defined with symbolic names of the form SIGxxxx.

Signals are sent to a process by the kernel, by another process (with suitable permissions), or by the process itself. For example, the kernel may send a signal to a process when one of the following occurs:

• the user typed the interrupt character (usually Control-C) on the keyboard;

• one of the process’s children has terminated;

• a timer (alarm clock) set by the process has expired; or

• the process attempted to access an invalid memory address.

Within the shell, the kill command can be used to send a signal to a process. The kill() system call provides the same facility within programs.

When a process receives a signal, it takes one of the following actions, depending on the signal:

• it ignores the signal;

• it is killed by the signal; or

• it is suspended until later being resumed by receipt of a special-purpose signal.

For most signal types, instead of accepting the default signal action, a program can choose to ignore the signal (useful if the default action for the signal is something other than being ignored), or to establish a signal handler. A signal handler is a programmer-defined function that is automatically invoked when the signal is delivered to the process. This function performs some action appropriate to the condition that generated the signal.

In the interval between the time it is generated and the time it is delivered, a signal is said to be pending for a process. Normally, a pending signal is delivered as soon as the receiving process is next scheduled to run, or immediately if the process is already running. However, it is also possible to block a signal by adding it to the process’s signal mask. If a signal is generated while it is blocked, it remains pending until it is later unblocked (i.e., removed from the signal mask).