A signal is a software interrupt delivered to a process. The
operating system uses signals to report exceptional situations to an
executing program. Some signals report errors such as references to
invalid memory addresses; others report asynchronous events, such as
disconnection of a phone line.
The GNU C library defines a variety of signal types, each for a
particular kind of event. Some kinds of events make it inadvisable or
impossible for the program to proceed as usual, and the corresponding
signals normally abort the program. Other kinds of signals that report
harmless events are ignored by default.
If you anticipate an event that causes signals, you can define a handler
function and tell the operating system to run it when that particular
type of signal arrives.
Finally, one process can send a signal to another process; this allows a
parent process to abort a child, or two related processes to communicate
and synchronize.
