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@node Signals

@chapter Signal Handling (@file{signal.h})

A @dfn{signal} is an event that interrupts the normal flow of control

in your program.  Your operating environment normally defines the full

set of signals available (see @file{sys/signal.h}), as well as the

default means of dealing with them---typically, either printing an

error message and aborting your program, or ignoring the signal.

All systems support at least the following signals:

@table @code

@item SIGABRT

Abnormal termination of a program; raised by the <<abort>> function.

@item SIGFPE

A domain error in arithmetic, such as overflow, or division by zero.

@item SIGILL

Attempt to execute as a function data that is not executable.

@item SIGINT

Interrupt; an interactive attention signal.

@item SIGSEGV

An attempt to access a memory location that is not available.

@item SIGTERM

A request that your program end execution.

@end table

Two functions are available for dealing with asynchronous

signals---one to allow your program to send signals to itself (this is

called @dfn{raising} a signal), and one to specify subroutines (called

@dfn{handlers} to handle particular signals that you anticipate may

occur---whether raised by your own program or the operating environment.

To support these functions, @file{signal.h} defines three macros:

@table @code

@item SIG_DFL

Used with the @code{signal} function in place of a pointer to a

handler subroutine, to select the operating environment's default

handling of a signal.

@item SIG_IGN

Used with the @code{signal} function in place of a pointer to a

handler, to ignore a particular signal.

@item SIG_ERR

Returned by the @code{signal} function in place of a pointer to a

handler, to indicate that your request to set up a handler could not

be honored for some reason.

@end table

@file{signal.h} also defines an integral type, @code{sig_atomic_t}.

This type is not used in any function declarations; it exists only to

allow your signal handlers to declare a static storage location where

they may store a signal value.  (Static storage is not otherwise

reliable from signal handlers.)

@menu

* raise::   Send a signal

* signal::  Specify handler subroutine for a signal

@end menu

@page

@include signal/raise.def

@page

@include signal/signal.def