7 Signal Handling (signal.h)

A 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 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:

SIGABRT

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

SIGFPE

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

SIGILL

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

SIGINT

Interrupt; an interactive attention signal.

SIGSEGV

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

SIGTERM

A request that your program end execution.

Two functions are available for dealing with asynchronous signals—one to allow your program to send signals to itself (this is called raising a signal), and one to specify subroutines (called handlers to handle particular signals that you anticipate may occur—whether raised by your own program or the operating environment.

To support these functions, signal.h defines three macros:

SIG_DFL

Used with the signal function in place of a pointer to a handler subroutine, to select the operating environment’s default handling of a signal.

SIG_IGN

Used with the signal function in place of a pointer to a handler, to ignore a particular signal.

SIG_ERR

Returned by the 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.

signal.h also defines an integral type, 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.)