On POSIX

POSIX

POSIX , an acronym for "Portable Operating System Interface", is a family of standards specified by the IEEE for maintaining compatibility between operating systems...

-compliant platforms, SIGALRM is the signal

Signal (computing)

A signal is a limited form of inter-process communication used in Unix, Unix-like, and other POSIX-compliant operating systems. Essentially it is an asynchronous notification sent to a process in order to notify it of an event that occurred. When a signal is sent to a process, the operating system...

 sent to a process

Process (computing)

In computing, a process is an instance of a computer program that is being executed. It contains the program code and its current activity. Depending on the operating system , a process may be made up of multiple threads of execution that execute instructions concurrently.A computer program is a...

  when a time limit has elapsed. The symbolic constant

C preprocessor

The C preprocessor is the preprocessor for the C and C++ computer programming languages. The preprocessor handles directives for source file inclusion , macro definitions , and conditional inclusion ....

 for SIGALRM is defined in the signal.h

Signal.h

signal.h is a header file defined in the C Standard Library to specify how a program handles signals while it executes. A signal can report some exceptional behavior within the program , or a signal can report some asynchronous event outside the program .A signal can be generated...

header file

Header file

Some programming languages use header files. These files allow programmers to separate certain elements of a program's source code into reusable files. Header files commonly contain forward declarations of classes, subroutines, variables, and other identifiers...

. Symbolic signal names are used because signal numbers can vary across platforms.

Etymology

SIG is a common prefix for signal names. ALRM stands for alarm.

Usage

Computer programs often use SIGALRM to make a long-running action time out, or to provide a way of performing an action at regular intervals.

SIGALRM is generally raised a whole number of seconds after an alarm system call

System call

In computing, a system call is how a program requests a service from an operating system's kernel. This may include hardware related services , creating and executing new processes, and communicating with integral kernel services...

is made. It is sometimes used to implement the sleep function; consequently, programs cannot reliably use alarm to "wake up" from a delay caused by sleep http://perldoc.perl.org/functions/sleep.html.