Editing Signal (IPC) (section)

{{short description|Form of inter-process communication in computer systems}}
{{About|form of inter-process communication|event-driven processing concept|Signal programming}}
{{Use dmy dates|date=February 2021}}
{{More citations needed|date=August 2012}}

'''Signals''' are standardized messages sent to a running [[Computer program|program]] to trigger specific behavior, such as quitting or error handling. They are a limited form of [[inter-process communication]] (IPC), typically used in [[Unix]], [[Unix-like]], and other [[POSIX]]-compliant operating systems.

A signal is an [[Asynchrony (computer programming)|asynchronous]] notification sent to a [[Process (computing)|process]] or to a specific [[Thread (computing)|thread]] within the same process to notify it of an event. Common uses of signals are to interrupt, suspend, terminate or [[kill (command)|kill]] a process.  Signals originated in 1970s [[Bell Labs]] Unix and were later specified in the [[POSIX]] standard.

When a signal is sent, the operating system interrupts the target process's normal [[Control flow|flow of execution]] to deliver the signal. Execution can be interrupted during any [[Atomic operation|non-atomic instruction]]. If the process has previously registered a '''signal handler''', that routine is executed. Otherwise, the default signal handler is executed.

Embedded programs may find signals useful for inter-process communications, as signals are notable for their [[algorithmic efficiency]].

Signals are similar to [[interrupt]]s, the difference being that interrupts are mediated by the [[Central_processing_unit|CPU]] and handled by the [[Kernel (operating system)|kernel]] while signals are mediated by the kernel (possibly via system calls) and handled by individual [[Process_(computing)|processes]].{{cn |date=May 2022}} The kernel may pass an interrupt as a signal to the process that caused it (typical examples are [[SIGSEGV]], [[SIGBUS]], [[#SIGILL|SIGILL]] and [[#SIGFPE|SIGFPE]]).