Signals are standardized messages sent to a running
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 (; trademarked as UNIX) is a family of multitasking, multiuser computer operating systems that derive from the original AT&T Unix, whose development started in 1969 at the Bell Labs research center by Ken Thompson, Dennis Ritchie, an ...
,
Unix-like
A Unix-like (sometimes referred to as UN*X or *nix) operating system is one that behaves in a manner similar to a Unix system, although not necessarily conforming to or being certified to any version of the Single UNIX Specification. A Unix-li ...
, and other
POSIX
The Portable Operating System Interface (POSIX) is a family of standards specified by the IEEE Computer Society for maintaining compatibility between operating systems. POSIX defines both the system- and user-level application programming inter ...
-compliant operating systems.
A signal is an
asynchronous notification sent to a
process
A process is a series or set of activities that interact to produce a result; it may occur once-only or be recurrent or periodic.
Things called a process include:
Business and management
*Business process, activities that produce a specific se ...
or to a specific
thread within the same process to notify it of an event. Common uses of signals are to interrupt, suspend, terminate or
kill
Kill often refers to:
*Homicide, one human killing another
*cause death, to kill a living organism, to cause its death
Kill may also refer to:
Media
*'' Kill!'', a 1968 film directed by Kihachi Okamoto
* ''Kill'' (Cannibal Corpse album), 2006
* ...
a process. Signals originated in 1970s
Bell Labs
Nokia Bell Labs, originally named Bell Telephone Laboratories (1925–1984),
then AT&T Bell Laboratories (1984–1996)
and Bell Labs Innovations (1996–2007),
is an American industrial Research and development, research and scientific developm ...
Unix and were later specified in the
POSIX
The Portable Operating System Interface (POSIX) is a family of standards specified by the IEEE Computer Society for maintaining compatibility between operating systems. POSIX defines both the system- and user-level application programming inter ...
standard.
When a signal is sent, the operating system interrupts the target process' normal
flow of execution to deliver the signal. Execution can be interrupted during any
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
In computer science, algorithmic efficiency is a property of an algorithm which relates to the amount of computational resources used by the algorithm. An algorithm must be analyzed to determine its resource usage, and the efficiency of an algo ...
.
Signals are similar to
interrupt
In digital computers, an interrupt (sometimes referred to as a trap) is a request for the processor to ''interrupt'' currently executing code (when permitted), so that the event can be processed in a timely manner. If the request is accepted, ...
s, the difference being that interrupts are mediated by the
CPU and handled by the
kernel
Kernel may refer to:
Computing
* Kernel (operating system), the central component of most operating systems
* Kernel (image processing), a matrix used for image convolution
* Compute kernel, in GPGPU programming
* Kernel method, in machine learn ...
while signals are mediated by the kernel (possibly via system calls) and handled by individual
processes. The kernel may pass an interrupt as a signal to the process that caused it (typical examples are
SIGSEGV
In computing, a segmentation fault (often shortened to segfault) or access violation is a fault, or failure condition, raised by hardware with memory protection, notifying an operating system (OS) the software has attempted to access a restrict ...
,
SIGBUS,
SIGILL
Signals are standardized messages sent to a running 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-comp ...
and
SIGFPE
Signals are standardized messages sent to a running 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-compli ...
).
History
Version 1 Unix
The term "Research Unix" refers to early versions of the Unix operating system for DEC PDP-7, PDP-11, VAX and Interdata 7/32 and 8/32 computers, developed in the Bell Labs Computing Sciences Research Center (CSRC).
History
The term ''Resear ...
had separate
system call
In computing, a system call (commonly abbreviated to syscall) is the programmatic way in which a computer program requests a service from the operating system on which it is executed. This may include hardware-related services (for example, acc ...
s to catch interrupts, quits, and machine traps.
Version 4 combined all traps into one call, , and each numbered trap received a symbolic name in
Version 7. appeared in
Version 2
Version may refer to:
Computing
* Software version, a set of numbers that identify a unique evolution of a computer program
* VERSION (CONFIG.SYS directive), a configuration directive in FreeDOS
Music
* Cover version
* Dub version
* Remix
* ' ...
, and in
Version 5 could send arbitrary signals.
Plan 9 from Bell Labs
Plan 9 from Bell Labs is a distributed operating system which originated from the Computing Science Research Center (CSRC) at Bell Labs in the mid-1980s and built on UNIX concepts first developed there in the late 1960s. Since 2000, Plan 9 has be ...
replaced signals with ''notes'', which permit sending short, arbitrary strings.
Sending signals
The system call sends a specified signal to a specified process, if permissions allow. Similarly, the command allows a user to send signals to processes. The library function sends the specified signal to the current process.
Exceptions such as
division by zero
In mathematics, division by zero is division where the divisor (denominator) is zero. Such a division can be formally expressed as \tfrac, where is the dividend (numerator). In ordinary arithmetic, the expression has no meaning, as there is ...
,
segmentation violation
In computing, a segmentation fault (often shortened to segfault) or access violation is a fault, or failure condition, raised by hardware with memory protection, notifying an operating system (OS) the software has attempted to access a restricte ...
(
SIGSEGV
In computing, a segmentation fault (often shortened to segfault) or access violation is a fault, or failure condition, raised by hardware with memory protection, notifying an operating system (OS) the software has attempted to access a restrict ...
), and floating point exception (
SIGFPE
Signals are standardized messages sent to a running 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-compli ...
) will cause a
core dump
In computing, a core dump, memory dump, crash dump, storage dump, system dump, or ABEND dump consists of the recorded state of the working memory of a computer program at a specific time, generally when the program has crashed or otherwise termina ...
and terminate the program.
The kernel can generate signals to notify processes of events. For example,
SIGPIPE will be generated when a process writes to a pipe which has been closed by the reader; by default, this causes the process to terminate, which is convenient when constructing
shell pipelines.
Typing certain key combinations at the
controlling terminal of a running process causes the system to send it certain signals:
*
Ctrl-C (in older Unixes, DEL) sends an INT signal ("interrupt",
SIGINT
Signals intelligence (SIGINT) is intelligence-gathering by interception of '' signals'', whether communications between people (communications intelligence—abbreviated to COMINT) or from electronic signals not directly used in communication ...
); by default, this causes the process to terminate.
*
Ctrl-Z
In computer data, a substitute character (␚) is a control character that is used to pad transmitted data in order to send it in blocks of fixed size, or to stand in place of a character that is recognized to be invalid, erroneous or unreprese ...
sends a TSTP signal ("terminal stop",
SIGTSTP); by default, this causes the process to suspend execution.
*
Ctrl-\ sends a QUIT signal (
SIGQUIT
Signals are standardized messages sent to a running 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-comp ...
); by default, this causes the process to terminate and dump core.
*
Ctrl-T (not supported on all UNIXes) sends an INFO signal (
SIGINFO); by default, and if supported by the command, this causes the operating system to show information about the running command.
These default key combinations with modern operating systems can be changed with the command.
Handling signals
Signal handlers can be installed with the
or system call. If a signal handler is not installed for a particular signal, the default handler is used. Otherwise the signal is intercepted and the signal handler is invoked. The process can also specify two default behaviors, without creating a handler: ignore the signal (SIG_IGN) and use the default signal handler (SIG_DFL). There are two signals which cannot be intercepted and handled:
SIGKILL
Signals are standardized messages sent to a running 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-comp ...
and
SIGSTOP
In Unix and Unix-like operating systems, job control refers to control of jobs by a shell, especially interactively, where a "job" is a shell's representation for a process group. Basic job control features are the suspending, resuming, or termi ...
.
Risks
Signal handling is vulnerable to
race condition
A race condition or race hazard is the condition of an electronics, software, or other system where the system's substantive behavior is Sequential logic, dependent on the sequence or timing of other uncontrollable events. It becomes a software ...
s. As signals are asynchronous, another signal (even of the same type) can be delivered to the process during execution of the signal handling routine.
The call can be used to block and unblock delivery of signals. Blocked signals are not delivered to the process until unblocked. Signals that cannot be ignored (SIGKILL and SIGSTOP) cannot be blocked.
Signals can cause the interruption of a system call in progress, leaving it to the application to manage a
non-transparent restart.
Signal handlers should be written in a way that does not result in any unwanted side-effects, e.g. alteration, signal mask alteration, signal disposition change, and other global
process
A process is a series or set of activities that interact to produce a result; it may occur once-only or be recurrent or periodic.
Things called a process include:
Business and management
*Business process, activities that produce a specific se ...
attribute changes. Use of non-
reentrant functions, e.g., or , inside signal handlers is also unsafe. In particular, the POSIX specification and the Linux man page require that all system functions directly or ''indirectly'' called from a signal function are ''async-signal safe''. The man page gives a list of such async-signal safe system functions (practically the
system call
In computing, a system call (commonly abbreviated to syscall) is the programmatic way in which a computer program requests a service from the operating system on which it is executed. This may include hardware-related services (for example, acc ...
s), otherwise it is an
undefined behavior
In computer programming, undefined behavior (UB) is the result of executing a program whose behavior is prescribed to be unpredictable, in the language specification to which the computer code adheres. This is different from unspecified behavior ...
. It is suggested to simply set some variable in a signal handler, and to test it elsewhere.
Signal handlers can instead put the signal into a
queue __NOTOC__
Queue () may refer to:
* Queue area, or queue, a line or area where people wait for goods or services
Arts, entertainment, and media
*''ACM Queue'', a computer magazine
* ''The Queue'' (Sorokin novel), a 1983 novel by Russian author ...
and immediately return. The main thread will then continue "uninterrupted" until signals are taken from the queue, such as in an
event loop In computer science, the event loop is a programming construct or design pattern that waits for and dispatches events or messages in a program. The event loop works by making a request to some internal or external "event provider" (that generally ...
. "Uninterrupted" here means that operations that
block
Block or blocked may refer to:
Arts, entertainment and media Broadcasting
* Block programming, the result of a programming strategy in broadcasting
* W242BX, a radio station licensed to Greenville, South Carolina, United States known as ''96.3 ...
may return prematurely and
must be resumed, as mentioned above. Signals should be processed from the queue on the main thread and not by
worker pools, as that reintroduces the problem of asynchronicity. However, managing a queue is not possible in an async-signal safe way with only , as only single reads and writes to such variables are guaranteed to be atomic, not increments or (fetch-and)-decrements, as would be required for a queue. Thus, effectively, only one signal per handler can be queued safely with until it has been processed.
Relationship with hardware exceptions
A
process
A process is a series or set of activities that interact to produce a result; it may occur once-only or be recurrent or periodic.
Things called a process include:
Business and management
*Business process, activities that produce a specific se ...
's execution may result in the generation of a hardware
exception, for instance, if the process attempts to divide by zero or incurs a
page fault
In computing, a page fault (sometimes called PF or hard fault) is an exception that the memory management unit (MMU) raises when a process accesses a memory page without proper preparations. Accessing the page requires a mapping to be added t ...
.
In
Unix-like
A Unix-like (sometimes referred to as UN*X or *nix) operating system is one that behaves in a manner similar to a Unix system, although not necessarily conforming to or being certified to any version of the Single UNIX Specification. A Unix-li ...
operating systems, this event automatically changes the processor
context
Context may refer to:
* Context (language use), the relevant constraints of the communicative situation that influence language use, language variation, and discourse summary
Computing
* Context (computing), the virtual environment required to su ...
to start executing a
kernel
Kernel may refer to:
Computing
* Kernel (operating system), the central component of most operating systems
* Kernel (image processing), a matrix used for image convolution
* Compute kernel, in GPGPU programming
* Kernel method, in machine learn ...
exception handler
In computing and computer programming, exception handling is the process of responding to the occurrence of ''exceptions'' – anomalous or exceptional conditions requiring special processing – during the Execution (computing), execution of a C ...
. In case of some exceptions, such as a
page fault
In computing, a page fault (sometimes called PF or hard fault) is an exception that the memory management unit (MMU) raises when a process accesses a memory page without proper preparations. Accessing the page requires a mapping to be added t ...
, the kernel has sufficient information to fully handle the event itself and resume the process's execution.
Other exceptions, however, the kernel cannot process intelligently and it must instead defer the exception handling operation to the faulting process. This deferral is achieved via the signal mechanism, wherein the kernel sends to the process a signal corresponding to the current exception. For example, if a process attempted integer divide by zero on an
x86
x86 (also known as 80x86 or the 8086 family) is a family of complex instruction set computer (CISC) instruction set architectures initially developed by Intel based on the Intel 8086 microprocessor and its 8088 variant. The 8086 was intr ...
CPU, a ''divide error'' exception would be generated and cause the kernel to send the
SIGFPE
Signals are standardized messages sent to a running 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-compli ...
signal to the process.
Similarly, if the process attempted to access a memory address outside of its
virtual address space
In computing, a virtual address space (VAS) or address space is the set of ranges of virtual addresses that an operating system makes available to a process. The range of virtual addresses usually starts at a low address and can extend to the hig ...
, the kernel would notify the process of this violation via a
SIGSEGV
In computing, a segmentation fault (often shortened to segfault) or access violation is a fault, or failure condition, raised by hardware with memory protection, notifying an operating system (OS) the software has attempted to access a restrict ...
(
segmentation violation
In computing, a segmentation fault (often shortened to segfault) or access violation is a fault, or failure condition, raised by hardware with memory protection, notifying an operating system (OS) the software has attempted to access a restricte ...
signal). The exact mapping between signal names and exceptions is obviously dependent upon the CPU, since exception types differ between architectures.
POSIX signals
The list below documents the signals specified in the
Single Unix Specification. All signals are defined as macro constants in the
<signal.h>
header file. The name of the macro constant consists of a "SIG"
prefix followed by a mnemonic name for the signal.
; and
: ''"Signal abort", "signal input/output trap"''
: The SIGABRT and SIGIOT signal is sent to a process to tell it to
abort, i.e. to terminate. The signal is usually initiated by the process itself when it calls
abort()
function of the
C Standard Library, but it can be sent to the process from outside like any other signal.
; , and
: ''"Signal alarm", "signal VT alarm", "signal profiling timer alarm"''
: The SIGALRM, SIGVTALRM and SIGPROF signal is sent to a process when the time limit specified in a call to a preceding alarm setting function (such as
setitimer
) elapses. SIGALRM is sent when real or clock time elapses. SIGVTALRM is sent when CPU time used by the process elapses. SIGPROF is sent when CPU time used by the process and by the system on behalf of the process elapses, i.e. when the profiling timer expires.
;
: ''"Signal bus"''
: The
SIGBUS signal is sent to a process when it causes a
bus error
In computing, a bus error is a fault raised by hardware, notifying an operating system (OS) that a process is trying to access memory that the CPU cannot physically address: an invalid address for the address bus, hence the name. In modern use on ...
. The conditions that lead to the signal being sent are, for example, incorrect memory access alignment or non-existent physical address.
;
: ''"Signal child"''
: The
SIGCHLD signal is sent to a process when a
child process
A child process in computing is a process created by another process (the parent process). This technique pertains to multitasking operating systems, and is sometimes called a subprocess or traditionally a subtask.
There are two major procedure ...
terminates, is interrupted, or resumes after being interrupted. One common usage of the signal is to instruct the operating system to clean up the resources used by a child process after its termination without an explicit call to the
wait
Wait or WAIT may refer to:
Music
* Wait (musician), British town pipers
Albums and EPs
* ''Wait'' (The Polyphonic Spree EP), by The Polyphonic Spree
* ''Wait'' (Emanuel Nice EP), a 2002 EP released by the band Emanuel Nice
* ''Wait'' (Stee ...
system call.
;
: ''"Signal continue"''
: The
SIGCONT signal instructs the operating system to continue (restart) a process previously paused by the SIGSTOP or SIGTSTP signal. One important use of this signal is in
job control in the
Unix shell
A Unix shell is a command-line interpreter or shell that provides a command line user interface for Unix-like operating systems. The shell is both an interactive command language and a scripting language, and is used by the operating syste ...
.
;
: ''"Signal
floating-point error"''
: The SIGFPE signal is sent to a process when an exceptional (but not necessarily erroneous) condition has been detected in the floating point or integer arithmetic hardware. This may include
division by zero
In mathematics, division by zero is division where the divisor (denominator) is zero. Such a division can be formally expressed as \tfrac, where is the dividend (numerator). In ordinary arithmetic, the expression has no meaning, as there is ...
, floating point underflow or overflow,
integer overflow
In computer programming, an integer overflow occurs when an arithmetic operation attempts to create a numeric value that is outside of the range that can be represented with a given number of digits – either higher than the maximum or lower ...
, an invalid operation or an inexact computation. Behaviour may differ depending on hardware.
;
: ''"Signal hangup"''
: The
SIGHUP On POSIX-compliant platforms, SIGHUP ("signal hang up") is a signal sent to a process when its controlling terminal is closed. It was originally designed to notify the process of a serial line drop. SIGHUP is a symbolic constant defined in the h ...
signal is sent to a process when its controlling terminal is closed. It was originally designed to notify the process of a
serial line
Serial may refer to:
Arts, entertainment, and media The presentation of works in sequential segments
* Serial (literature), serialised literature in print
* Serial (publishing), periodical publications and newspapers
* Serial (radio and televisio ...
drop (a hangup). In modern systems, this signal usually means that the controlling
pseudo or virtual terminal has been closed.
Many
daemons (who have no controlling terminal) interpret receipt of this signal as a request to reload their configuration files and flush/reopen their logfiles instead of exiting.
nohup
nohup is a POSIX command which means "no hang up". Its purpose is to execute a command such that it ignores the HUP (hangup) signal and therefore does not stop when the user logs out.
Output that would normally go to the terminal goes to a file ...
is a command to make a command ignore the signal.
;
: ''"Signal illegal"''
: The SIGILL signal is sent to a process when it attempts to execute an illegal, malformed, unknown, or privileged
instruction.
;
: ''"Signal interrupt"''
: The SIGINT signal is sent to a process by its controlling terminal when a user wishes to interrupt the process. This is typically initiated by pressing
, but on some systems, the "
delete" character or "
break
Break or Breaks or The Break may refer to:
Time off from duties
* Recess (break), time in which a group of people is temporarily dismissed from its duties
* Break (work), time off during a shift/recess
** Coffee break, a short mid-morning res ...
" key can be used.
;
: ''"Signal kill"''
: The SIGKILL signal is sent to a process to cause it to terminate immediately (kill). In contrast to SIGTERM and SIGINT, this signal cannot be caught or ignored, and the receiving process cannot perform any clean-up upon receiving this signal. The following exceptions apply:
:*
Zombie process
On Unix and Unix-like computer operating systems, a zombie process or defunct process is a process that has completed execution (via the exit system call) but still has an entry in the process table: it is a process in the " Terminated state". ...
es cannot be killed since they are already dead and waiting for their parent processes to reap them.
:* Processes that are in the blocked state will not die until they wake up again.
:* The ''
init'' process is special: It does not get signals that it does not want to handle, and thus it can ignore SIGKILL. An exception from this rule is while init is
ptrace
ptrace is a system call found in Unix and several Unix-like operating systems. By using ptrace (the name is an abbreviation of "process trace") one process can control another, enabling the controller to inspect and manipulate the internal state ...
d on Linux.
:* An
uninterruptibly sleeping process may not terminate (and free its resources) even when sent SIGKILL. This is one of the few cases in which a UNIX system may have to be rebooted to solve a temporary software problem.
:SIGKILL is used as a last resort when terminating processes in most system
shutdown procedures if it does not voluntarily exit in response to SIGTERM. To speed the computer shutdown procedure, Mac OS X 10.6, aka
Snow Leopard
The snow leopard (''Panthera uncia''), also known as the ounce, is a felid in the genus '' Panthera'' native to the mountain ranges of Central and South Asia. It is listed as Vulnerable on the IUCN Red List because the global population is es ...
, will send SIGKILL to applications that have marked themselves "clean" resulting in faster shutdown times with, presumably, no ill effects. The command has a similar, while dangerous effect, when executed e.g. in Linux; it doesn't let programs save unsaved data. It has other options, and with none, uses the safer SIGTERM signal.
;
: ''"Signal pipe"''
: The SIGPIPE signal is sent to a process when it attempts to write to a
pipe
Pipe(s), PIPE(S) or piping may refer to:
Objects
* Pipe (fluid conveyance), a hollow cylinder following certain dimension rules
** Piping, the use of pipes in industry
* Smoking pipe
** Tobacco pipe
* Half-pipe and quarter pipe, semi-circular ...
without a process connected to the other end.
;
: ''"Signal poll"''
: The SIGPOLL signal is sent when an event occurred on an explicitly watched file descriptor. Using it effectively leads to making
asynchronous I/O
In computer science, asynchronous I/O (also non-sequential I/O) is a form of input/output processing that permits other processing to continue before the transmission has finished. A name used for asynchronous I/O in the Windows API is overlappe ...
requests since the kernel will poll the descriptor in place of the caller. It provides an alternative to active
polling
Poll, polled, or polling may refer to:
Figurative head counts
* Poll, a formal election
** Election verification exit poll, a survey taken to verify election counts
** Polling, voting to make decisions or determine opinions
** Polling places o ...
.
; to
: ''"Signal real-time minimum", "signal real-time maximum"''
: The SIGRTMIN to SIGRTMAX signals are intended to be used for user-defined purposes. They are real-time signals.
;
: ''"Signal quit"''
: The SIGQUIT signal is sent to a process by its controlling terminal when the user requests that the process quit and perform a
core dump
In computing, a core dump, memory dump, crash dump, storage dump, system dump, or ABEND dump consists of the recorded state of the working memory of a computer program at a specific time, generally when the program has crashed or otherwise termina ...
.
;
: ''"Signal segmentation violation"''
: The
SIGSEGV
In computing, a segmentation fault (often shortened to segfault) or access violation is a fault, or failure condition, raised by hardware with memory protection, notifying an operating system (OS) the software has attempted to access a restrict ...
signal is sent to a process when it makes an invalid virtual memory reference, or
segmentation fault
In computing, a segmentation fault (often shortened to segfault) or access violation is a fault, or failure condition, raised by hardware with memory protection, notifying an operating system (OS) the software has attempted to access a restrict ...
, i.e. when it performs a ''seg''mentation ''v''iolation.
;
: ''"Signal stop"''
: The
SIGSTOP
In Unix and Unix-like operating systems, job control refers to control of jobs by a shell, especially interactively, where a "job" is a shell's representation for a process group. Basic job control features are the suspending, resuming, or termi ...
signal instructs the operating system to stop a process for later resumption.
;
: ''"Signal system call"''
: The SIGSYS signal is sent to a process when it passes a bad argument to a
system call
In computing, a system call (commonly abbreviated to syscall) is the programmatic way in which a computer program requests a service from the operating system on which it is executed. This may include hardware-related services (for example, acc ...
. In practice, this kind of signal is rarely encountered since applications rely on libraries (e.g.
libc
The C standard library or libc is the standard library for the C programming language, as specified in the ISO C standard. ISO/IEC (2018). '' ISO/IEC 9899:2018(E): Programming Languages - C §7'' Starting from the original ANSI C standard, it was ...
) to make the call for them. SIGSYS can be received by applications violating the Linux
Seccomp
seccomp (short for secure computing mode) is a computer security facility in the Linux kernel. seccomp allows a process to make a one-way transition into a "secure" state where it cannot make any system calls except exit(), sigreturn(), read() ...
security rules configured to restrict them. SIGSYS can also be used to emulate foreign system calls, e.g. emulate Windows system calls on Linux.
;
: ''"Signal terminate"''
: The SIGTERM signal is sent to a process to request its termination. Unlike the SIGKILL signal, it can be caught and interpreted or ignored by the process. This allows the process to perform nice termination releasing resources and saving state if appropriate. SIGINT is nearly identical to SIGTERM.
;
: ''"Signal terminal stop"''
: The
SIGTSTP signal is sent to a process by its controlling terminal to request it to stop (terminal stop). It is commonly initiated by the user pressing
. Unlike SIGSTOP, the process can register a signal handler for, or ignore, the signal.
; and
: The
SIGTTIN
In Unix and Unix-like operating systems, job control refers to control of jobs by a shell, especially interactively, where a "job" is a shell's representation for a process group. Basic job control features are the suspending, resuming, or termina ...
and
SIGTTOU
In Unix and Unix-like operating systems, job control refers to control of jobs by a shell, especially interactively, where a "job" is a shell's representation for a process group. Basic job control features are the suspending, resuming, or termina ...
signals are sent to a process when it attempts to read in or write out respectively from the
tty while in the
background
Background may refer to:
Performing arts and stagecraft
* Background actor
* Background artist
* Background light
* Background music
* Background story
* Background vocals
* ''Background'' (play), a 1950 play by Warren Chetham-Strode
Record ...
. Typically, these signals are received only by processes under
job control;
daemons do not have controlling terminals and, therefore, should never receive these signals.
;
: ''"Signal trap"''
: The SIGTRAP signal is sent to a process when an exception (or trap) occurs: a condition that a
debugger
A debugger or debugging tool is a computer program used to test and debug other programs (the "target" program). The main use of a debugger is to run the target program under controlled conditions that permit the programmer to track its executi ...
has requested to be informed of for example, when a particular
function
Function or functionality may refer to:
Computing
* Function key, a type of key on computer keyboards
* Function model, a structured representation of processes in a system
* Function object or functor or functionoid, a concept of object-oriente ...
is executed, or when a particular
variable
Variable may refer to:
* Variable (computer science), a symbolic name associated with a value and whose associated value may be changed
* Variable (mathematics), a symbol that represents a quantity in a mathematical expression, as used in many ...
changes value.
;
: ''"Signal urgent"''
: The
SIGURG signal is sent to a process when a
socket
Socket may refer to:
Mechanics
* Socket wrench, a type of wrench that uses separate, removable sockets to fit different sizes of nuts and bolts
* Socket head screw, a screw (or bolt) with a cylindrical head containing a socket into which the hexag ...
has urgent or
out-of-band data available to read.
; and
: ''"Signal user 1", "signal user 2""''
: The SIGUSR1 and SIGUSR2 signals are sent to a process to indicate user-defined conditions.
;
: ''"Signal exceeded CPU"''
: The SIGXCPU signal is sent to a process when it has used up the
CPU for a duration that exceeds a certain predetermined user-settable value.
The arrival of a SIGXCPU signal provides the receiving process a chance to quickly save any intermediate results and to exit gracefully, before it is terminated by the operating system using the SIGKILL signal.
;
: ''"Signal excess file size"''
: The SIGXFSZ signal is sent to a process when it grows a file that exceeds the maximum allowed size.
;
: ''"Signal window change"''
: The SIGWINCH signal is sent to a process when its controlling terminal changes its size (a window change).
Default action
A process can define
how to handle incoming POSIX signals. If a process does not define a behaviour for a signal, then the ''default handler'' for that signal is being used. The table below lists some default actions for POSIX-compliant UNIX systems, such as
FreeBSD,
OpenBSD and
Linux
Linux ( or ) is a family of open-source Unix-like operating systems based on the Linux kernel, an operating system kernel first released on September 17, 1991, by Linus Torvalds. Linux is typically packaged as a Linux distribution, w ...
.
; ''Portable number:''
: For most signals the corresponding signal number is implementation-defined. This column lists the numbers specified in the POSIX standard.
; ''Actions explained:''
: Terminate Abnormal termination of the process. The process is terminated with all the consequences of _exit() except that the status made available to wait() and waitpid() indicates abnormal termination by the specified signal.
: Terminate (core dump) Abnormal termination of the process. Additionally, implementation-defined abnormal termination actions, such as creation of a core file, may occur.
: Ignore Ignore the signal.
: Stop Stop (or suspend) the process.
: Continue Continue the process, if it is stopped; otherwise, ignore the signal.
Miscellaneous signals
The following signals are not specified in the
POSIX
The Portable Operating System Interface (POSIX) is a family of standards specified by the IEEE Computer Society for maintaining compatibility between operating systems. POSIX defines both the system- and user-level application programming inter ...
specification. They are, however, sometimes used on various systems.
;
: The SIGEMT signal is sent to a process when an
emulator
In computing, an emulator is hardware or software that enables one computer system (called the ''host'') to behave like another computer system (called the ''guest''). An emulator typically enables the host system to run software or use pe ...
trap occurs.
;
: The SIGINFO signal is sent to a process when a status (info) request is received from the controlling terminal.
;
: The SIGPWR signal is sent to a process when the system experiences a
power failure
A power outage (also called a powercut, a power out, a power failure, a power blackout, a power loss, or a blackout) is the loss of the electrical power network supply to an end user.
There are many causes of power failures in an electricity ...
.
;
: The SIGLOST signal is sent to a process when a file lock is lost.
;
: The SIGSTKFLT signal is sent to a process when the coprocessor experiences a stack fault (i.e. popping when the stack is empty or pushing when it is full).
It is defined by, but not used on Linux, where a
x87 coprocessor stack fault will generate SIGFPE instead.
;
: The SIGUNUSED signal is sent to a process when a system call with an unused system call number is made. It is synonymous with SIGSYS on most architectures.
;
: The SIGCLD signal is synonymous with SIGCHLD.
See also
*
C signal handling
References
*
*
External links
Unix Signals Table, Ali Alanjawi, University of Pittsburgh* Introduction To Unix Signals Programming
Another Introduction to Unix Signals Programming(blog post, 2009)
UNIX and Reliable POSIX Signalsby Baris Simsek
by Henning Brauer
{{Inter-process communication
Inter-process communication
Control flow