SIGHUP
Encyclopedia
On POSIX
-compliant platforms, SIGHUP 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 header file
of hang up.
Access to computer systems
for many years consisted of connecting a terminal
to a mainframe system via a serial line and the RS-232
protocol
. For this reason, when a system of software interrupt
s, called signals, were being developed, a signal was designated for use on "Hangup".
SIGHUP would be sent to programs
when the serial line was dropped, often because the connected user terminated the connection by hanging up the modem. The system would detect the line was dropped via the lost DCD "carrier detect" signal.
Signals have always been a convenient method of inter-process communication
(IPC), but in early implementations there were no user-definable signals (such as the later additions of SIGUSR1 and SIGUSR2) that programs could intercept and interpret for their own purposes. For this reason, applications that did not require a controlling terminal, such as daemons
, would re-purpose SIGHUP as a signal to re-read configuration files, or reinitialize. This convention survives to this day in packages such as Apache
and Sendmail
.
has been closed.
If the program receiving SIGHUP is a Unix shell
, it will often intercept the signal and ensure all stopped programs are continued before sending the signal to child jobs.
The Single UNIX Specification
describes a shell utility called nohup
, which can be used as a wrapper to start a program and make it immune to SIGHUP.
Different shells also have other methods of controlling and managing SIGHUP, such as the disown facility of GNU bash. Most modern Linux distribution
s documentation specify using kill -HUP
Daemon
programs sometimes use SIGHUP as a signal to restart themselves, the most common reason for this being to re-read a configuration file which has been changed.
-conformant systems allow the use of the numeric constant 1 to be used to indicate a SIGHUP, which the vast majority of systems in fact use.
SIGHUP can be handled. That is, programmer
s can define the action they want to occur upon receiving a SIGHUP, such as calling a function
, ignoring it, or restoring the default action.
The default action on POSIX
-compliant systems is an abnormal termination.
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, SIGHUP is a 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 its controlling terminal is closed. (It was originally designed to notify the process of a serial line
RS-232
In telecommunications, RS-232 is the traditional name for a series of standards for serial binary single-ended data and control signals connecting between a DTE and a DCE . It is commonly used in computer serial ports...
drop). SIGHUP is a 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 ....
defined in the 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...
signal.hSignal.hsignal.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...
.Etymology
SIG is a common prefix for signal names; HUP is a contractionContraction (grammar)
A contraction is a shortened version of the written and spoken forms of a word, syllable, or word group, created by omission of internal letters....
of hang up.
History
Mainframe computer
Mainframes are powerful computers used primarily by corporate and governmental organizations for critical applications, bulk data processing such as census, industry and consumer statistics, enterprise resource planning, and financial transaction processing.The term originally referred to the...
for many years consisted of connecting a terminal
Computer terminal
A computer terminal is an electronic or electromechanical hardware device that is used for entering data into, and displaying data from, a computer or a computing system...
to a mainframe system via a serial line and the RS-232
RS-232
In telecommunications, RS-232 is the traditional name for a series of standards for serial binary single-ended data and control signals connecting between a DTE and a DCE . It is commonly used in computer serial ports...
protocol
Communications protocol
A communications protocol is a system of digital message formats and rules for exchanging those messages in or between computing systems and in telecommunications...
. For this reason, when a system of software interrupt
Interrupt
In computing, an interrupt is an asynchronous signal indicating the need for attention or a synchronous event in software indicating the need for a change in execution....
s, called signals, were being developed, a signal was designated for use on "Hangup".
SIGHUP would be sent to programs
Computer program
A computer program is a sequence of instructions written to perform a specified task with a computer. A computer requires programs to function, typically executing the program's instructions in a central processor. The program has an executable form that the computer can use directly to execute...
when the serial line was dropped, often because the connected user terminated the connection by hanging up the modem. The system would detect the line was dropped via the lost DCD "carrier detect" signal.
Signals have always been a convenient method of inter-process communication
Inter-process communication
In computing, Inter-process communication is a set of methods for the exchange of data among multiple threads in one or more processes. Processes may be running on one or more computers connected by a network. IPC methods are divided into methods for message passing, synchronization, shared...
(IPC), but in early implementations there were no user-definable signals (such as the later additions of SIGUSR1 and SIGUSR2) that programs could intercept and interpret for their own purposes. For this reason, applications that did not require a controlling terminal, such as daemons
Daemon (computer software)
In Unix and other multitasking computer operating systems, a daemon is a computer program that runs as a background process, rather than being under the direct control of an interactive user...
, would re-purpose SIGHUP as a signal to re-read configuration files, or reinitialize. This convention survives to this day in packages such as Apache
Apache HTTP Server
The Apache HTTP Server, commonly referred to as Apache , is web server software notable for playing a key role in the initial growth of the World Wide Web. In 2009 it became the first web server software to surpass the 100 million website milestone...
and Sendmail
Sendmail
Sendmail is a general purpose internetwork email routing facility that supports many kinds of mail-transfer and -delivery methods, including the Simple Mail Transfer Protocol used for email transport over the Internet....
.
Modern usage
With the decline of access via serial line, the meaning of SIGHUP has changed somewhat on modern systems, often meaning a controlling pseudo or virtual terminalTerminal emulator
A terminal emulator, terminal application, term, or tty for short, is a program that emulates a video terminal within some other display architecture....
has been closed.
If the program receiving SIGHUP is a Unix shell
Unix shell
A Unix shell is a command-line interpreter or shell that provides a traditional user interface for the Unix operating system and for Unix-like systems...
, it will often intercept the signal and ensure all stopped programs are continued before sending the signal to child jobs.
The Single UNIX Specification
Single UNIX Specification
The Single UNIX Specification is the collective name of a family of standards for computer operating systems to qualify for the name "Unix"...
describes a shell utility called nohup
Nohup
nohup is a POSIX command to ignore the HUP signal, enabling the command to keep running after the user who issues the command has logged out. The HUP signal is by convention the way a terminal warns depending processes of logout....
, which can be used as a wrapper to start a program and make it immune to SIGHUP.
Different shells also have other methods of controlling and managing SIGHUP, such as the disown facility of GNU bash. Most modern Linux distribution
Linux distribution
A Linux distribution is a member of the family of Unix-like operating systems built on top of the Linux kernel. Such distributions are operating systems including a large collection of software applications such as word processors, spreadsheets, media players, and database applications...
s documentation specify using kill -HUP
Daemon
Daemon (computer software)
In Unix and other multitasking computer operating systems, a daemon is a computer program that runs as a background process, rather than being under the direct control of an interactive user...
programs sometimes use SIGHUP as a signal to restart themselves, the most common reason for this being to re-read a configuration file which has been changed.
Details
Symbolic signal names are used because signal numbers can vary across platforms, but XSISingle UNIX Specification
The Single UNIX Specification is the collective name of a family of standards for computer operating systems to qualify for the name "Unix"...
-conformant systems allow the use of the numeric constant 1 to be used to indicate a SIGHUP, which the vast majority of systems in fact use.
SIGHUP can be handled. That is, programmer
Programmer
A programmer, computer programmer or coder is someone who writes computer software. The term computer programmer can refer to a specialist in one area of computer programming or to a generalist who writes code for many kinds of software. One who practices or professes a formal approach to...
s can define the action they want to occur upon receiving a SIGHUP, such as calling a function
Subroutine
In computer science, a subroutine is a portion of code within a larger program that performs a specific task and is relatively independent of the remaining code....
, ignoring it, or restoring the default action.
The default action 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 systems is an abnormal termination.