General rules¶

Thesignal.signal() function allows defining custom handlers to beexecuted when a signal is received. A small number of default handlers areinstalled:SIGPIPE is ignored (so write errors on pipes and socketscan be reported as ordinary Python exceptions) andSIGINT istranslated into aKeyboardInterrupt exception if the parent processhas not changed it.

A handler for a particular signal, once set, remains installed until it isexplicitly reset (Python emulates the BSD style interface regardless of theunderlying implementation), with the exception of the handler forSIGCHLD, which follows the underlying implementation.

On WebAssembly platforms, signals are emulated and therefore behavedifferently. Several functions and signals are not available on theseplatforms.

Module contents¶

The signal module defines three enums:

classsignal.Signals¶

enum.IntEnum collection of SIG* constants and the CTRL_* constants.

Added in version 3.5.

classsignal.Handlers¶

enum.IntEnum collection the constantsSIG_DFL andSIG_IGN.

Added in version 3.5.

classsignal.Sigmasks¶

enum.IntEnum collection the constantsSIG_BLOCK,SIG_UNBLOCK andSIG_SETMASK.

Availability: Unix.

See the man pagesigprocmask(2) andpthread_sigmask(3) for further information.

Added in version 3.5.

The variables defined in thesignal module are:

signal.SIG_DFL¶

This is one of two standard signal handling options; it will simply performthe default function for the signal. For example, on most systems thedefault action forSIGQUIT is to dump core and exit, while thedefault action forSIGCHLD is to simply ignore it.

signal.SIG_IGN¶

This is another standard signal handler, which will simply ignore the givensignal.

signal.SIGABRT¶

Abort signal fromabort(3).

signal.SIGALRM¶

Timer signal fromalarm(2).

Availability: Unix.

signal.SIGBREAK¶

Interrupt from keyboard (CTRL + BREAK).

Availability: Windows.

signal.SIGBUS¶

Bus error (bad memory access).

Availability: Unix.

signal.SIGCHLD¶

Child process stopped or terminated.

Availability: Unix.

signal.SIGCLD¶

Alias toSIGCHLD.

Availability: not macOS.

signal.SIGCONT¶

Continue the process if it is currently stopped

Availability: Unix.

signal.SIGFPE¶

Floating-point exception. For example, division by zero.

See also

ZeroDivisionError is raised when the second argument of a divisionor modulo operation is zero.

signal.SIGHUP¶

Hangup detected on controlling terminal or death of controlling process.

Availability: Unix.

signal.SIGILL¶

Illegal instruction.

signal.SIGINT¶

Interrupt from keyboard (CTRL + C).

Default action is to raiseKeyboardInterrupt.

signal.SIGKILL¶

Kill signal.

It cannot be caught, blocked, or ignored.

Availability: Unix.

signal.SIGPIPE¶

Broken pipe: write to pipe with no readers.

Default action is to ignore the signal.

Availability: Unix.

signal.SIGSEGV¶

Segmentation fault: invalid memory reference.

signal.SIGSTKFLT¶

Stack fault on coprocessor. The Linux kernel does not raise this signal: itcan only be raised in user space.

Availability: Linux.

On architectures where the signal is available. Seethe man pagesignal(7) for further information.

Added in version 3.11.

signal.SIGTERM¶

Termination signal.

signal.SIGUSR1¶

User-defined signal 1.

Availability: Unix.

signal.SIGUSR2¶

User-defined signal 2.

Availability: Unix.

signal.SIGWINCH¶

Window resize signal.

Availability: Unix.

SIG*

All the signal numbers are defined symbolically. For example, the hangup signalis defined assignal.SIGHUP; the variable names are identical to thenames used in C programs, as found in<signal.h>. The Unix man page for‘signal()’ lists the existing signals (on some systems this issignal(2), on others the list is insignal(7)). Note thatnot all systems define the same set of signal names; only those names defined bythe system are defined by this module.

signal.CTRL_C_EVENT¶

The signal corresponding to theCtrl+C keystroke event. This signal canonly be used withos.kill().

Availability: Windows.

Added in version 3.2.

signal.CTRL_BREAK_EVENT¶

The signal corresponding to theCtrl+Break keystroke event. This signal canonly be used withos.kill().

Availability: Windows.

Added in version 3.2.

signal.NSIG¶

One more than the number of the highest signal number.Usevalid_signals() to get valid signal numbers.

signal.ITIMER_REAL¶

Decrements interval timer in real time, and deliversSIGALRM uponexpiration.

signal.ITIMER_VIRTUAL¶

Decrements interval timer only when the process is executing, and deliversSIGVTALRM upon expiration.

signal.ITIMER_PROF¶

Decrements interval timer both when the process executes and when thesystem is executing on behalf of the process. Coupled with ITIMER_VIRTUAL,this timer is usually used to profile the time spent by the applicationin user and kernel space. SIGPROF is delivered upon expiration.

signal.SIG_BLOCK¶

A possible value for thehow parameter topthread_sigmask()indicating that signals are to be blocked.

Added in version 3.3.

signal.SIG_UNBLOCK¶

A possible value for thehow parameter topthread_sigmask()indicating that signals are to be unblocked.

Added in version 3.3.

signal.SIG_SETMASK¶

A possible value for thehow parameter topthread_sigmask()indicating that the signal mask is to be replaced.

Added in version 3.3.

Thesignal module defines one exception:

exceptionsignal.ItimerError¶

Raised to signal an error from the underlyingsetitimer() orgetitimer() implementation. Expect this error if an invalidinterval timer or a negative time is passed tosetitimer().This error is a subtype ofOSError.

Added in version 3.3:This error used to be a subtype ofIOError, which is now analias ofOSError.

Thesignal module defines the following functions:

signal.alarm(time)¶

Iftime is non-zero, this function requests that aSIGALRM signal besent to the process intime seconds. Any previously scheduled alarm iscanceled (only one alarm can be scheduled at any time). The returned value isthen the number of seconds before any previously set alarm was to have beendelivered. Iftime is zero, no alarm is scheduled, and any scheduled alarm iscanceled. If the return value is zero, no alarm is currently scheduled.

Availability: Unix.

See the man pagealarm(2) for further information.

signal.getsignal(signalnum)¶

Return the current signal handler for the signalsignalnum. The returned valuemay be a callable Python object, or one of the special valuessignal.SIG_IGN,signal.SIG_DFL orNone. Here,signal.SIG_IGN means that the signal was previously ignored,signal.SIG_DFL means that the default way of handling the signal waspreviously in use, andNone means that the previous signal handler was notinstalled from Python.

signal.strsignal(signalnum)¶

Returns the description of signalsignalnum, such as “Interrupt”forSIGINT. ReturnsNone ifsignalnum has nodescription. RaisesValueError ifsignalnum is invalid.

Added in version 3.8.

signal.valid_signals()¶

Return the set of valid signal numbers on this platform. This can beless thanrange(1,NSIG) if some signals are reserved by the systemfor internal use.

Added in version 3.8.

signal.pause()¶

Cause the process to sleep until a signal is received; the appropriate handlerwill then be called. Returns nothing.

Availability: Unix.

See the man pagesignal(2) for further information.

See alsosigwait(),sigwaitinfo(),sigtimedwait() andsigpending().

signal.raise_signal(signum)¶

Sends a signal to the calling process. Returns nothing.

Added in version 3.8.

signal.pidfd_send_signal(pidfd,sig,siginfo=None,flags=0)¶

Send signalsig to the process referred to by file descriptorpidfd.Python does not currently support thesiginfo parameter; it must beNone. Theflags argument is provided for future extensions; no flagvalues are currently defined.

See thepidfd_send_signal(2) man page for more information.

Availability: Linux >= 5.1, Android >=build-time API level 31

Added in version 3.9.

signal.pthread_kill(thread_id,signalnum)¶

Send the signalsignalnum to the threadthread_id, another thread in thesame process as the caller. The target thread can be executing any code(Python or not). However, if the target thread is executing the Pythoninterpreter, the Python signal handlers will beexecuted by the mainthread of the main interpreter. Therefore, the only point of sending asignal to a particular Python thread would be to force a running system callto fail withInterruptedError.

Usethreading.get_ident() or theidentattribute ofthreading.Thread objects to get a suitable valueforthread_id.

Ifsignalnum is 0, then no signal is sent, but error checking is stillperformed; this can be used to check if the target thread is still running.

Raises anauditing eventsignal.pthread_kill with argumentsthread_id,signalnum.

Availability: Unix.

See the man pagepthread_kill(3) for further information.

See alsoos.kill().

Added in version 3.3.

signal.pthread_sigmask(how,mask)¶

Fetch and/or change the signal mask of the calling thread. The signal maskis the set of signals whose delivery is currently blocked for the caller.Return the old signal mask as a set of signals.

The behavior of the call is dependent on the value ofhow, as follows.

• SIG_BLOCK: The set of blocked signals is the union of the currentset and themask argument.

• SIG_UNBLOCK: The signals inmask are removed from the currentset of blocked signals. It is permissible to attempt to unblock asignal which is not blocked.

• SIG_SETMASK: The set of blocked signals is set to themaskargument.

mask is a set of signal numbers (e.g. {signal.SIGINT,signal.SIGTERM}). Usevalid_signals() for a fullmask including all signals.

For example,signal.pthread_sigmask(signal.SIG_BLOCK,[]) reads thesignal mask of the calling thread.

SIGKILL andSIGSTOP cannot be blocked.

Availability: Unix.

See the man pagesigprocmask(2) andpthread_sigmask(3) for further information.

See alsopause(),sigpending() andsigwait().

Added in version 3.3.

signal.setitimer(which,seconds,interval=0.0)¶

Sets given interval timer (one ofsignal.ITIMER_REAL,signal.ITIMER_VIRTUAL orsignal.ITIMER_PROF) specifiedbywhich to fire afterseconds (float is accepted, different fromalarm()) and after that everyinterval seconds (ifintervalis non-zero). The interval timer specified bywhich can be cleared bysettingseconds to zero.

When an interval timer fires, a signal is sent to the process.The signal sent is dependent on the timer being used;signal.ITIMER_REAL will deliverSIGALRM,signal.ITIMER_VIRTUAL sendsSIGVTALRM,andsignal.ITIMER_PROF will deliverSIGPROF.

The old values are returned as a tuple: (delay, interval).

Attempting to pass an invalid interval timer will cause anItimerError.

Availability: Unix.

signal.getitimer(which)¶

Returns current value of a given interval timer specified bywhich.

Availability: Unix.

signal.set_wakeup_fd(fd,*,warn_on_full_buffer=True)¶

Set the wakeup file descriptor tofd. When a signal your program hasregistered a signal handler for is received, the signal number is written asa single byte into the fd. If you haven’t registered a signal handler forthe signals you care about, then nothing will be written to the wakeup fd.This can be used by a library to wakeup a poll or select call, allowing thesignal to be fully processed.

The old wakeup fd is returned (or -1 if file descriptor wakeup was notenabled). Iffd is -1, file descriptor wakeup is disabled.If not -1,fd must be non-blocking. It is up to the library to removeany bytes fromfd before calling poll or select again.

When threads are enabled, this function can only be calledfromthe main thread of the main interpreter;attempting to call it from other threads will cause aValueErrorexception to be raised.

There are two common ways to use this function. In both approaches,you use the fd to wake up when a signal arrives, but then theydiffer in how they determinewhich signal or signals havearrived.

In the first approach, we read the data out of the fd’s buffer, andthe byte values give you the signal numbers. This is simple, but inrare cases it can run into a problem: generally the fd will have alimited amount of buffer space, and if too many signals arrive tooquickly, then the buffer may become full, and some signals may belost. If you use this approach, then you should setwarn_on_full_buffer=True, which will at least cause a warningto be printed to stderr when signals are lost.

In the second approach, we use the wakeup fdonly for wakeups,and ignore the actual byte values. In this case, all we care aboutis whether the fd’s buffer is empty or non-empty; a full bufferdoesn’t indicate a problem at all. If you use this approach, thenyou should setwarn_on_full_buffer=False, so that your usersare not confused by spurious warning messages.

Changed in version 3.5:On Windows, the function now also supports socket handles.

Changed in version 3.7:Addedwarn_on_full_buffer parameter.

signal.siginterrupt(signalnum,flag)¶

Change system call restart behaviour: ifflag isFalse, systemcalls will be restarted when interrupted by signalsignalnum, otherwisesystem calls will be interrupted. Returns nothing.

Availability: Unix.

See the man pagesiginterrupt(3) for further information.

Note that installing a signal handler withsignal() will reset therestart behaviour to interruptible by implicitly callingsiginterrupt() with a trueflag value for the given signal.

signal.signal(signalnum,handler)¶

Set the handler for signalsignalnum to the functionhandler.handler canbe a callable Python object taking two arguments (see below), or one of thespecial valuessignal.SIG_IGN orsignal.SIG_DFL. The previoussignal handler will be returned (see the description ofgetsignal()above). (See the Unix man pagesignal(2) for further information.)

When threads are enabled, this function can only be calledfromthe main thread of the main interpreter;attempting to call it from other threads will cause aValueErrorexception to be raised.

Thehandler is called with two arguments: the signal number and the currentstack frame (None or a frame object; for a description of frame objects,see thedescription in the type hierarchy or see theattribute descriptions in theinspect module).

On Windows,signal() can only be called withSIGABRT,SIGFPE,SIGILL,SIGINT,SIGSEGV,SIGTERM, orSIGBREAK.AValueError will be raised in any other case.Note that not all systems define the same set of signal names; anAttributeError will be raised if a signal name is not defined asSIG* module level constant.

signal.sigpending()¶

Examine the set of signals that are pending for delivery to the callingthread (i.e., the signals which have been raised while blocked). Return theset of the pending signals.

Availability: Unix.

See the man pagesigpending(2) for further information.

See alsopause(),pthread_sigmask() andsigwait().

Added in version 3.3.

signal.sigwait(sigset)¶

Suspend execution of the calling thread until the delivery of one of thesignals specified in the signal setsigset. The function accepts the signal(removes it from the pending list of signals), and returns the signal number.

Availability: Unix.

See the man pagesigwait(3) for further information.

See alsopause(),pthread_sigmask(),sigpending(),sigwaitinfo() andsigtimedwait().

Added in version 3.3.

signal.sigwaitinfo(sigset)¶

Suspend execution of the calling thread until the delivery of one of thesignals specified in the signal setsigset. The function accepts thesignal and removes it from the pending list of signals. If one of thesignals insigset is already pending for the calling thread, the functionwill return immediately with information about that signal. The signalhandler is not called for the delivered signal. The function raises anInterruptedError if it is interrupted by a signal that is not insigset.

The return value is an object representing the data contained in thesiginfo_t structure, namely:si_signo,si_code,si_errno,si_pid,si_uid,si_status,si_band.

Availability: Unix.

See the man pagesigwaitinfo(2) for further information.

See alsopause(),sigwait() andsigtimedwait().

Added in version 3.3.

Changed in version 3.5:The function is now retried if interrupted by a signal not insigsetand the signal handler does not raise an exception (seePEP 475 forthe rationale).

signal.sigtimedwait(sigset,timeout)¶

Likesigwaitinfo(), but takes an additionaltimeout argumentspecifying a timeout. Iftimeout is specified as0, a poll isperformed. ReturnsNone if a timeout occurs.

Availability: Unix.

See the man pagesigtimedwait(2) for further information.

See alsopause(),sigwait() andsigwaitinfo().

Added in version 3.3.

Changed in version 3.5:The function is now retried with the recomputedtimeout if interruptedby a signal not insigset and the signal handler does not raise anexception (seePEP 475 for the rationale).

Examples¶

Here is a minimal example program. It uses thealarm() function to limitthe time spent waiting to open a file; this is useful if the file is for aserial device that may not be turned on, which would normally cause theos.open() to hang indefinitely. The solution is to set a 5-second alarmbefore opening the file; if the operation takes too long, the alarm signal willbe sent, and the handler raises an exception.

importsignal,osdefhandler(signum,frame):signame=signal.Signals(signum).nameprint(f'Signal handler called with signal{signame} ({signum})')raiseOSError("Couldn't open device!")# Set the signal handler and a 5-second alarmsignal.signal(signal.SIGALRM,handler)signal.alarm(5)# This open() may hang indefinitelyfd=os.open('/dev/ttyS0',os.O_RDWR)signal.alarm(0)# Disable the alarm

Note on SIGPIPE¶

Piping output of your program to tools likehead(1) willcause aSIGPIPE signal to be sent to your process when the receiverof its standard output closes early. This results in an exceptionlikeBrokenPipeError:[Errno32]Brokenpipe. To handle thiscase, wrap your entry point to catch this exception as follows:

importosimportsysdefmain():try:# simulate large output (your code replaces this loop)forxinrange(10000):print("y")# flush output here to force SIGPIPE to be triggered# while inside this try block.sys.stdout.flush()exceptBrokenPipeError:# Python flushes standard streams on exit; redirect remaining output# to devnull to avoid another BrokenPipeError at shutdowndevnull=os.open(os.devnull,os.O_WRONLY)os.dup2(devnull,sys.stdout.fileno())sys.exit(1)# Python exits with error code 1 on EPIPEif__name__=='__main__':main()

Do not setSIGPIPE’s disposition toSIG_DFL inorder to avoidBrokenPipeError. Doing that would causeyour program to exit unexpectedly whenever any socketconnection is interrupted while your program is still writing toit.

Note on Signal Handlers and Exceptions¶

If a signal handler raises an exception, the exception will be propagated tothe main thread and may be raised after anybytecode instruction. Mostnotably, aKeyboardInterrupt may appear at any point during execution.Most Python code, including the standard library, cannot be made robust againstthis, and so aKeyboardInterrupt (or any other exception resulting froma signal handler) may on rare occasions put the program in an unexpected state.

To illustrate this issue, consider the following code:

classSpamContext:def__init__(self):self.lock=threading.Lock()def__enter__(self):# If KeyboardInterrupt occurs here, everything is fineself.lock.acquire()# If KeyboardInterrupt occurs here, __exit__ will not be called...# KeyboardInterrupt could occur just before the function returnsdef__exit__(self,exc_type,exc_val,exc_tb):...self.lock.release()

For many programs, especially those that merely want to exit onKeyboardInterrupt, this is not a problem, but applications that arecomplex or require high reliability should avoid raising exceptions from signalhandlers. They should also avoid catchingKeyboardInterrupt as a meansof gracefully shutting down. Instead, they should install their ownSIGINT handler. Below is an example of an HTTP server that avoidsKeyboardInterrupt:

importsignalimportsocketfromselectorsimportDefaultSelector,EVENT_READfromhttp.serverimportHTTPServer,SimpleHTTPRequestHandlerinterrupt_read,interrupt_write=socket.socketpair()defhandler(signum,frame):print('Signal handler called with signal',signum)interrupt_write.send(b'\0')signal.signal(signal.SIGINT,handler)defserve_forever(httpd):sel=DefaultSelector()sel.register(interrupt_read,EVENT_READ)sel.register(httpd,EVENT_READ)whileTrue:forkey,_insel.select():ifkey.fileobj==interrupt_read:interrupt_read.recv(1)returnifkey.fileobj==httpd:httpd.handle_request()print("Serving on port 8000")httpd=HTTPServer(('',8000),SimpleHTTPRequestHandler)serve_forever(httpd)print("Shutdown...")