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:mod:`!signal` --- Set handlers for asynchronous events
=======================================================
.. module:: signal
:synopsis: Set handlers for asynchronous events.
**Source code:** :source:`Lib/signal.py`
--------------
This module provides mechanisms to use signal handlers in Python.
General rules
-------------
The :func:`signal.signal` function allows defining custom handlers to be
executed when a signal is received. A small number of default handlers are
installed: :const:`SIGPIPE` is ignored (so write errors on pipes and sockets
can be reported as ordinary Python exceptions) and :const:`SIGINT` is
translated into a :exc:`KeyboardInterrupt` exception if the parent process
has not changed it.
A handler for a particular signal, once set, remains installed until it is
explicitly reset (Python emulates the BSD style interface regardless of the
underlying implementation), with the exception of the handler for
:const:`SIGCHLD`, which follows the underlying implementation.
On WebAssembly platforms, signals are emulated and therefore behave
differently. Several functions and signals are not available on these
platforms.
Execution of Python signal handlers
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
A Python signal handler does not get executed inside the low-level (C) signal
handler. Instead, the low-level signal handler sets a flag which tells the
:term:`virtual machine` to execute the corresponding Python signal handler
at a later point(for example at the next :term:`bytecode` instruction).
This has consequences:
* It makes little sense to catch synchronous errors like :const:`SIGFPE` or
:const:`SIGSEGV` that are caused by an invalid operation in C code. Python
will return from the signal handler to the C code, which is likely to raise
the same signal again, causing Python to apparently hang. From Python 3.3
onwards, you can use the :mod:`faulthandler` module to report on synchronous
errors.
* A long-running calculation implemented purely in C (such as regular
expression matching on a large body of text) may run uninterrupted for an
arbitrary amount of time, regardless of any signals received. The Python
signal handlers will be called when the calculation finishes.
* If the handler raises an exception, it will be raised "out of thin air" in
the main thread. See the :ref:`note below <handlers-and-exceptions>` for a
discussion.
.. _signals-and-threads:
Signals and threads
^^^^^^^^^^^^^^^^^^^
Python signal handlers are always executed in the main Python thread of the main interpreter,
even if the signal was received in another thread. This means that signals
can't be used as a means of inter-thread communication. You can use
the synchronization primitives from the :mod:`threading` module instead.
Besides, only the main thread of the main interpreter is allowed to set a new signal handler.
Module contents
---------------
.. versionchanged:: 3.5
signal (SIG*), handler (:const:`SIG_DFL`, :const:`SIG_IGN`) and sigmask
(:const:`SIG_BLOCK`, :const:`SIG_UNBLOCK`, :const:`SIG_SETMASK`)
related constants listed below were turned into
:class:`enums <enum.IntEnum>` (:class:`Signals`, :class:`Handlers` and :class:`Sigmasks` respectively).
:func:`getsignal`, :func:`pthread_sigmask`, :func:`sigpending` and
:func:`sigwait` functions return human-readable
:class:`enums <enum.IntEnum>` as :class:`Signals` objects.
The signal module defines three enums:
.. class:: Signals
:class:`enum.IntEnum` collection of SIG* constants and the CTRL_* constants.
.. versionadded:: 3.5
.. class:: Handlers
:class:`enum.IntEnum` collection the constants :const:`SIG_DFL` and :const:`SIG_IGN`.
.. versionadded:: 3.5
.. class:: Sigmasks
:class:`enum.IntEnum` collection the constants :const:`SIG_BLOCK`, :const:`SIG_UNBLOCK` and :const:`SIG_SETMASK`.
.. availability:: Unix.
See the man page :manpage:`sigprocmask(2)` and
:manpage:`pthread_sigmask(3)` for further information.
.. versionadded:: 3.5
The variables defined in the :mod:`signal` module are:
.. data:: SIG_DFL
This is one of two standard signal handling options; it will simply perform
the default function for the signal. For example, on most systems the
default action for :const:`SIGQUIT` is to dump core and exit, while the
default action for :const:`SIGCHLD` is to simply ignore it.
.. data:: SIG_IGN
This is another standard signal handler, which will simply ignore the given
signal.
.. data:: SIGABRT
Abort signal from :manpage:`abort(3)`.
.. data:: SIGALRM
Timer signal from :manpage:`alarm(2)`.
.. availability:: Unix.
.. data:: SIGBREAK
Interrupt from keyboard (CTRL + BREAK).
.. availability:: Windows.
.. data:: SIGBUS
Bus error (bad memory access).
.. availability:: Unix.
.. data:: SIGCHLD
Child process stopped or terminated.
.. availability:: Unix.
.. data:: SIGCLD
Alias to :data:`SIGCHLD`.
.. availability:: not macOS.
.. data:: SIGCONT
Continue the process if it is currently stopped
.. availability:: Unix.
.. data:: SIGFPE
Floating-point exception. For example, division by zero.
.. seealso::
:exc:`ZeroDivisionError` is raised when the second argument of a division
or modulo operation is zero.
.. data:: SIGHUP
Hangup detected on controlling terminal or death of controlling process.
.. availability:: Unix.
.. data:: SIGILL
Illegal instruction.
.. data:: SIGINT
Interrupt from keyboard (CTRL + C).
Default action is to raise :exc:`KeyboardInterrupt`.
.. data:: SIGKILL
Kill signal.
It cannot be caught, blocked, or ignored.
.. availability:: Unix.
.. data:: SIGPIPE
Broken pipe: write to pipe with no readers.
Default action is to ignore the signal.
.. availability:: Unix.
.. data:: SIGSEGV
Segmentation fault: invalid memory reference.
.. data:: SIGSTKFLT
Stack fault on coprocessor. The Linux kernel does not raise this signal: it
can only be raised in user space.
.. availability:: Linux.
On architectures where the signal is available. See
the man page :manpage:`signal(7)` for further information.
.. versionadded:: 3.11
.. data:: SIGTERM
Termination signal.
.. data:: SIGUSR1
User-defined signal 1.
.. availability:: Unix.
.. data:: SIGUSR2
User-defined signal 2.
.. availability:: Unix.
.. data:: SIGWINCH
Window resize signal.
.. availability:: Unix.
.. data:: SIG*
All the signal numbers are defined symbolically. For example, the hangup signal
is defined as :const:`signal.SIGHUP`; the variable names are identical to the
names used in C programs, as found in ``<signal.h>``. The Unix man page for
':c:func:`signal`' lists the existing signals (on some systems this is
:manpage:`signal(2)`, on others the list is in :manpage:`signal(7)`). Note that
not all systems define the same set of signal names; only those names defined by
the system are defined by this module.
.. data:: CTRL_C_EVENT
The signal corresponding to the :kbd:`Ctrl+C` keystroke event. This signal can
only be used with :func:`os.kill`.
.. availability:: Windows.
.. versionadded:: 3.2
.. data:: CTRL_BREAK_EVENT
The signal corresponding to the :kbd:`Ctrl+Break` keystroke event. This signal can
only be used with :func:`os.kill`.
.. availability:: Windows.
.. versionadded:: 3.2
.. data:: NSIG
One more than the number of the highest signal number.
Use :func:`valid_signals` to get valid signal numbers.
.. data:: ITIMER_REAL
Decrements interval timer in real time, and delivers :const:`SIGALRM` upon
expiration.
.. data:: ITIMER_VIRTUAL
Decrements interval timer only when the process is executing, and delivers
SIGVTALRM upon expiration.
.. data:: ITIMER_PROF
Decrements interval timer both when the process executes and when the
system is executing on behalf of the process. Coupled with ITIMER_VIRTUAL,
this timer is usually used to profile the time spent by the application
in user and kernel space. SIGPROF is delivered upon expiration.
.. data:: SIG_BLOCK
A possible value for the *how* parameter to :func:`pthread_sigmask`
indicating that signals are to be blocked.
.. versionadded:: 3.3
.. data:: SIG_UNBLOCK
A possible value for the *how* parameter to :func:`pthread_sigmask`
indicating that signals are to be unblocked.
.. versionadded:: 3.3
.. data:: SIG_SETMASK
A possible value for the *how* parameter to :func:`pthread_sigmask`
indicating that the signal mask is to be replaced.
.. versionadded:: 3.3
The :mod:`signal` module defines one exception:
.. exception:: ItimerError
Raised to signal an error from the underlying :func:`setitimer` or
:func:`getitimer` implementation. Expect this error if an invalid
interval timer or a negative time is passed to :func:`setitimer`.
This error is a subtype of :exc:`OSError`.
.. versionadded:: 3.3
This error used to be a subtype of :exc:`IOError`, which is now an
alias of :exc:`OSError`.
The :mod:`signal` module defines the following functions:
.. function:: alarm(time)
If *time* is non-zero, this function requests that a :const:`SIGALRM` signal be
sent to the process in *time* seconds. Any previously scheduled alarm is
canceled (only one alarm can be scheduled at any time). The returned value is
then the number of seconds before any previously set alarm was to have been
delivered. If *time* is zero, no alarm is scheduled, and any scheduled alarm is
canceled. If the return value is zero, no alarm is currently scheduled.
.. availability:: Unix.
See the man page :manpage:`alarm(2)` for further information.
.. function:: getsignal(signalnum)
Return the current signal handler for the signal *signalnum*. The returned value
may be a callable Python object, or one of the special values
:const:`signal.SIG_IGN`, :const:`signal.SIG_DFL` or :const:`None`. Here,
:const:`signal.SIG_IGN` means that the signal was previously ignored,
:const:`signal.SIG_DFL` means that the default way of handling the signal was
previously in use, and ``None`` means that the previous signal handler was not
installed from Python.
.. function:: strsignal(signalnum)
Returns the description of signal *signalnum*, such as "Interrupt"
for :const:`SIGINT`. Returns :const:`None` if *signalnum* has no
description. Raises :exc:`ValueError` if *signalnum* is invalid.
.. versionadded:: 3.8
.. function:: valid_signals()
Return the set of valid signal numbers on this platform. This can be
less than ``range(1, NSIG)`` if some signals are reserved by the system
for internal use.
.. versionadded:: 3.8
.. function:: pause()
Cause the process to sleep until a signal is received; the appropriate handler
will then be called. Returns nothing.
.. availability:: Unix.
See the man page :manpage:`signal(2)` for further information.
See also :func:`sigwait`, :func:`sigwaitinfo`, :func:`sigtimedwait` and
:func:`sigpending`.
.. function:: raise_signal(signum)
Sends a signal to the calling process. Returns nothing.
.. versionadded:: 3.8
.. function:: pidfd_send_signal(pidfd, sig, siginfo=None, flags=0)
Send signal *sig* to the process referred to by file descriptor *pidfd*.
Python does not currently support the *siginfo* parameter; it must be
``None``. The *flags* argument is provided for future extensions; no flag
values are currently defined.
See the :manpage:`pidfd_send_signal(2)` man page for more information.
.. availability:: Linux >= 5.1, Android >= :func:`build-time <sys.getandroidapilevel>` API level 31
.. versionadded:: 3.9
.. function:: pthread_kill(thread_id, signalnum)
Send the signal *signalnum* to the thread *thread_id*, another thread in the
same process as the caller. The target thread can be executing any code
(Python or not). However, if the target thread is executing the Python
interpreter, the Python signal handlers will be :ref:`executed by the main
thread of the main interpreter <signals-and-threads>`. Therefore, the only point of sending a
signal to a particular Python thread would be to force a running system call
to fail with :exc:`InterruptedError`.
Use :func:`threading.get_ident` or the :attr:`~threading.Thread.ident`
attribute of :class:`threading.Thread` objects to get a suitable value
for *thread_id*.
If *signalnum* is 0, then no signal is sent, but error checking is still
performed; this can be used to check if the target thread is still running.
.. audit-event:: signal.pthread_kill thread_id,signalnum signal.pthread_kill
.. availability:: Unix.
See the man page :manpage:`pthread_kill(3)` for further information.
See also :func:`os.kill`.
.. versionadded:: 3.3
.. function:: pthread_sigmask(how, mask)
Fetch and/or change the signal mask of the calling thread. The signal mask
is 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 of *how*, as follows.
* :data:`SIG_BLOCK`: The set of blocked signals is the union of the current
set and the *mask* argument.
* :data:`SIG_UNBLOCK`: The signals in *mask* are removed from the current
set of blocked signals. It is permissible to attempt to unblock a
signal which is not blocked.
* :data:`SIG_SETMASK`: The set of blocked signals is set to the *mask*
argument.
*mask* is a set of signal numbers (e.g. {:const:`signal.SIGINT`,
:const:`signal.SIGTERM`}). Use :func:`~signal.valid_signals` for a full
mask including all signals.
For example, ``signal.pthread_sigmask(signal.SIG_BLOCK, [])`` reads the
signal mask of the calling thread.
:data:`SIGKILL` and :data:`SIGSTOP` cannot be blocked.
.. availability:: Unix.
See the man page :manpage:`sigprocmask(2)` and
:manpage:`pthread_sigmask(3)` for further information.
See also :func:`pause`, :func:`sigpending` and :func:`sigwait`.
.. versionadded:: 3.3
.. function:: setitimer(which, seconds, interval=0.0)
Sets given interval timer (one of :const:`signal.ITIMER_REAL`,
:const:`signal.ITIMER_VIRTUAL` or :const:`signal.ITIMER_PROF`) specified
by *which* to fire after *seconds* (float is accepted, different from
:func:`alarm`) and after that every *interval* seconds (if *interval*
is non-zero). The interval timer specified by *which* can be cleared by
setting *seconds* to zero.
When an interval timer fires, a signal is sent to the process.
The signal sent is dependent on the timer being used;
:const:`signal.ITIMER_REAL` will deliver :const:`SIGALRM`,
:const:`signal.ITIMER_VIRTUAL` sends :const:`SIGVTALRM`,
and :const:`signal.ITIMER_PROF` will deliver :const:`SIGPROF`.
The old values are returned as a tuple: (delay, interval).
Attempting to pass an invalid interval timer will cause an
:exc:`ItimerError`.
.. availability:: Unix.
.. function:: getitimer(which)
Returns current value of a given interval timer specified by *which*.
.. availability:: Unix.
.. function:: set_wakeup_fd(fd, *, warn_on_full_buffer=True)
Set the wakeup file descriptor to *fd*. When a signal is received, the
signal number is written as a single byte into the fd. This can be used by
a library to wakeup a poll or select call, allowing the signal to be fully
processed.
The old wakeup fd is returned (or -1 if file descriptor wakeup was not
enabled). If *fd* is -1, file descriptor wakeup is disabled.
If not -1, *fd* must be non-blocking. It is up to the library to remove
any bytes from *fd* before calling poll or select again.
When threads are enabled, this function can only be called
from :ref:`the main thread of the main interpreter <signals-and-threads>`;
attempting to call it from other threads will cause a :exc:`ValueError`
exception 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 they
differ in how they determine *which* signal or signals have
arrived.
In the first approach, we read the data out of the fd's buffer, and
the byte values give you the signal numbers. This is simple, but in
rare cases it can run into a problem: generally the fd will have a
limited amount of buffer space, and if too many signals arrive too
quickly, then the buffer may become full, and some signals may be
lost. If you use this approach, then you should set
``warn_on_full_buffer=True``, which will at least cause a warning
to be printed to stderr when signals are lost.
In the second approach, we use the wakeup fd *only* for wakeups,
and ignore the actual byte values. In this case, all we care about
is whether the fd's buffer is empty or non-empty; a full buffer
doesn't indicate a problem at all. If you use this approach, then
you should set ``warn_on_full_buffer=False``, so that your users
are not confused by spurious warning messages.
.. versionchanged:: 3.5
On Windows, the function now also supports socket handles.
.. versionchanged:: 3.7
Added ``warn_on_full_buffer`` parameter.
.. function:: siginterrupt(signalnum, flag)
Change system call restart behaviour: if *flag* is :const:`False`, system
calls will be restarted when interrupted by signal *signalnum*, otherwise
system calls will be interrupted. Returns nothing.
.. availability:: Unix.
See the man page :manpage:`siginterrupt(3)` for further information.
Note that installing a signal handler with :func:`signal` will reset the
restart behaviour to interruptible by implicitly calling
:c:func:`!siginterrupt` with a true *flag* value for the given signal.
.. function:: signal(signalnum, handler)
Set the handler for signal *signalnum* to the function *handler*. *handler* can
be a callable Python object taking two arguments (see below), or one of the
special values :const:`signal.SIG_IGN` or :const:`signal.SIG_DFL`. The previous
signal handler will be returned (see the description of :func:`getsignal`
above). (See the Unix man page :manpage:`signal(2)` for further information.)
When threads are enabled, this function can only be called
from :ref:`the main thread of the main interpreter <signals-and-threads>`;
attempting to call it from other threads will cause a :exc:`ValueError`
exception to be raised.
The *handler* is called with two arguments: the signal number and the current
stack frame (``None`` or a frame object; for a description of frame objects,
see the :ref:`description in the type hierarchy <frame-objects>` or see the
attribute descriptions in the :mod:`inspect` module).
On Windows, :func:`signal` can only be called with :const:`SIGABRT`,
:const:`SIGFPE`, :const:`SIGILL`, :const:`SIGINT`, :const:`SIGSEGV`,
:const:`SIGTERM`, or :const:`SIGBREAK`.
A :exc:`ValueError` will be raised in any other case.
Note that not all systems define the same set of signal names; an
:exc:`AttributeError` will be raised if a signal name is not defined as
``SIG*`` module level constant.
.. function:: sigpending()
Examine the set of signals that are pending for delivery to the calling
thread (i.e., the signals which have been raised while blocked). Return the
set of the pending signals.
.. availability:: Unix.
See the man page :manpage:`sigpending(2)` for further information.
See also :func:`pause`, :func:`pthread_sigmask` and :func:`sigwait`.
.. versionadded:: 3.3
.. function:: sigwait(sigset)
Suspend execution of the calling thread until the delivery of one of the
signals specified in the signal set *sigset*. The function accepts the signal
(removes it from the pending list of signals), and returns the signal number.
.. availability:: Unix.
See the man page :manpage:`sigwait(3)` for further information.
See also :func:`pause`, :func:`pthread_sigmask`, :func:`sigpending`,
:func:`sigwaitinfo` and :func:`sigtimedwait`.
.. versionadded:: 3.3
.. function:: sigwaitinfo(sigset)
Suspend execution of the calling thread until the delivery of one of the
signals specified in the signal set *sigset*. The function accepts the
signal and removes it from the pending list of signals. If one of the
signals in *sigset* is already pending for the calling thread, the function
will return immediately with information about that signal. The signal
handler is not called for the delivered signal. The function raises an
:exc:`InterruptedError` if it is interrupted by a signal that is not in
*sigset*.
The return value is an object representing the data contained in the
:c:type:`siginfo_t` structure, namely: :attr:`si_signo`, :attr:`si_code`,
:attr:`si_errno`, :attr:`si_pid`, :attr:`si_uid`, :attr:`si_status`,
:attr:`si_band`.
.. availability:: Unix.
See the man page :manpage:`sigwaitinfo(2)` for further information.
See also :func:`pause`, :func:`sigwait` and :func:`sigtimedwait`.
.. versionadded:: 3.3
.. versionchanged:: 3.5
The function is now retried if interrupted by a signal not in *sigset*
and the signal handler does not raise an exception (see :pep:`475` for
the rationale).
.. function:: sigtimedwait(sigset, timeout)
Like :func:`sigwaitinfo`, but takes an additional *timeout* argument
specifying a timeout. If *timeout* is specified as ``0``, a poll is
performed. Returns :const:`None` if a timeout occurs.
.. availability:: Unix.
See the man page :manpage:`sigtimedwait(2)` for further information.
See also :func:`pause`, :func:`sigwait` and :func:`sigwaitinfo`.
.. versionadded:: 3.3
.. versionchanged:: 3.5
The function is now retried with the recomputed *timeout* if interrupted
by a signal not in *sigset* and the signal handler does not raise an
exception (see :pep:`475` for the rationale).
.. _signal-example:
Examples
--------
Here is a minimal example program. It uses the :func:`alarm` function to limit
the time spent waiting to open a file; this is useful if the file is for a
serial device that may not be turned on, which would normally cause the
:func:`os.open` to hang indefinitely. The solution is to set a 5-second alarm
before opening the file; if the operation takes too long, the alarm signal will
be sent, and the handler raises an exception. ::
import signal, os
def handler(signum, frame):
signame = signal.Signals(signum).name
print(f'Signal handler called with signal {signame} ({signum})')
raise OSError("Couldn't open device!")
# Set the signal handler and a 5-second alarm
signal.signal(signal.SIGALRM, handler)
signal.alarm(5)
# This open() may hang indefinitely
fd = os.open('/dev/ttyS0', os.O_RDWR)
signal.alarm(0) # Disable the alarm
Note on SIGPIPE
---------------
Piping output of your program to tools like :manpage:`head(1)` will
cause a :const:`SIGPIPE` signal to be sent to your process when the receiver
of its standard output closes early. This results in an exception
like :code:`BrokenPipeError: [Errno 32] Broken pipe`. To handle this
case, wrap your entry point to catch this exception as follows::
import os
import sys
def main():
try:
# simulate large output (your code replaces this loop)
for x in range(10000):
print("y")
# flush output here to force SIGPIPE to be triggered
# while inside this try block.
sys.stdout.flush()
except BrokenPipeError:
# Python flushes standard streams on exit; redirect remaining output
# to devnull to avoid another BrokenPipeError at shutdown
devnull = os.open(os.devnull, os.O_WRONLY)
os.dup2(devnull, sys.stdout.fileno())
sys.exit(1) # Python exits with error code 1 on EPIPE
if __name__ == '__main__':
main()
Do not set :const:`SIGPIPE`'s disposition to :const:`SIG_DFL` in
order to avoid :exc:`BrokenPipeError`. Doing that would cause
your program to exit unexpectedly whenever any socket
connection is interrupted while your program is still writing to
it.
.. _handlers-and-exceptions:
Note on Signal Handlers and Exceptions
--------------------------------------
If a signal handler raises an exception, the exception will be propagated to
the main thread and may be raised after any :term:`bytecode` instruction. Most
notably, a :exc:`KeyboardInterrupt` may appear at any point during execution.
Most Python code, including the standard library, cannot be made robust against
this, and so a :exc:`KeyboardInterrupt` (or any other exception resulting from
a signal handler) may on rare occasions put the program in an unexpected state.
To illustrate this issue, consider the following code::
class SpamContext:
def __init__(self):
self.lock = threading.Lock()
def __enter__(self):
# If KeyboardInterrupt occurs here, everything is fine
self.lock.acquire()
# If KeyboardInterrupt occurs here, __exit__ will not be called
...
# KeyboardInterrupt could occur just before the function returns
def __exit__(self, exc_type, exc_val, exc_tb):
...
self.lock.release()
For many programs, especially those that merely want to exit on
:exc:`KeyboardInterrupt`, this is not a problem, but applications that are
complex or require high reliability should avoid raising exceptions from signal
handlers. They should also avoid catching :exc:`KeyboardInterrupt` as a means
of gracefully shutting down. Instead, they should install their own
:const:`SIGINT` handler. Below is an example of an HTTP server that avoids
:exc:`KeyboardInterrupt`::
import signal
import socket
from selectors import DefaultSelector, EVENT_READ
from http.server import HTTPServer, SimpleHTTPRequestHandler
interrupt_read, interrupt_write = socket.socketpair()
def handler(signum, frame):
print('Signal handler called with signal', signum)
interrupt_write.send(b'\0')
signal.signal(signal.SIGINT, handler)
def serve_forever(httpd):
sel = DefaultSelector()
sel.register(interrupt_read, EVENT_READ)
sel.register(httpd, EVENT_READ)
while True:
for key, _ in sel.select():
if key.fileobj == interrupt_read:
interrupt_read.recv(1)
return
if key.fileobj == httpd:
httpd.handle_request()
print("Serving on port 8000")
httpd = HTTPServer(('', 8000), SimpleHTTPRequestHandler)
serve_forever(httpd)
print("Shutdown...")
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