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signal — Set handlers for asynchronous events¶
Source code: Lib/signal.py
This module provides mechanisms to use signal handlers in Python.
General rules¶
The signal.signal() function allows defining custom handlers to be
executed when a signal is received. A small number of default handlers are
installed: SIGPIPE is ignored (so write errors on pipes and sockets
can be reported as ordinary Python exceptions) and SIGINT is
translated into a 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
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
virtual machine to execute the corresponding Python signal handler
at a later point (for example, at the next bytecode instruction).
This has consequences:
It makes little sense to catch synchronous errors like SIGFPE or
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 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 note below for a
discussion.
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 threading module instead.
Besides, only the main thread of the main interpreter is allowed to set a new signal handler.
Warning
Synchronization primitives such as threading.Lock should not be used
within signal handlers. Doing so can lead to unexpected deadlocks.
Module contents¶
The signal module defines three enums:
-
class signal.Signals¶
enum.IntEnum collection of SIG* constants and the CTRL_* constants.
-
class signal.Handlers¶
enum.IntEnum collection of the constants SIG_DFL and SIG_IGN.
-
class signal.Sigmasks¶
enum.IntEnum collection of the constants SIG_BLOCK, SIG_UNBLOCK and SIG_SETMASK.
The variables defined in the signal module are:
-
signal.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 SIGQUIT is to dump core and exit, while the
default action for SIGCHLD is to simply ignore it.
-
signal.SIG_IGN¶
This is another standard signal handler, which will simply ignore the given
signal.
-
signal.SIGABRT¶
Abort signal from abort(3).
-
signal.SIGALRM¶
Timer signal from alarm(2).
-
signal.SIGBREAK¶
Interrupt from keyboard (CTRL + BREAK).
-
signal.SIGBUS¶
Bus error (bad memory access).
-
signal.SIGCHLD¶
Child process stopped or terminated.
-
signal.SIGCLD¶
Alias to SIGCHLD.
-
signal.SIGCONT¶
Continue the process if it is currently stopped
-
signal.SIGFPE¶
Floating-point exception. For example, division by zero.
See also
ZeroDivisionError is raised when the second argument of a division
or modulo operation is zero.
-
signal.SIGHUP¶
Hangup detected on controlling terminal or death of controlling process.
-
signal.SIGILL¶
Illegal instruction.
-
signal.SIGINT¶
Interrupt from keyboard (CTRL + C).
Default action is to raise KeyboardInterrupt.
-
signal.SIGKILL¶
Kill signal.
It cannot be caught, blocked, or ignored.
-
signal.SIGPIPE¶
Broken pipe: write to pipe with no readers.
Default action is to ignore the signal.
-
signal.SIGPROF¶
Profiling timer expired.
-
signal.SIGQUIT¶
Terminal quit signal.
-
signal.SIGSEGV¶
Segmentation fault: invalid memory reference.
-
signal.SIGSTOP¶
Stop executing (cannot be caught or ignored).
-
signal.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 signal(7) for further information.
-
signal.SIGTERM¶
Termination signal.
-
signal.SIGUSR1¶
User-defined signal 1.
-
signal.SIGUSR2¶
User-defined signal 2.
-
signal.SIGVTALRM¶
Virtual timer expired.
-
signal.SIGWINCH¶
Window resize signal.
-
signal.SIGXCPU¶
CPU time limit exceeded.
-
SIG*
All the signal numbers are defined symbolically. For example, the hangup signal
is defined as signal.SIGHUP; the variable names are identical to the
names used in C programs, as found in <signal.h>. The Unix man page for
‘signal’ lists the existing signals (on some systems this is
signal(2), on others the list is in 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.
-
signal.CTRL_C_EVENT¶
The signal corresponding to the Ctrl+C keystroke event. This signal can
only be used with os.kill().
-
signal.CTRL_BREAK_EVENT¶
The signal corresponding to the Ctrl+Break keystroke event. This signal can
only be used with os.kill().
-
signal.NSIG¶
One more than the number of the highest signal number.
Use valid_signals() to get valid signal numbers.
-
signal.ITIMER_REAL¶
Decrements interval timer in real time, and delivers SIGALRM upon
expiration.
-
signal.ITIMER_VIRTUAL¶
Decrements interval timer only when the process is executing, and delivers
SIGVTALRM upon expiration.
-
signal.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.
-
signal.SIG_BLOCK¶
A possible value for the how parameter to pthread_sigmask()
indicating that signals are to be blocked.
-
signal.SIG_UNBLOCK¶
A possible value for the how parameter to pthread_sigmask()
indicating that signals are to be unblocked.
-
signal.SIG_SETMASK¶
A possible value for the how parameter to pthread_sigmask()
indicating that the signal mask is to be replaced.
The signal module defines one exception:
-
exception signal.ItimerError¶
Raised to signal an error from the underlying setitimer() or
getitimer() implementation. Expect this error if an invalid
interval timer or a negative time is passed to setitimer().
This error is a subtype of OSError.
Added in version 3.3: This error used to be a subtype of IOError, which is now an
alias of OSError.
The signal module defines the following functions:
-
signal.alarm(time)¶
If time is non-zero, this function requests that a 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.
-
signal.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
signal.SIG_IGN, signal.SIG_DFL or None. Here,
signal.SIG_IGN means that the signal was previously ignored,
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.
-
signal.strsignal(signalnum)¶
Returns the description of signal signalnum, such as “Interrupt”
for SIGINT. Returns None if signalnum has no
description. Raises ValueError if signalnum is invalid.
-
signal.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.
-
signal.pause()¶
Cause the process to sleep until a signal is received; the appropriate handler
will then be called. Returns nothing.
See also sigwait(), sigwaitinfo(), sigtimedwait() and
sigpending().
-
signal.raise_signal(signum)¶
Sends a signal to the calling process. Returns nothing.
-
signal.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 pidfd_send_signal(2) man page for more information.
-
signal.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 executed by the main
thread of the main interpreter. Therefore, the only point of sending a
signal to a particular Python thread would be to force a running system call
to fail with InterruptedError.
Use threading.get_ident() or the ident
attribute of 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.
Raises an auditing event signal.pthread_kill with arguments thread_id, signalnum.
See also os.kill().
-
signal.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.
SIG_BLOCK: The set of blocked signals is the union of the current
set and the mask argument.
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.
SIG_SETMASK: The set of blocked signals is set to the mask
argument.
mask is a set of signal numbers (e.g. {signal.SIGINT,
signal.SIGTERM}). Use 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.
SIGKILL and SIGSTOP cannot be blocked.
See also pause(), sigpending() and sigwait().
-
signal.setitimer(which, seconds, interval=0)¶
Sets given interval timer (one of signal.ITIMER_REAL,
signal.ITIMER_VIRTUAL or signal.ITIMER_PROF) specified
by which to fire after seconds (rounded up to microseconds, different from
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;
signal.ITIMER_REAL will deliver SIGALRM,
signal.ITIMER_VIRTUAL sends SIGVTALRM,
and signal.ITIMER_PROF will deliver SIGPROF.
The old values are returned as a two-tuple of floats:
(delay, interval).
Attempting to pass an invalid interval timer will cause an
ItimerError.
Changed in version 3.15: Accepts any real numbers as seconds and interval, not only integers
or floats.
-
signal.getitimer(which)¶
Returns current value of a given interval timer specified by which.
-
signal.set_wakeup_fd(fd, *, warn_on_full_buffer=True)¶
Set the wakeup file descriptor to fd. When a signal your program has
registered a signal handler for is received, the signal number is written as
a single byte into the fd. If you haven’t registered a signal handler for
the 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 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 the main thread of the main interpreter;
attempting to call it from other threads will cause a 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.
Changed in version 3.5: On Windows, the function now also supports socket handles.
Changed in version 3.7: Added warn_on_full_buffer parameter.
-
signal.siginterrupt(signalnum, flag)¶
Change system call restart behaviour: if flag is False, system
calls will be restarted when interrupted by signal signalnum, otherwise
system calls will be interrupted. Returns nothing.
Note that installing a signal handler with signal() will reset
the restart behaviour to interruptible by implicitly calling
siginterrupt() with a true flag value for the given signal.
-
signal.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 signal.SIG_IGN or signal.SIG_DFL. The previous
signal handler will be returned (see the description of getsignal()
above). (See the Unix man page signal(2) for further information.)
When threads are enabled, this function can only be called
from the main thread of the main interpreter;
attempting to call it from other threads will cause a 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 description in the type hierarchy or see the
attribute descriptions in the inspect module).
On Windows, signal() can only be called with SIGABRT,
SIGFPE, SIGILL, SIGINT, SIGSEGV,
SIGTERM, or SIGBREAK.
A ValueError will be raised in any other case.
Note that not all systems define the same set of signal names; an
AttributeError will be raised if a signal name is not defined as
SIG* module level constant.
-
signal.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.
See also pause(), pthread_sigmask() and sigwait().
-
signal.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.
See also pause(), pthread_sigmask(), sigpending(),
sigwaitinfo() and sigtimedwait().
-
signal.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
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
siginfo_t structure, namely: si_signo, si_code,
si_errno, si_pid, si_uid, si_status, si_band.
See also pause(), sigwait() and sigtimedwait().
Changed in version 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).
-
signal.sigtimedwait(sigset, timeout)¶
Like sigwaitinfo(), but takes an additional timeout argument
specifying a timeout. If timeout is specified as 0, a poll is
performed. Returns None if a timeout occurs.
See also pause(), sigwait() and sigwaitinfo().
Changed in version 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).
Changed in version 3.15: Accepts any real number as timeout, not only integer or float.
Examples¶
Here is a minimal example program. It uses the 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
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 head(1) will
cause a SIGPIPE signal to be sent to your process when the receiver
of its standard output closes early. This results in an exception
like 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 SIGPIPE’s disposition to SIG_DFL in
order to avoid BrokenPipeError. Doing that would cause
your program to exit unexpectedly whenever any socket
connection is interrupted while your program is still writing to
it.
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 bytecode instruction. Most
notably, a KeyboardInterrupt may appear at any point during execution.
Most Python code, including the standard library, cannot be made robust against
this, and so a 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
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 KeyboardInterrupt as a means
of gracefully shutting down. Instead, they should install their own
SIGINT handler. Below is an example of an HTTP server that avoids
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...")
© Copyright 2001 Python Software Foundation.
This page is licensed under the Python Software Foundation License Version 2.
Examples, recipes, and other code in the documentation are additionally licensed under the Zero Clause BSD License.
See History and License for more information.
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Please donate.
Last updated on Nov 13, 2026 (04:52 UTC).
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