532 lines
20 KiB
ReStructuredText
532 lines
20 KiB
ReStructuredText
:mod:`signal` --- Set handlers for asynchronous events
|
|
======================================================
|
|
|
|
.. module:: signal
|
|
:synopsis: Set handlers for asynchronous events.
|
|
|
|
--------------
|
|
|
|
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.
|
|
|
|
|
|
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.
|
|
|
|
|
|
.. _signals-and-threads:
|
|
|
|
|
|
Signals and threads
|
|
^^^^^^^^^^^^^^^^^^^
|
|
|
|
Python signal handlers are always executed in the main Python thread,
|
|
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 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>`.
|
|
:func:`getsignal`, :func:`pthread_sigmask`, :func:`sigpending` and
|
|
:func:`sigwait` functions return human-readable
|
|
:class:`enums <enum.IntEnum>`.
|
|
|
|
|
|
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:: 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.
|
|
|
|
|
|
.. 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:: 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:: 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 <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.
|
|
|
|
.. 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 ``range(1, signal.NSIG)`` for a full mask
|
|
including all signals.
|
|
|
|
For example, ``signal.pthread_sigmask(signal.SIG_BLOCK, [])`` reads the
|
|
signal mask of the calling thread.
|
|
|
|
.. availability:: Unix. See the man page :manpage:`sigprocmask(3)` 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 the main thread;
|
|
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 the main thread;
|
|
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 :const:`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:
|
|
|
|
Example
|
|
-------
|
|
|
|
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):
|
|
print('Signal handler called with signal', 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 also whenever any socket connection
|
|
is interrupted while your program is still writing to it.
|