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: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. Some general
rules for working with signals and their handlers:
* 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.
* There is no way to "block" signals temporarily from critical sections (since
this is not supported by all Unix flavors).
* Although Python signal handlers are called asynchronously as far as the Python
user is concerned, they can only occur between the "atomic" instructions of the
Python interpreter. This means that signals arriving during long calculations
implemented purely in C (such as regular expression matches on large bodies of
text) may be delayed for an arbitrary amount of time.
* When a signal arrives during an I/O operation, it is possible that the I/O
operation raises an exception after the signal handler returns. This is
dependent on the underlying Unix system's semantics regarding interrupted system
calls.
* Because the C signal handler always returns, it makes little sense to catch
synchronous errors like :const:`SIGFPE` or :const:`SIGSEGV`.
* Python installs a small number of signal handlers by default: :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. All of these can be overridden.
* Some care must be taken if both signals and threads are used in the same
program. The fundamental thing to remember in using signals and threads
simultaneously is: always perform :func:`signal` operations in the main thread
of execution. Any thread can perform an :func:`alarm`, :func:`getsignal`, or
:func:`pause`; only the main thread can set a new signal handler, and the main
thread will be the only one to receive signals (this is enforced by the Python
:mod:`signal` module, even if the underlying thread implementation supports
sending signals to individual threads). This means that signals can't be used
as a means of inter-thread communication. Use locks instead.
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:`SIGCLD` 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
':cfunc:`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:: NSIG
One more than the number of the highest signal number.
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. (See
the Unix man page :manpage:`alarm(2)`.) Availability: Unix.
.. 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. Not on Windows. (See the Unix man page
:manpage:`signal(2)`.)
.. 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)`.)
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 reference manual section on the standard type hierarchy or see the attribute
descriptions in the :mod:`inspect` module).
.. _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 IOError("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
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