cpython/Doc/library/signal.rst

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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.
* 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`,
:func:`pause`, :func:`setitimer` or :func:`getitimer`; 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:`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 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 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:`IOError`.
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)`.)
See also :func:`sigwait` and :func:`sigpending`.
.. function:: pthread_kill(thread_id, signum)
Send the signal *signum* to the thread *thread_id*, another thread in the same
process as the caller. The signal is asynchronously directed to thread.
*thread_id* can be read from the :attr:`~threading.Thread.ident` attribute
of :attr:`threading.Thread`. For example,
``threading.current_thread().ident`` gives the identifier of the current
thread.
If *signum* is 0, then no signal is sent, but error checking is still
performed; this can be used to check if a 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])
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. 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)
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. *fd* must be non-blocking. It is up to the
library to remove any bytes before calling poll or select again.
Use for example ``struct.unpack('%uB' % len(data), data)`` to decode the
signal numbers list.
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.
.. 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)`.)
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`, or
:const:`SIGTERM`. A :exc:`ValueError` will be raised in any other case.
.. 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` and :func:`sigpending`.
.. versionadded:: 3.3
.. _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