440 lines
16 KiB
ReStructuredText
440 lines
16 KiB
ReStructuredText
:mod:`signal` --- Set handlers for asynchronous events
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======================================================
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.. module:: signal
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:synopsis: Set handlers for asynchronous events.
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This module provides mechanisms to use signal handlers in Python.
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General rules
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-------------
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The :func:`signal.signal` function allows to define custom handlers to be
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executed when a signal is received. A small number of default handlers are
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installed: :const:`SIGPIPE` is ignored (so write errors on pipes and sockets
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can be reported as ordinary Python exceptions) and :const:`SIGINT` is
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translated into a :exc:`KeyboardInterrupt` exception.
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A handler for a particular signal, once set, remains installed until it is
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explicitly reset (Python emulates the BSD style interface regardless of the
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underlying implementation), with the exception of the handler for
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:const:`SIGCHLD`, which follows the underlying implementation.
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There is no way to "block" signals temporarily from critical sections (since
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this is not supported by all Unix flavors).
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Execution of Python signal handlers
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^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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A Python signal handler does not get executed inside the low-level (C) signal
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handler. Instead, the low-level signal handler sets a flag which tells the
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:term:`virtual machine` to execute the corresponding Python signal handler
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at a later point(for example at the next :term:`bytecode` instruction).
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This has consequences:
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* It makes little sense to catch synchronous errors like :const:`SIGFPE` or
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:const:`SIGSEGV` that are caused by an invalid operation in C code. Python
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will return from the signal handler to the C code, which is likely to raise
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the same signal again, causing Python to apparently hang. From Python 3.3
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onwards, you can use the :mod:`faulthandler` module to report on synchronous
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errors.
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* A long-running calculation implemented purely in C (such as regular
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expression matching on a large body of text) may run uninterrupted for an
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arbitrary amount of time, regardless of any signals received. The Python
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signal handlers will be called when the calculation finishes.
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.. _signals-and-threads:
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Signals and threads
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^^^^^^^^^^^^^^^^^^^
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Python signal handlers are always executed in the main Python thread,
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even if the signal was received in another thread. This means that signals
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can't be used as a means of inter-thread communication. You can use
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the synchronization primitives from the :mod:`threading` module instead.
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Besides, only the main thread is allowed to set a new signal handler.
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Module contents
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---------------
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The variables defined in the :mod:`signal` module are:
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.. data:: SIG_DFL
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This is one of two standard signal handling options; it will simply perform
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the default function for the signal. For example, on most systems the
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default action for :const:`SIGQUIT` is to dump core and exit, while the
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default action for :const:`SIGCHLD` is to simply ignore it.
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.. data:: SIG_IGN
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This is another standard signal handler, which will simply ignore the given
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signal.
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.. data:: SIG*
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All the signal numbers are defined symbolically. For example, the hangup signal
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is defined as :const:`signal.SIGHUP`; the variable names are identical to the
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names used in C programs, as found in ``<signal.h>``. The Unix man page for
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':c:func:`signal`' lists the existing signals (on some systems this is
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:manpage:`signal(2)`, on others the list is in :manpage:`signal(7)`). Note that
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not all systems define the same set of signal names; only those names defined by
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the system are defined by this module.
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.. data:: CTRL_C_EVENT
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The signal corresponding to the :kbd:`Ctrl+C` keystroke event. This signal can
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only be used with :func:`os.kill`.
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Availability: Windows.
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.. versionadded:: 3.2
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.. data:: CTRL_BREAK_EVENT
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The signal corresponding to the :kbd:`Ctrl+Break` keystroke event. This signal can
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only be used with :func:`os.kill`.
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Availability: Windows.
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.. versionadded:: 3.2
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.. data:: NSIG
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One more than the number of the highest signal number.
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.. data:: ITIMER_REAL
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Decrements interval timer in real time, and delivers :const:`SIGALRM` upon
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expiration.
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.. data:: ITIMER_VIRTUAL
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Decrements interval timer only when the process is executing, and delivers
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SIGVTALRM upon expiration.
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.. data:: ITIMER_PROF
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Decrements interval timer both when the process executes and when the
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system is executing on behalf of the process. Coupled with ITIMER_VIRTUAL,
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this timer is usually used to profile the time spent by the application
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in user and kernel space. SIGPROF is delivered upon expiration.
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.. data:: SIG_BLOCK
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A possible value for the *how* parameter to :func:`pthread_sigmask`
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indicating that signals are to be blocked.
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.. versionadded:: 3.3
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.. data:: SIG_UNBLOCK
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A possible value for the *how* parameter to :func:`pthread_sigmask`
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indicating that signals are to be unblocked.
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.. versionadded:: 3.3
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.. data:: SIG_SETMASK
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A possible value for the *how* parameter to :func:`pthread_sigmask`
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indicating that the signal mask is to be replaced.
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.. versionadded:: 3.3
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The :mod:`signal` module defines one exception:
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.. exception:: ItimerError
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Raised to signal an error from the underlying :func:`setitimer` or
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:func:`getitimer` implementation. Expect this error if an invalid
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interval timer or a negative time is passed to :func:`setitimer`.
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This error is a subtype of :exc:`OSError`.
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.. versionadded:: 3.3
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This error used to be a subtype of :exc:`IOError`, which is now an
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alias of :exc:`OSError`.
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The :mod:`signal` module defines the following functions:
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.. function:: alarm(time)
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If *time* is non-zero, this function requests that a :const:`SIGALRM` signal be
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sent to the process in *time* seconds. Any previously scheduled alarm is
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canceled (only one alarm can be scheduled at any time). The returned value is
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then the number of seconds before any previously set alarm was to have been
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delivered. If *time* is zero, no alarm is scheduled, and any scheduled alarm is
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canceled. If the return value is zero, no alarm is currently scheduled. (See
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the Unix man page :manpage:`alarm(2)`.) Availability: Unix.
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.. function:: getsignal(signalnum)
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Return the current signal handler for the signal *signalnum*. The returned value
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may be a callable Python object, or one of the special values
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:const:`signal.SIG_IGN`, :const:`signal.SIG_DFL` or :const:`None`. Here,
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:const:`signal.SIG_IGN` means that the signal was previously ignored,
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:const:`signal.SIG_DFL` means that the default way of handling the signal was
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previously in use, and ``None`` means that the previous signal handler was not
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installed from Python.
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.. function:: pause()
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Cause the process to sleep until a signal is received; the appropriate handler
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will then be called. Returns nothing. Not on Windows. (See the Unix man page
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:manpage:`signal(2)`.)
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See also :func:`sigwait`, :func:`sigwaitinfo`, :func:`sigtimedwait` and
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:func:`sigpending`.
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.. function:: pthread_kill(thread_id, signum)
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Send the signal *signum* to the thread *thread_id*, another thread in the
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same process as the caller. The target thread can be executing any code
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(Python or not). However, if the target thread is executing the Python
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interpreter, the Python signal handlers will be :ref:`executed by the main
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thread <signals-and-threads>`. Therefore, the only point of sending a signal to a particular
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Python thread would be to force a running system call to fail with
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:exc:`InterruptedError`.
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Use :func:`threading.get_ident()` or the :attr:`~threading.Thread.ident`
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attribute of :class:`threading.Thread` objects to get a suitable value
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for *thread_id*.
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If *signum* is 0, then no signal is sent, but error checking is still
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performed; this can be used to check if the target thread is still running.
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Availability: Unix (see the man page :manpage:`pthread_kill(3)` for further
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information).
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See also :func:`os.kill`.
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.. versionadded:: 3.3
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.. function:: pthread_sigmask(how, mask)
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Fetch and/or change the signal mask of the calling thread. The signal mask
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is the set of signals whose delivery is currently blocked for the caller.
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Return the old signal mask as a set of signals.
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The behavior of the call is dependent on the value of *how*, as follows.
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* :data:`SIG_BLOCK`: The set of blocked signals is the union of the current
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set and the *mask* argument.
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* :data:`SIG_UNBLOCK`: The signals in *mask* are removed from the current
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set of blocked signals. It is permissible to attempt to unblock a
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signal which is not blocked.
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* :data:`SIG_SETMASK`: The set of blocked signals is set to the *mask*
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argument.
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*mask* is a set of signal numbers (e.g. {:const:`signal.SIGINT`,
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:const:`signal.SIGTERM`}). Use ``range(1, signal.NSIG)`` for a full mask
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including all signals.
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For example, ``signal.pthread_sigmask(signal.SIG_BLOCK, [])`` reads the
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signal mask of the calling thread.
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Availability: Unix. See the man page :manpage:`sigprocmask(3)` and
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:manpage:`pthread_sigmask(3)` for further information.
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See also :func:`pause`, :func:`sigpending` and :func:`sigwait`.
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.. versionadded:: 3.3
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.. function:: setitimer(which, seconds[, interval])
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Sets given interval timer (one of :const:`signal.ITIMER_REAL`,
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:const:`signal.ITIMER_VIRTUAL` or :const:`signal.ITIMER_PROF`) specified
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by *which* to fire after *seconds* (float is accepted, different from
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:func:`alarm`) and after that every *interval* seconds. The interval
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timer specified by *which* can be cleared by setting seconds to zero.
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When an interval timer fires, a signal is sent to the process.
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The signal sent is dependent on the timer being used;
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:const:`signal.ITIMER_REAL` will deliver :const:`SIGALRM`,
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:const:`signal.ITIMER_VIRTUAL` sends :const:`SIGVTALRM`,
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and :const:`signal.ITIMER_PROF` will deliver :const:`SIGPROF`.
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The old values are returned as a tuple: (delay, interval).
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Attempting to pass an invalid interval timer will cause an
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:exc:`ItimerError`. Availability: Unix.
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.. function:: getitimer(which)
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Returns current value of a given interval timer specified by *which*.
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Availability: Unix.
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.. function:: set_wakeup_fd(fd)
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Set the wakeup file descriptor to *fd*. When a signal is received, the
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signal number is written as a single byte into the fd. This can be used by
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a library to wakeup a poll or select call, allowing the signal to be fully
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processed.
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The old wakeup fd is returned. *fd* must be non-blocking. It is up to the
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library to remove any bytes before calling poll or select again.
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Use for example ``struct.unpack('%uB' % len(data), data)`` to decode the
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signal numbers list.
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When threads are enabled, this function can only be called from the main thread;
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attempting to call it from other threads will cause a :exc:`ValueError`
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exception to be raised.
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.. function:: siginterrupt(signalnum, flag)
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Change system call restart behaviour: if *flag* is :const:`False`, system
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calls will be restarted when interrupted by signal *signalnum*, otherwise
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system calls will be interrupted. Returns nothing. Availability: Unix (see
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the man page :manpage:`siginterrupt(3)` for further information).
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Note that installing a signal handler with :func:`signal` will reset the
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restart behaviour to interruptible by implicitly calling
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:c:func:`siginterrupt` with a true *flag* value for the given signal.
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.. function:: signal(signalnum, handler)
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Set the handler for signal *signalnum* to the function *handler*. *handler* can
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be a callable Python object taking two arguments (see below), or one of the
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special values :const:`signal.SIG_IGN` or :const:`signal.SIG_DFL`. The previous
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signal handler will be returned (see the description of :func:`getsignal`
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above). (See the Unix man page :manpage:`signal(2)`.)
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When threads are enabled, this function can only be called from the main thread;
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attempting to call it from other threads will cause a :exc:`ValueError`
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exception to be raised.
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The *handler* is called with two arguments: the signal number and the current
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stack frame (``None`` or a frame object; for a description of frame objects,
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see the :ref:`description in the type hierarchy <frame-objects>` or see the
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attribute descriptions in the :mod:`inspect` module).
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On Windows, :func:`signal` can only be called with :const:`SIGABRT`,
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:const:`SIGFPE`, :const:`SIGILL`, :const:`SIGINT`, :const:`SIGSEGV`, or
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:const:`SIGTERM`. A :exc:`ValueError` will be raised in any other case.
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.. function:: sigpending()
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Examine the set of signals that are pending for delivery to the calling
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thread (i.e., the signals which have been raised while blocked). Return the
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set of the pending signals.
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Availability: Unix (see the man page :manpage:`sigpending(2)` for further
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information).
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See also :func:`pause`, :func:`pthread_sigmask` and :func:`sigwait`.
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.. versionadded:: 3.3
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.. function:: sigwait(sigset)
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Suspend execution of the calling thread until the delivery of one of the
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signals specified in the signal set *sigset*. The function accepts the signal
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(removes it from the pending list of signals), and returns the signal number.
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Availability: Unix (see the man page :manpage:`sigwait(3)` for further
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information).
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See also :func:`pause`, :func:`pthread_sigmask`, :func:`sigpending`,
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:func:`sigwaitinfo` and :func:`sigtimedwait`.
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.. versionadded:: 3.3
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.. function:: sigwaitinfo(sigset)
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Suspend execution of the calling thread until the delivery of one of the
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signals specified in the signal set *sigset*. The function accepts the
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signal and removes it from the pending list of signals. If one of the
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signals in *sigset* is already pending for the calling thread, the function
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will return immediately with information about that signal. The signal
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handler is not called for the delivered signal. The function raises an
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:exc:`InterruptedError` if it is interrupted by a signal that is not in
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*sigset*.
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The return value is an object representing the data contained in the
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:c:type:`siginfo_t` structure, namely: :attr:`si_signo`, :attr:`si_code`,
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:attr:`si_errno`, :attr:`si_pid`, :attr:`si_uid`, :attr:`si_status`,
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:attr:`si_band`.
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Availability: Unix (see the man page :manpage:`sigwaitinfo(2)` for further
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information).
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See also :func:`pause`, :func:`sigwait` and :func:`sigtimedwait`.
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.. versionadded:: 3.3
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.. function:: sigtimedwait(sigset, timeout)
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Like :func:`sigwaitinfo`, but takes an additional *timeout* argument
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specifying a timeout. If *timeout* is specified as :const:`0`, a poll is
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performed. Returns :const:`None` if a timeout occurs.
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Availability: Unix (see the man page :manpage:`sigtimedwait(2)` for further
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information).
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See also :func:`pause`, :func:`sigwait` and :func:`sigwaitinfo`.
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.. versionadded:: 3.3
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.. _signal-example:
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Example
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-------
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Here is a minimal example program. It uses the :func:`alarm` function to limit
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the time spent waiting to open a file; this is useful if the file is for a
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serial device that may not be turned on, which would normally cause the
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:func:`os.open` to hang indefinitely. The solution is to set a 5-second alarm
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before opening the file; if the operation takes too long, the alarm signal will
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be sent, and the handler raises an exception. ::
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import signal, os
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def handler(signum, frame):
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print('Signal handler called with signal', signum)
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raise OSError("Couldn't open device!")
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# Set the signal handler and a 5-second alarm
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signal.signal(signal.SIGALRM, handler)
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signal.alarm(5)
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# This open() may hang indefinitely
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fd = os.open('/dev/ttyS0', os.O_RDWR)
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signal.alarm(0) # Disable the alarm
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