158 lines
6.6 KiB
ReStructuredText
158 lines
6.6 KiB
ReStructuredText
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: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. Some general
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rules for working with signals and their handlers:
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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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* Although Python signal handlers are called asynchronously as far as the Python
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user is concerned, they can only occur between the "atomic" instructions of the
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Python interpreter. This means that signals arriving during long calculations
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implemented purely in C (such as regular expression matches on large bodies of
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text) may be delayed for an arbitrary amount of time.
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* When a signal arrives during an I/O operation, it is possible that the I/O
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operation raises an exception after the signal handler returns. This is
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dependent on the underlying Unix system's semantics regarding interrupted system
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calls.
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* Because the C signal handler always returns, it makes little sense to catch
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synchronous errors like :const:`SIGFPE` or :const:`SIGSEGV`.
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* Python installs a small number of signal handlers by default: :const:`SIGPIPE`
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is ignored (so write errors on pipes and sockets can be reported as ordinary
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Python exceptions) and :const:`SIGINT` is translated into a
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:exc:`KeyboardInterrupt` exception. All of these can be overridden.
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* Some care must be taken if both signals and threads are used in the same
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program. The fundamental thing to remember in using signals and threads
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simultaneously is: always perform :func:`signal` operations in the main thread
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of execution. Any thread can perform an :func:`alarm`, :func:`getsignal`, or
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:func:`pause`; only the main thread can set a new signal handler, and the main
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thread will be the only one to receive signals (this is enforced by the Python
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:mod:`signal` module, even if the underlying thread implementation supports
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sending signals to individual threads). This means that signals can't be used
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as a means of inter-thread communication. Use locks instead.
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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 the
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default function for the signal. For example, on most systems the default
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action for :const:`SIGQUIT` is to dump core and exit, while the default action
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for :const:`SIGCLD` 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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':cfunc:`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:: NSIG
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One more than the number of the highest signal number.
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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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.. 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, see
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the reference manual section on the standard type hierarchy or see the attribute
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descriptions in the :mod:`inspect` module).
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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 IOError("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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