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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.
+
+* 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
+