
``signal`` --- Set handlers for asynchronous events
***************************************************

This module provides mechanisms to use signal handlers in Python.


General rules
=============

The ``signal.signal()`` function allows to define custom handlers to
be executed when a signal is received.  A small number of default
handlers are installed: ``SIGPIPE`` is ignored (so write errors on
pipes and sockets can be reported as ordinary Python exceptions) and
``SIGINT`` is translated into a ``KeyboardInterrupt`` exception.

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 ``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).


Execution of Python signal handlers
-----------------------------------

A Python signal handler does not get executed inside the low-level (C)
signal handler.  Instead, the low-level signal handler sets a flag
which tells the *virtual machine* to execute the corresponding Python
signal handler at a later point(for example at the next *bytecode*
instruction). This has consequences:

* It makes little sense to catch synchronous errors like ``SIGFPE`` or
  ``SIGSEGV``.

* A long-running calculation implemented purely in C (such as regular
  expression matching on a large body of text) may run uninterrupted
  for an arbitrary amount of time, regardless of any signals received.
  The Python signal handlers will be called when the calculation
  finishes.


Signals and threads
-------------------

Python signal handlers are always executed in the main Python thread,
even if the signal was received in another thread.  This means that
signals can't be used as a means of inter-thread communication.  You
can use the synchronization primitives from the ``threading`` module
instead.

Besides, only the main thread is allowed to set a new signal handler.


Module contents
===============

The variables defined in the ``signal`` module are:

signal.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 ``SIGQUIT`` is to dump core and
   exit, while the default action for ``SIGCHLD`` is to simply ignore
   it.

signal.SIG_IGN

   This is another standard signal handler, which will simply ignore
   the given signal.

SIG*

   All the signal numbers are defined symbolically.  For example, the
   hangup signal is defined as ``signal.SIGHUP``; the variable names
   are identical to the names used in C programs, as found in
   ``<signal.h>``. The Unix man page for '``signal()``' lists the
   existing signals (on some systems this is *signal(2)*, on others
   the list is in *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.

signal.CTRL_C_EVENT

   The signal corresponding to the CTRL+C keystroke event. This signal
   can only be used with ``os.kill()``.

   Availability: Windows.

   New in version 3.2.

signal.CTRL_BREAK_EVENT

   The signal corresponding to the CTRL+BREAK keystroke event. This
   signal can only be used with ``os.kill()``.

   Availability: Windows.

   New in version 3.2.

signal.NSIG

   One more than the number of the highest signal number.

signal.ITIMER_REAL

   Decrements interval timer in real time, and delivers ``SIGALRM``
   upon expiration.

signal.ITIMER_VIRTUAL

   Decrements interval timer only when the process is executing, and
   delivers SIGVTALRM upon expiration.

signal.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.

The ``signal`` module defines one exception:

exception exception signal.ItimerError

   Raised to signal an error from the underlying ``setitimer()`` or
   ``getitimer()`` implementation. Expect this error if an invalid
   interval timer or a negative time is passed to ``setitimer()``.
   This error is a subtype of ``IOError``.

The ``signal`` module defines the following functions:

signal.alarm(time)

   If *time* is non-zero, this function requests that a ``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 *alarm(2)*.) Availability: Unix.

signal.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 ``signal.SIG_IGN``, ``signal.SIG_DFL`` or ``None``.
   Here, ``signal.SIG_IGN`` means that the signal was previously
   ignored, ``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.

signal.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 *signal(2)*.)

signal.setitimer(which, seconds[, interval])

   Sets given interval timer (one of ``signal.ITIMER_REAL``,
   ``signal.ITIMER_VIRTUAL`` or ``signal.ITIMER_PROF``) specified by
   *which* to fire after *seconds* (float is accepted, different from
   ``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;
   ``signal.ITIMER_REAL`` will deliver ``SIGALRM``,
   ``signal.ITIMER_VIRTUAL`` sends ``SIGVTALRM``, and
   ``signal.ITIMER_PROF`` will deliver ``SIGPROF``.

   The old values are returned as a tuple: (delay, interval).

   Attempting to pass an invalid interval timer will cause an
   ``ItimerError``.  Availability: Unix.

signal.getitimer(which)

   Returns current value of a given interval timer specified by
   *which*. Availability: Unix.

signal.set_wakeup_fd(fd)

   Set the wakeup fd to *fd*.  When a signal is received, a ``'\0'``
   byte is written to 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.

   When threads are enabled, this function can only be called from the
   main thread; attempting to call it from other threads will cause a
   ``ValueError`` exception to be raised.

signal.siginterrupt(signalnum, flag)

   Change system call restart behaviour: if *flag* is ``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 *siginterrupt(3)*
   for further information).

   Note that installing a signal handler with ``signal()`` will reset
   the restart behaviour to interruptible by implicitly calling
   ``siginterrupt()`` with a true *flag* value for the given signal.

signal.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 ``signal.SIG_IGN`` or
   ``signal.SIG_DFL``.  The previous signal handler will be returned
   (see the description of ``getsignal()`` above).  (See the Unix man
   page *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
   ``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 *description in the type
   hierarchy* or see the attribute descriptions in the ``inspect``
   module).

   On Windows, ``signal()`` can only be called with ``SIGABRT``,
   ``SIGFPE``, ``SIGILL``, ``SIGINT``, ``SIGSEGV``, or ``SIGTERM``. A
   ``ValueError`` will be raised in any other case.


Example
=======

Here is a minimal example program. It uses the ``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 ``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
