17.4. "signal" — 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 "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 "SIGFPE" or "SIGSEGV".

* Python installs a small number of signal handlers by default:
  "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.  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 "signal()"
  operations in the main thread of execution.  Any thread can perform
  an "alarm()", "getsignal()", "pause()", "setitimer()" or
  "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 "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 "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 2.7.

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 2.7.

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 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.

   New in version 2.6.

signal.getitimer(which)

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

   New in version 2.6.

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 (or -1 if file descriptor wakeup was
   not enabled).  If *fd* is -1, file descriptor wakeup is disabled.
   If not -1, *fd* must be non-blocking.  It is up to the library to
   remove any bytes from *fd* 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.

   New in version 2.6.

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.

   New in version 2.6.

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.


17.4.1. 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
