18.8. "signal" — Set handlers for asynchronous events
*****************************************************

======================================================================

This module provides mechanisms to use signal handlers in Python.


18.8.1. General rules
=====================

The "signal.signal()" function allows defining 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.


18.8.1.1. 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" that are caused by an invalid operation in C code.  Python
  will return from the signal handler to the C code, which is likely
  to raise the same signal again, causing Python to apparently hang.
  From Python 3.3 onwards, you can use the "faulthandler" module to
  report on synchronous errors.

* 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.


18.8.1.2. 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.


18.8.2. Module contents
=======================

Changed in version 3.5: signal (SIG*), handler ("SIG_DFL", "SIG_IGN")
and sigmask ("SIG_BLOCK", "SIG_UNBLOCK", "SIG_SETMASK") related
constants listed below were turned into "enums". "getsignal()",
"pthread_sigmask()", "sigpending()" and "sigwait()" functions return
human-readable "enums".

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.

signal.SIG_BLOCK

   A possible value for the *how* parameter to "pthread_sigmask()"
   indicating that signals are to be blocked.

   New in version 3.3.

signal.SIG_UNBLOCK

   A possible value for the *how* parameter to "pthread_sigmask()"
   indicating that signals are to be unblocked.

   New in version 3.3.

signal.SIG_SETMASK

   A possible value for the *how* parameter to "pthread_sigmask()"
   indicating that the signal mask is to be replaced.

   New in version 3.3.

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 "OSError".

   New in version 3.3: This error used to be a subtype of "IOError",
   which is now an alias of "OSError".

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)*.)

   See also "sigwait()", "sigwaitinfo()", "sigtimedwait()" and
   "sigpending()".

signal.pthread_kill(thread_id, signalnum)

   Send the signal *signalnum* to the thread *thread_id*, another
   thread in the same process as the caller.  The target thread can be
   executing any code (Python or not).  However, if the target thread
   is executing the Python interpreter, the Python signal handlers
   will be executed by the main thread.  Therefore, the only point of
   sending a signal to a particular Python thread would be to force a
   running system call to fail with "InterruptedError".

   Use "threading.get_ident()" or the "ident" attribute of
   "threading.Thread" objects to get a suitable value for *thread_id*.

   If *signalnum* is 0, then no signal is sent, but error checking is
   still performed; this can be used to check if the target thread is
   still running.

   Availability: Unix (see the man page *pthread_kill(3)* for further
   information).

   See also "os.kill()".

   New in version 3.3.

signal.pthread_sigmask(how, mask)

   Fetch and/or change the signal mask of the calling thread.  The
   signal mask is the set of signals whose delivery is currently
   blocked for the caller. Return the old signal mask as a set of
   signals.

   The behavior of the call is dependent on the value of *how*, as
   follows.

   * "SIG_BLOCK": The set of blocked signals is the union of the
     current set and the *mask* argument.

   * "SIG_UNBLOCK": The signals in *mask* are removed from the
     current set of blocked signals.  It is permissible to attempt to
     unblock a signal which is not blocked.

   * "SIG_SETMASK": The set of blocked signals is set to the *mask*
     argument.

   *mask* is a set of signal numbers (e.g. {"signal.SIGINT",
   "signal.SIGTERM"}). Use "range(1, signal.NSIG)" for a full mask
   including all signals.

   For example, "signal.pthread_sigmask(signal.SIG_BLOCK, [])" reads
   the signal mask of the calling thread.

   Availability: Unix. See the man page *sigprocmask(3)* and
   *pthread_sigmask(3)* for further information.

   See also "pause()", "sigpending()" and "sigwait()".

   New in version 3.3.

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 file descriptor to *fd*.  When a signal is received,
   the signal number is written as a single byte into 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.

   Use for example "struct.unpack('%uB' % len(data), data)" to decode
   the signal numbers list.

   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.

   Changed in version 3.5: On Windows, the function now also supports
   socket handles.

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", "SIGTERM", or "SIGBREAK". A
   "ValueError" will be raised in any other case. Note that not all
   systems define the same set of signal names; an "AttributeError"
   will be raised if a signal name is not defined as "SIG*" module
   level constant.

signal.sigpending()

   Examine the set of signals that are pending for delivery to the
   calling thread (i.e., the signals which have been raised while
   blocked).  Return the set of the pending signals.

   Availability: Unix (see the man page *sigpending(2)* for further
   information).

   See also "pause()", "pthread_sigmask()" and "sigwait()".

   New in version 3.3.

signal.sigwait(sigset)

   Suspend execution of the calling thread until the delivery of one
   of the signals specified in the signal set *sigset*.  The function
   accepts the signal (removes it from the pending list of signals),
   and returns the signal number.

   Availability: Unix (see the man page *sigwait(3)* for further
   information).

   See also "pause()", "pthread_sigmask()", "sigpending()",
   "sigwaitinfo()" and "sigtimedwait()".

   New in version 3.3.

signal.sigwaitinfo(sigset)

   Suspend execution of the calling thread until the delivery of one
   of the signals specified in the signal set *sigset*.  The function
   accepts the signal and removes it from the pending list of signals.
   If one of the signals in *sigset* is already pending for the
   calling thread, the function will return immediately with
   information about that signal. The signal handler is not called for
   the delivered signal. The function raises an "InterruptedError" if
   it is interrupted by a signal that is not in *sigset*.

   The return value is an object representing the data contained in
   the "siginfo_t" structure, namely: "si_signo", "si_code",
   "si_errno", "si_pid", "si_uid", "si_status", "si_band".

   Availability: Unix (see the man page *sigwaitinfo(2)* for further
   information).

   See also "pause()", "sigwait()" and "sigtimedwait()".

   New in version 3.3.

   Changed in version 3.5: The function is now retried if interrupted
   by a signal not in *sigset* and the signal handler does not raise
   an exception (see **PEP 475** for the rationale).

signal.sigtimedwait(sigset, timeout)

   Like "sigwaitinfo()", but takes an additional *timeout* argument
   specifying a timeout. If *timeout* is specified as "0", a poll is
   performed. Returns "None" if a timeout occurs.

   Availability: Unix (see the man page *sigtimedwait(2)* for further
   information).

   See also "pause()", "sigwait()" and "sigwaitinfo()".

   New in version 3.3.

   Changed in version 3.5: The function is now retried with the
   recomputed *timeout* if interrupted by a signal not in *sigset* and
   the signal handler does not raise an exception (see **PEP 475** for
   the rationale).


18.8.3. 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 OSError("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
