# 17.3 `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()`, or `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 `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:

`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 `SIGCLD` is to simply ignore it.

`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.

`NSIG`
  One more than the number of the highest signal number.

The `signal` module defines the following functions:

`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.

`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.

`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`( | 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 reference manual section on the standard type hierarchy or see the attribute descriptions in the `inspect` module).