: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:
+This module provides mechanisms to use signal handlers in Python.
-* 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.
-* 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
+The :func:`signal.signal` function allows to define custom handlers to be
+executed when a signal is received. A small number of default handlers are
+installed: :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.
-* Because the C signal handler always returns, it makes little sense to catch
- synchronous errors like :const:`SIGFPE` or :const:`SIGSEGV`.
+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.
-* 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.
+There is no way to "block" signals temporarily from critical sections (since
+this is not supported by all Unix flavors).
-* 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`,
- :func:`pause`, :func:`setitimer` or :func:`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 :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.
+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
+:term:`virtual machine` to execute the corresponding Python signal handler
+at a later point(for example at the next :term:`bytecode` instruction).
+* It makes little sense to catch synchronous errors like :const:`SIGFPE` or
+* 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.
+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 :mod:`threading` module instead.
+Besides, only the main thread is allowed to set a new signal handler.
The variables defined in the :mod:`signal` module are:
.. function:: pthread_kill(thread_id, signum)
- Send the signal *signum* to the thread *thread_id*, another thread in the same
- process as the caller. The signal is asynchronously directed to thread.
+ Send the signal *signum* 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 :ref:`executed by the main
+ thread <signals-and-threads>`. Therefore, the only point of sending a signal to a particular
+ Python thread would be to force a running system call to fail with
Use :func:`threading.get_ident()` or the :attr:`~threading.Thread.ident`
- attribute of :attr:`threading.Thread` to get a 'thread identifier' for
+ attribute of :class:`threading.Thread` objects to get a suitable value
If *signum* is 0, then no signal is sent, but error checking is still
- performed; this can be used to check if
a thread is still running.
+ performed; this can be used to check if thread is still running.
Availability: Unix (see the man page :manpage:`pthread_kill(3)` for further