Movatterモバイル変換
[0]ホーム
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 untilit is explicitly reset (Python emulates the BSD style interfaceregardless of the underlying implementation), with the exception ofthe handler forSIGCHLD, which follows the underlyingimplementation.
- There is no way to ``block'' signals temporarily from criticalsections (since this is not supported by all Unix flavors).
- Although Python signal handlers are called asynchronously as far asthe Python user is concerned, they can only occur between the``atomic'' instructions of the Python interpreter. This means thatsignals arriving during long calculations implemented purely in C(such as regular expression matches on large bodies of text) may bedelayed for an arbitrary amount of time.
- When a signal arrives during an I/O operation, it is possible that theI/O operation raises an exception after the signal handler returns.This is dependent on the underlying Unix system's semantics regardinginterrupted system calls.
- Because the C signal handler always returns, it makes little sense tocatch synchronous errors likeSIGFPE orSIGSEGV.
- Python installs a small number of signal handlers by default:SIGPIPE is ignored (so write errors on pipes and sockets can bereported as ordinary Python exceptions) andSIGINT is translatedinto aKeyboardInterrupt exception. All of these can beoverridden.
- Some care must be taken if both signals and threads are used in thesame program. The fundamental thing to remember in using signals andthreads simultaneously is: always performsignal() operationsin the main thread of execution. Any thread can perform analarm(),getsignal(), orpause();only the main thread can set a new signal handler, and the main threadwill be the only one to receive signals (this is enforced by thePythonsignal module, even if the underlying threadimplementation supports sending signals to individual threads). Thismeans that signals can't be used as a means of inter-threadcommunication. Use locks instead.
The variables defined in thesignal 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 forSIGQUIT is to dump core and exit, while the default action forSIGCLD 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 assignal.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 issignal(2), on others the list is insignal(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.
Thesignal module defines the following functions:
- alarm(time)
- Iftime is non-zero, this function requests that aSIGALRM signal be sent to the process intime 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. Iftime is zero, no alarm id scheduled, and any scheduled alarm is canceled. The return value is the number of seconds remaining before a previously scheduled alarm. If the return value is zero, no alarm is currently scheduled. (See the Unix man pagealarm(2).) Availability: Unix.
- getsignal(signalnum)
- Return the current signal handler for the signalsignalnum. The returned value may be a callable Python object, or one of the special valuessignal.SIG_IGN,signal.SIG_DFL orNone. 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. (See the Unix man pagesignal(2).)
- signal(signalnum, handler)
- Set the handler for signalsignalnum to the functionhandler.handler can be a callable Python object taking two arguments (see below), or one of the special valuessignal.SIG_IGN orsignal.SIG_DFL. The previous signal handler will be returned (see the description ofgetsignal() above). (See the Unix man pagesignal(2).)
When threads are enabled, this function can only be called from the main thread; attempting to call it from other threads will cause aValueError exception to be raised.
Thehandler is called with two arguments: the signal number and the current stack frame (None or a frame object; see the reference manual for a description of frame objects).
SubsectionsSeeAbout this document... for information on suggesting changes.
[8]ページ先頭
