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(python2.1-lib.info)signal
Set handlers for asynchronous events
====================================
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 (i.e. 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
(e.g. 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 (i.e. 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 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 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. (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; see the
reference manual for a description of frame objects).
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