130 lines
5.3 KiB
TeX
130 lines
5.3 KiB
TeX
\section{Built-in Module \sectcode{signal}}
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\bimodindex{signal}
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This module provides mechanisms to use signal handlers in Python.
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Some general rules for working with signals handlers:
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\begin{itemize}
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\item
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A handler for a particular signal, once set, remains installed until
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it is explicitly reset (i.e. Python uses the BSD style interface).
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\item
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There is no way to ``block'' signals temporarily from critical
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sections (since this is not supported by all \UNIX{} flavors).
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\item
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Although Python signal handlers are called asynchronously as far as
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the Python user is concerned, they can only occur between the
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``atomic'' instructions of the Python interpreter. This means that
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signals arriving during long calculations implemented purely in C
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(e.g.\ regular expression matches on large bodies of text) may be
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delayed for an arbitrary time.
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\item
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When a signal arrives during an I/O operation, it is possible that the
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I/O operation raises an exception after the signal handler returns.
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This is dependent on the underlying \UNIX{} system's semantics regarding
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interrupted system calls.
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\item
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Because the C signal handler always returns, it makes little sense to
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catch synchronous errors like \code{SIGFPE} or \code{SIGSEGV}.
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\item
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Python installs a small number of signal handlers by default:
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\code{SIGPIPE} is ignored (so write errors on pipes and sockets can be
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reported as ordinary Python exceptions), \code{SIGINT} is translated
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into a \code{KeyboardInterrupt} exception, and \code{SIGTERM} is
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caught so that necessary cleanup (especially \code{sys.exitfunc}) can
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be performed before actually terminating. All of these can be
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overridden.
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\item
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Some care must be taken if both signals and threads are used in the
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same program. The fundamental thing to remember in using signals and
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threads simultaneously is:\ always perform \code{signal()} operations
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in the main thread of execution. Any thread can perform an
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\code{alarm()}, \code{getsignal()}, or \code{pause()}; only the main
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thread can set a new signal handler, and the main thread will be the
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only one to receive signals. This means that signals can't be used as
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a means of interthread communication. Use locks instead.
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\end{itemize}
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The variables defined in the signal module are:
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\renewcommand{\indexsubitem}{(in module signal)}
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\begin{datadesc}{SIG_DFL}
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This is one of two standard signal handling options; it will simply
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perform the default function for the signal. For example, on most
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systems the default action for SIGQUIT is to dump core and exit,
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while the default action for SIGCLD is to simply ignore it.
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\end{datadesc}
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\begin{datadesc}{SIG_IGN}
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This is another standard signal handler, which will simply ignore
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the given signal.
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\end{datadesc}
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\begin{datadesc}{SIG*}
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All the signal numbers are defined symbolically. For example, the
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hangup signal is defined as \code{signal.SIGHUP}; the variable names
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are identical to the names used in C programs, as found in
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\file{signal.h}.
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The \UNIX{} man page for \file{signal} lists the existing signals (on
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some systems this is \file{signal(2)}, on others the list is in
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\file{signal(7)}).
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Note that not all systems define the same set of signal names; only
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those names defined by the system are defined by this module.
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\end{datadesc}
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\begin{datadesc}{NSIG}
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One more than the number of the highest signal number.
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\end{datadesc}
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The signal module defines the following functions:
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\begin{funcdesc}{alarm}{time}
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If \var{time} is non-zero, this function requests that a
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\code{SIGALRM} signal be sent to the process in \var{time} seconds.
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Any previously scheduled alarm is canceled (i.e.\ only one alarm can
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be scheduled at any time). The returned value is then the number of
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seconds before any previously set alarm was to have been delivered.
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If \var{time} is zero, no alarm id scheduled, and any scheduled
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alarm is canceled. The return value is the number of seconds
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remaining before a previously scheduled alarm. If the return value
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is zero, no alarm is currently scheduled. (See the \UNIX{} man page
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\code{alarm(2)}.)
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\end{funcdesc}
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\begin{funcdesc}{getsignal}{signalnum}
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Return the current signal handler for the signal \var{signalnum}.
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The returned value may be a callable Python object, or one of the
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special values \code{signal.SIG_IGN}, \code{signal.SIG_DFL} or
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\code{None}. Here, \code{signal.SIG_IGN} means that the signal was
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previously ignored, \code{signal.SIG_DFL} means that the default way
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of handling the signal was previously in use, and \code{None} means
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that the previous signal handler was not installed from Python.
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\end{funcdesc}
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\begin{funcdesc}{pause}{}
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Cause the process to sleep until a signal is received; the
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appropriate handler will then be called. Returns nothing. (See the
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\UNIX{} man page \code{signal(2)}.)
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\end{funcdesc}
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\begin{funcdesc}{signal}{signalnum\, handler}
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Set the handler for signal \var{signalnum} to the function
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\var{handler}. \var{handler} can be any callable Python object, or
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one of the special values \code{signal.SIG_IGN} or
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\code{signal.SIG_DFL}. The previous signal handler will be returned
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(see the description of \code{getsignal()} above). (See the \UNIX{}
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man page \code{signal(2)}.)
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When threads are enabled, this function can only be called from the
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main thread; attempting to call it from other threads will cause a
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\code{ValueError} exception to be raised.
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\end{funcdesc}
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