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81 lines
3.4 KiB
TeX
81 lines
3.4 KiB
TeX
\section{Built-in Module \sectcode{signal}}
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\bimodindex{signal}
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This module provides mechanisms to write signal handlers in Python.
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{\bf Warning:} Some care must be taken if both signals and threads
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will be used in the same program. The fundamental thing to remember
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in using signals and threads simultaneously is: always perform
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\code{signal()} operations in the main thread of execution. Any
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thread can perform a \code{alarm()}, \code{getsignal()}, or
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\code{pause()}; only the main thread can set a new signal handler, and
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the main thread will be the only one to receive signals. This means
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that signals can't be used as a means of interthread communication.
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Use locks instead.
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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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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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Returns 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} or \code{signal.SIG_DFL}.
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\end{funcdesc}
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\begin{funcdesc}{pause}{}
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Causes 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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Sets 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
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returned. (See the UNIX man page \code{signal(2)}.)
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If 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 will be raised.
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\end{funcdesc}
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