Doc/lib/libsignal.tex

   1 \section{\module{signal} ---
   2          Set handlers for asynchronous events.}
   3 \declaremodule{builtin}{signal}
   4
   5
   6 \modulesynopsis{Set handlers for asynchronous events.}
   7
   8 This module provides mechanisms to use signal handlers in Python.
   9 Some general rules for working with signals and their handlers:
  10
  11 \begin{itemize}
  12
  13 \item
  14 A handler for a particular signal, once set, remains installed until
  15 it is explicitly reset (i.e. Python emulates the BSD style interface
  16 regardless of the underlying implementation), with the exception of
  17 the handler for \constant{SIGCHLD}, which follows the underlying
  18 implementation.
  19
  20 \item
  21 There is no way to ``block'' signals temporarily from critical
  22 sections (since this is not supported by all \UNIX{} flavors).
  23
  24 \item
  25 Although Python signal handlers are called asynchronously as far as
  26 the Python user is concerned, they can only occur between the
  27 ``atomic'' instructions of the Python interpreter.  This means that
  28 signals arriving during long calculations implemented purely in \C{}
  29 (e.g.\ regular expression matches on large bodies of text) may be
  30 delayed for an arbitrary amount of time.
  31
  32 \item
  33 When a signal arrives during an I/O operation, it is possible that the
  34 I/O operation raises an exception after the signal handler returns.
  35 This is dependent on the underlying \UNIX{} system's semantics regarding
  36 interrupted system calls.
  37
  38 \item
  39 Because the \C{} signal handler always returns, it makes little sense to
  40 catch synchronous errors like \constant{SIGFPE} or \constant{SIGSEGV}.
  41
  42 \item
  43 Python installs a small number of signal handlers by default:
  44 \constant{SIGPIPE} is ignored (so write errors on pipes and sockets can be
  45 reported as ordinary Python exceptions) and \constant{SIGINT} is translated
  46 into a \exception{KeyboardInterrupt} exception.  All of these can be
  47 overridden.
  48
  49 \item
  50 Some care must be taken if both signals and threads are used in the
  51 same program.  The fundamental thing to remember in using signals and
  52 threads simultaneously is:\ always perform \function{signal()} operations
  53 in the main thread of execution.  Any thread can perform an
  54 \function{alarm()}, \function{getsignal()}, or \function{pause()};
  55 only the main thread can set a new signal handler, and the main thread
  56 will be the only one to receive signals (this is enforced by the
  57 Python \module{signal} module, even if the underlying thread
  58 implementation supports sending signals to individual threads).  This
  59 means that signals can't be used as a means of interthread
  60 communication.  Use locks instead.
  61
  62 \end{itemize}
  63
  64 The variables defined in the \module{signal} module are:
  65
  66 \begin{datadesc}{SIG_DFL}
  67   This is one of two standard signal handling options; it will simply
  68   perform the default function for the signal.  For example, on most
  69   systems the default action for \constant{SIGQUIT} is to dump core
  70   and exit, while the default action for \constant{SIGCLD} is to
  71   simply ignore it.
  72 \end{datadesc}
  73
  74 \begin{datadesc}{SIG_IGN}
  75   This is another standard signal handler, which will simply ignore
  76   the given signal.
  77 \end{datadesc}
  78
  79 \begin{datadesc}{SIG*}
  80   All the signal numbers are defined symbolically.  For example, the
  81   hangup signal is defined as \constant{signal.SIGHUP}; the variable names
  82   are identical to the names used in C programs, as found in
  83   \code{<signal.h>}.
  84   The \UNIX{} man page for `\cfunction{signal()}' lists the existing
  85   signals (on some systems this is \manpage{signal}{2}, on others the
  86   list is in \manpage{signal}{7}).
  87   Note that not all systems define the same set of signal names; only
  88   those names defined by the system are defined by this module.
  89 \end{datadesc}
  90
  91 \begin{datadesc}{NSIG}
  92   One more than the number of the highest signal number.
  93 \end{datadesc}
  94
  95 The \module{signal} module defines the following functions:
  96
  97 \begin{funcdesc}{alarm}{time}
  98   If \var{time} is non-zero, this function requests that a
  99   \constant{SIGALRM} signal be sent to the process in \var{time} seconds.
 100   Any previously scheduled alarm is canceled (i.e.\ only one alarm can
 101   be scheduled at any time).  The returned value is then the number of
 102   seconds before any previously set alarm was to have been delivered.
 103   If \var{time} is zero, no alarm id scheduled, and any scheduled
 104   alarm is canceled.  The return value is the number of seconds
 105   remaining before a previously scheduled alarm.  If the return value
 106   is zero, no alarm is currently scheduled.  (See the \UNIX{} man page
 107   \manpage{alarm}{2}.)
 108 \end{funcdesc}
 109
 110 \begin{funcdesc}{getsignal}{signalnum}
 111   Return the current signal handler for the signal \var{signalnum}.
 112   The returned value may be a callable Python object, or one of the
 113   special values \constant{signal.SIG_IGN}, \constant{signal.SIG_DFL} or
 114   \constant{None}.  Here, \constant{signal.SIG_IGN} means that the
 115   signal was previously ignored, \constant{signal.SIG_DFL} means that the
 116   default way of handling the signal was previously in use, and
 117   \code{None} means that the previous signal handler was not installed
 118   from Python.
 119 \end{funcdesc}
 120
 121 \begin{funcdesc}{pause}{}
 122   Cause the process to sleep until a signal is received; the
 123   appropriate handler will then be called.  Returns nothing.  (See the
 124   \UNIX{} man page \manpage{signal}{2}.)
 125 \end{funcdesc}
 126
 127 \begin{funcdesc}{signal}{signalnum, handler}
 128   Set the handler for signal \var{signalnum} to the function
 129   \var{handler}.  \var{handler} can be a callable Python object
 130   taking two arguments (see below), or
 131   one of the special values \constant{signal.SIG_IGN} or
 132   \constant{signal.SIG_DFL}.  The previous signal handler will be returned
 133   (see the description of \function{getsignal()} above).  (See the
 134   \UNIX{} man page \manpage{signal}{2}.)
 135
 136   When threads are enabled, this function can only be called from the
 137   main thread; attempting to call it from other threads will cause a
 138   \exception{ValueError} exception to be raised.
 139
 140   The \var{handler} is called with two arguments: the signal number
 141   and the current stack frame (\code{None} or a frame object; see the
 142   reference manual for a description of frame objects).
 143 \obindex{frame}
 144 \end{funcdesc}
 145
 146 \subsection{Example}
 147 \nodename{Signal Example}
 148
 149 Here is a minimal example program. It uses the \function{alarm()}
 150 function to limit the time spent waiting to open a file; this is
 151 useful if the file is for a serial device that may not be turned on,
 152 which would normally cause the \function{os.open()} to hang
 153 indefinitely.  The solution is to set a 5-second alarm before opening
 154 the file; if the operation takes too long, the alarm signal will be
 155 sent, and the handler raises an exception.
 156
 157 \begin{verbatim}
 158 import signal, os, FCNTL
 159
 160 def handler(signum, frame):
 161     print 'Signal handler called with signal', signum
 162     raise IOError, "Couldn't open device!"
 163
 164 # Set the signal handler and a 5-second alarm
 165 signal.signal(signal.SIGALRM, handler)
 166 signal.alarm(5)
 167
 168 # This open() may hang indefinitely
 169 fd = os.open('/dev/ttyS0', FCNTL.O_RDWR)
 170
 171 signal.alarm(0)          # Disable the alarm
 172 \end{verbatim}