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author Guido van Rossum <guido@python.org>
date Thu, 17 Jul 1997 16:29:42 +0000
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1 \section{Built-in Module \sectcode{signal}}
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2
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3 \bimodindex{signal}
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4 This module provides mechanisms to use signal handlers in Python.
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5 Some general rules for working with signals handlers:
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6
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7 \begin{itemize}
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8
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9 \item
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10 A handler for a particular signal, once set, remains installed until
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11 it is explicitly reset (i.e. Python emulates the BSD style interface
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12 regardless of the underlying implementation), with the exception of
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13 the handler for \code{SIGCHLD}, which follows the underlying
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14 implementation.
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15
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16 \item
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17 There is no way to ``block'' signals temporarily from critical
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18 sections (since this is not supported by all \UNIX{} flavors).
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19
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20 \item
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21 Although Python signal handlers are called asynchronously as far as
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22 the Python user is concerned, they can only occur between the
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23 ``atomic'' instructions of the Python interpreter. This means that
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24 signals arriving during long calculations implemented purely in C
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25 (e.g.\ regular expression matches on large bodies of text) may be
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26 delayed for an arbitrary amount of time.
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27
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28 \item
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29 When a signal arrives during an I/O operation, it is possible that the
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30 I/O operation raises an exception after the signal handler returns.
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31 This is dependent on the underlying \UNIX{} system's semantics regarding
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32 interrupted system calls.
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33
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34 \item
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35 Because the C signal handler always returns, it makes little sense to
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36 catch synchronous errors like \code{SIGFPE} or \code{SIGSEGV}.
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37
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38 \item
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39 Python installs a small number of signal handlers by default:
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40 \code{SIGPIPE} is ignored (so write errors on pipes and sockets can be
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41 reported as ordinary Python exceptions), \code{SIGINT} is translated
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42 into a \code{KeyboardInterrupt} exception, and \code{SIGTERM} is
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43 caught so that necessary cleanup (especially \code{sys.exitfunc}) can
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44 be performed before actually terminating. All of these can be
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45 overridden.
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46
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47 \item
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48 Some care must be taken if both signals and threads are used in the
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49 same program. The fundamental thing to remember in using signals and
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50 threads simultaneously is:\ always perform \code{signal()} operations
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51 in the main thread of execution. Any thread can perform an
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52 \code{alarm()}, \code{getsignal()}, or \code{pause()}; only the main
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53 thread can set a new signal handler, and the main thread will be the
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54 only one to receive signals (this is enforced by the Python signal
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55 module, even if the underlying thread implementation supports sending
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56 signals to individual threads). This means that signals can't be used
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57 as a means of interthread communication. Use locks instead.
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58
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59 \end{itemize}
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60
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61 The variables defined in the signal module are:
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62
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63 \renewcommand{\indexsubitem}{(in module signal)}
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64 \begin{datadesc}{SIG_DFL}
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65 This is one of two standard signal handling options; it will simply
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66 perform the default function for the signal. For example, on most
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67 systems the default action for SIGQUIT is to dump core and exit,
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68 while the default action for SIGCLD is to simply ignore it.
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69 \end{datadesc}
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70
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71 \begin{datadesc}{SIG_IGN}
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72 This is another standard signal handler, which will simply ignore
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73 the given signal.
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74 \end{datadesc}
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75
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76 \begin{datadesc}{SIG*}
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77 All the signal numbers are defined symbolically. For example, the
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78 hangup signal is defined as \code{signal.SIGHUP}; the variable names
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79 are identical to the names used in C programs, as found in
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80 \file{signal.h}.
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81 The \UNIX{} man page for \file{signal} lists the existing signals (on
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82 some systems this is \file{signal(2)}, on others the list is in
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83 \file{signal(7)}).
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84 Note that not all systems define the same set of signal names; only
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85 those names defined by the system are defined by this module.
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86 \end{datadesc}
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87
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88 \begin{datadesc}{NSIG}
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89 One more than the number of the highest signal number.
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90 \end{datadesc}
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91
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92 The signal module defines the following functions:
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93
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94 \begin{funcdesc}{alarm}{time}
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95 If \var{time} is non-zero, this function requests that a
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96 \code{SIGALRM} signal be sent to the process in \var{time} seconds.
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97 Any previously scheduled alarm is canceled (i.e.\ only one alarm can
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98 be scheduled at any time). The returned value is then the number of
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99 seconds before any previously set alarm was to have been delivered.
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100 If \var{time} is zero, no alarm id scheduled, and any scheduled
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101 alarm is canceled. The return value is the number of seconds
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102 remaining before a previously scheduled alarm. If the return value
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103 is zero, no alarm is currently scheduled. (See the \UNIX{} man page
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104 \code{alarm(2)}.)
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105 \end{funcdesc}
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106
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107 \begin{funcdesc}{getsignal}{signalnum}
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108 Return the current signal handler for the signal \var{signalnum}.
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109 The returned value may be a callable Python object, or one of the
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110 special values \code{signal.SIG_IGN}, \code{signal.SIG_DFL} or
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111 \code{None}. Here, \code{signal.SIG_IGN} means that the signal was
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112 previously ignored, \code{signal.SIG_DFL} means that the default way
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113 of handling the signal was previously in use, and \code{None} means
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114 that the previous signal handler was not installed from Python.
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115 \end{funcdesc}
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116
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117 \begin{funcdesc}{pause}{}
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118 Cause the process to sleep until a signal is received; the
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119 appropriate handler will then be called. Returns nothing. (See the
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120 \UNIX{} man page \code{signal(2)}.)
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121 \end{funcdesc}
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122
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123 \begin{funcdesc}{signal}{signalnum\, handler}
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124 Set the handler for signal \var{signalnum} to the function
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125 \var{handler}. \var{handler} can be any callable Python object, or
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126 one of the special values \code{signal.SIG_IGN} or
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127 \code{signal.SIG_DFL}. The previous signal handler will be returned
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128 (see the description of \code{getsignal()} above). (See the \UNIX{}
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129 man page \code{signal(2)}.)
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130
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131 When threads are enabled, this function can only be called from the
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132 main thread; attempting to call it from other threads will cause a
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133 \code{ValueError} exception to be raised.
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134
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135 The \var{handler} is called with two arguments: the signal number
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136 and the current stack frame (\code{None} or a frame object; see the
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137 reference manual for a description of frame objects).
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138 \obindex{frame}
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139 \end{funcdesc}