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author Johannes Gijsbers <jlg@dds.nl>
date Mon, 13 Dec 2004 13:11:42 +0000
parents e6a78dd425c8
children 21251e221e56
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1 \section{\module{thread} ---
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2 Multiple threads of control}
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3
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4 \declaremodule{builtin}{thread}
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5 \modulesynopsis{Create multiple threads of control within one interpreter.}
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6
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7
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8 This module provides low-level primitives for working with multiple
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9 threads (a.k.a.\ \dfn{light-weight processes} or \dfn{tasks}) --- multiple
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10 threads of control sharing their global data space. For
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11 synchronization, simple locks (a.k.a.\ \dfn{mutexes} or \dfn{binary
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12 semaphores}) are provided.
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13 \index{light-weight processes}
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14 \index{processes, light-weight}
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15 \index{binary semaphores}
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16 \index{semaphores, binary}
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17
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18 The module is optional. It is supported on Windows, Linux, SGI
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19 IRIX, Solaris 2.x, as well as on systems that have a \POSIX{} thread
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20 (a.k.a. ``pthread'') implementation. For systems lacking the \module{thread}
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21 module, the \refmodule[dummythread]{dummy_thread} module is available.
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22 It duplicates this module's interface and can be
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23 used as a drop-in replacement.
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24 \index{pthreads}
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25 \indexii{threads}{\POSIX}
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26
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27 It defines the following constant and functions:
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28
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29 \begin{excdesc}{error}
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30 Raised on thread-specific errors.
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31 \end{excdesc}
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32
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33 \begin{datadesc}{LockType}
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34 This is the type of lock objects.
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35 \end{datadesc}
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36
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37 \begin{funcdesc}{start_new_thread}{function, args\optional{, kwargs}}
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38 Start a new thread and return its identifier. The thread executes the function
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39 \var{function} with the argument list \var{args} (which must be a tuple). The
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40 optional \var{kwargs} argument specifies a dictionary of keyword arguments.
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41 When the function returns, the thread silently exits. When the function
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42 terminates with an unhandled exception, a stack trace is printed and
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43 then the thread exits (but other threads continue to run).
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44 \end{funcdesc}
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45
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46 \begin{funcdesc}{interrupt_main}{}
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47 Raise a KeyboardInterrupt in the main thread. A subthread can use this
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48 function to interrupt the main thread.
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49 \versionadded{2.3}
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50 \end{funcdesc}
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51
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52 \begin{funcdesc}{exit}{}
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53 Raise the \exception{SystemExit} exception. When not caught, this
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54 will cause the thread to exit silently.
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55 \end{funcdesc}
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56
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57 %\begin{funcdesc}{exit_prog}{status}
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58 %Exit all threads and report the value of the integer argument
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59 %\var{status} as the exit status of the entire program.
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60 %\strong{Caveat:} code in pending \keyword{finally} clauses, in this thread
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61 %or in other threads, is not executed.
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62 %\end{funcdesc}
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63
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64 \begin{funcdesc}{allocate_lock}{}
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65 Return a new lock object. Methods of locks are described below. The
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66 lock is initially unlocked.
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67 \end{funcdesc}
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68
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69 \begin{funcdesc}{get_ident}{}
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70 Return the `thread identifier' of the current thread. This is a
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71 nonzero integer. Its value has no direct meaning; it is intended as a
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72 magic cookie to be used e.g. to index a dictionary of thread-specific
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73 data. Thread identifiers may be recycled when a thread exits and
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74 another thread is created.
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75 \end{funcdesc}
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78 Lock objects have the following methods:
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79
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80 \begin{methoddesc}[lock]{acquire}{\optional{waitflag}}
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81 Without the optional argument, this method acquires the lock
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82 unconditionally, if necessary waiting until it is released by another
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83 thread (only one thread at a time can acquire a lock --- that's their
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84 reason for existence), and returns \code{None}. If the integer
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85 \var{waitflag} argument is present, the action depends on its
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86 value: if it is zero, the lock is only acquired if it can be acquired
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87 immediately without waiting, while if it is nonzero, the lock is
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88 acquired unconditionally as before. If an argument is present, the
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89 return value is \code{True} if the lock is acquired successfully,
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90 \code{False} if not.
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91 \end{methoddesc}
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92
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93 \begin{methoddesc}[lock]{release}{}
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94 Releases the lock. The lock must have been acquired earlier, but not
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95 necessarily by the same thread.
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96 \end{methoddesc}
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97
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98 \begin{methoddesc}[lock]{locked}{}
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99 Return the status of the lock:\ \code{True} if it has been acquired by
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100 some thread, \code{False} if not.
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101 \end{methoddesc}
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102
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103 \strong{Caveats:}
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104
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105 \begin{itemize}
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106 \item
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107 Threads interact strangely with interrupts: the
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108 \exception{KeyboardInterrupt} exception will be received by an
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109 arbitrary thread. (When the \refmodule{signal}\refbimodindex{signal}
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110 module is available, interrupts always go to the main thread.)
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111
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112 \item
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113 Calling \function{sys.exit()} or raising the \exception{SystemExit}
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114 exception is equivalent to calling \function{exit()}.
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115
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116 \item
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117 Not all built-in functions that may block waiting for I/O allow other
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118 threads to run. (The most popular ones (\function{time.sleep()},
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119 \method{\var{file}.read()}, \function{select.select()}) work as
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120 expected.)
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121
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122 \item
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123 It is not possible to interrupt the \method{acquire()} method on a lock
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124 --- the \exception{KeyboardInterrupt} exception will happen after the
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125 lock has been acquired.
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126
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127 \item
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128 When the main thread exits, it is system defined whether the other
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129 threads survive. On SGI IRIX using the native thread implementation,
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130 they survive. On most other systems, they are killed without
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131 executing \keyword{try} ... \keyword{finally} clauses or executing
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132 object destructors.
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133 \indexii{threads}{IRIX}
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134
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135 \item
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136 When the main thread exits, it does not do any of its usual cleanup
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137 (except that \keyword{try} ... \keyword{finally} clauses are honored),
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138 and the standard I/O files are not flushed.
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139
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140 \end{itemize}