Doc/lib/libthread.tex

   1 \section{\module{thread} ---
   2          Multiple threads of control}
   3
   4 \declaremodule{builtin}{thread}
   5 \modulesynopsis{Create multiple threads of control within one interpreter.}
   6
   7
   8 This module provides low-level primitives for working with multiple
   9 threads (a.k.a.\ \dfn{light-weight processes} or \dfn{tasks}) --- multiple
  10 threads of control sharing their global data space.  For
  11 synchronization, simple locks (a.k.a.\ \dfn{mutexes} or \dfn{binary
  12 semaphores}) are provided.
  13 \index{light-weight processes}
  14 \index{processes, light-weight}
  15 \index{binary semaphores}
  16 \index{semaphores, binary}
  17
  18 The module is optional.  It is supported on Windows, Linux, SGI
  19 IRIX, Solaris 2.x, as well as on systems that have a \POSIX{} thread
  20 (a.k.a. ``pthread'') implementation.  For systems lacking the \module{thread}
  21 module, the \refmodule[dummythread]{dummy_thread} module is available.
  22 It duplicates this module's interface and can be
  23 used as a drop-in replacement.
  24 \index{pthreads}
  25 \indexii{threads}{\POSIX}
  26
  27 It defines the following constant and functions:
  28
  29 \begin{excdesc}{error}
  30 Raised on thread-specific errors.
  31 \end{excdesc}
  32
  33 \begin{datadesc}{LockType}
  34 This is the type of lock objects.
  35 \end{datadesc}
  36
  37 \begin{funcdesc}{start_new_thread}{function, args\optional{, kwargs}}
  38 Start a new thread and return its identifier.  The thread executes the function
  39 \var{function} with the argument list \var{args} (which must be a tuple).  The
  40 optional \var{kwargs} argument specifies a dictionary of keyword arguments.
  41 When the function returns, the thread silently exits.  When the function
  42 terminates with an unhandled exception, a stack trace is printed and
  43 then the thread exits (but other threads continue to run).
  44 \end{funcdesc}
  45
  46 \begin{funcdesc}{interrupt_main}{}
  47 Raise a \exception{KeyboardInterrupt} exception in the main thread.  A subthread
  48 can use this function to interrupt the main thread.
  49 \versionadded{2.3}
  50 \end{funcdesc}
  51
  52 \begin{funcdesc}{exit}{}
  53 Raise the \exception{SystemExit} exception.  When not caught, this
  54 will cause the thread to exit silently.
  55 \end{funcdesc}
  56
  57 %\begin{funcdesc}{exit_prog}{status}
  58 %Exit all threads and report the value of the integer argument
  59 %\var{status} as the exit status of the entire program.
  60 %\strong{Caveat:} code in pending \keyword{finally} clauses, in this thread
  61 %or in other threads, is not executed.
  62 %\end{funcdesc}
  63
  64 \begin{funcdesc}{allocate_lock}{}
  65 Return a new lock object.  Methods of locks are described below.  The
  66 lock is initially unlocked.
  67 \end{funcdesc}
  68
  69 \begin{funcdesc}{get_ident}{}
  70 Return the `thread identifier' of the current thread.  This is a
  71 nonzero integer.  Its value has no direct meaning; it is intended as a
  72 magic cookie to be used e.g. to index a dictionary of thread-specific
  73 data.  Thread identifiers may be recycled when a thread exits and
  74 another thread is created.
  75 \end{funcdesc}
  76
  77 \begin{funcdesc}{stack_size}{\optional{size}}
  78 Return the thread stack size used when creating new threads.  The
  79 optional \var{size} argument specifies the stack size to be used for
  80 subsequently created threads, and must be 0 (use platform or
  81 configured default) or a positive integer value of at least 32,768 (32kB).
  82 If changing the thread stack size is unsupported, a \exception{ThreadError}
  83 is raised.  If the specified stack size is invalid, a \exception{ValueError}
  84 is raised and the stack size is unmodified.  32kB is currently the minimum
  85 supported stack size value to guarantee sufficient stack space for the
  86 interpreter itself.  Note that some platforms may have particular
  87 restrictions on values for the stack size, such as requiring a minimum
  88 stack size > 32kB or requiring allocation in multiples of the system
  89 memory page size - platform documentation should be referred to for
  90 more information (4kB pages are common; using multiples of 4096 for
  91 the stack size is the suggested approach in the absence of more
  92 specific information).
  93 Availability: Windows, systems with \POSIX{} threads.
  94 \versionadded{2.5}
  95 \end{funcdesc}
  96
  97
  98 Lock objects have the following methods:
  99
 100 \begin{methoddesc}[lock]{acquire}{\optional{waitflag}}
 101 Without the optional argument, this method acquires the lock
 102 unconditionally, if necessary waiting until it is released by another
 103 thread (only one thread at a time can acquire a lock --- that's their
 104 reason for existence).  If the integer
 105 \var{waitflag} argument is present, the action depends on its
 106 value: if it is zero, the lock is only acquired if it can be acquired
 107 immediately without waiting, while if it is nonzero, the lock is
 108 acquired unconditionally as before.  The
 109 return value is \code{True} if the lock is acquired successfully,
 110 \code{False} if not.
 111 \end{methoddesc}
 112
 113 \begin{methoddesc}[lock]{release}{}
 114 Releases the lock.  The lock must have been acquired earlier, but not
 115 necessarily by the same thread.
 116 \end{methoddesc}
 117
 118 \begin{methoddesc}[lock]{locked}{}
 119 Return the status of the lock:\ \code{True} if it has been acquired by
 120 some thread, \code{False} if not.
 121 \end{methoddesc}
 122
 123 In addition to these methods, lock objects can also be used via the
 124 \keyword{with} statement, e.g.:
 125
 126 \begin{verbatim}
 127 from __future__ import with_statement
 128 import thread
 129
 130 a_lock = thread.allocate_lock()
 131
 132 with a_lock:
 133     print "a_lock is locked while this executes"
 134 \end{verbatim}
 135
 136 \strong{Caveats:}
 137
 138 \begin{itemize}
 139 \item
 140 Threads interact strangely with interrupts: the
 141 \exception{KeyboardInterrupt} exception will be received by an
 142 arbitrary thread.  (When the \refmodule{signal}\refbimodindex{signal}
 143 module is available, interrupts always go to the main thread.)
 144
 145 \item
 146 Calling \function{sys.exit()} or raising the \exception{SystemExit}
 147 exception is equivalent to calling \function{exit()}.
 148
 149 \item
 150 Not all built-in functions that may block waiting for I/O allow other
 151 threads to run.  (The most popular ones (\function{time.sleep()},
 152 \method{\var{file}.read()}, \function{select.select()}) work as
 153 expected.)
 154
 155 \item
 156 It is not possible to interrupt the \method{acquire()} method on a lock
 157 --- the \exception{KeyboardInterrupt} exception will happen after the
 158 lock has been acquired.
 159
 160 \item
 161 When the main thread exits, it is system defined whether the other
 162 threads survive.  On SGI IRIX using the native thread implementation,
 163 they survive.  On most other systems, they are killed without
 164 executing \keyword{try} ... \keyword{finally} clauses or executing
 165 object destructors.
 166 \indexii{threads}{IRIX}
 167
 168 \item
 169 When the main thread exits, it does not do any of its usual cleanup
 170 (except that \keyword{try} ... \keyword{finally} clauses are honored),
 171 and the standard I/O files are not flushed.
 172
 173 \end{itemize}