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}{exit}{}
  47 Raise the \exception{SystemExit} exception.  When not caught, this
  48 will cause the thread to exit silently.
  49 \end{funcdesc}
  50
  51 \begin{funcdesc}{exit_thread}{}
  52 \deprecated{1.5.2}{Use \function{exit()}.}
  53 This is an obsolete synonym for \function{exit()}.
  54 \end{funcdesc}
  55
  56 %\begin{funcdesc}{exit_prog}{status}
  57 %Exit all threads and report the value of the integer argument
  58 %\var{status} as the exit status of the entire program.
  59 %\strong{Caveat:} code in pending \keyword{finally} clauses, in this thread
  60 %or in other threads, is not executed.
  61 %\end{funcdesc}
  62
  63 \begin{funcdesc}{allocate_lock}{}
  64 Return a new lock object.  Methods of locks are described below.  The
  65 lock is initially unlocked.
  66 \end{funcdesc}
  67
  68 \begin{funcdesc}{get_ident}{}
  69 Return the `thread identifier' of the current thread.  This is a
  70 nonzero integer.  Its value has no direct meaning; it is intended as a
  71 magic cookie to be used e.g. to index a dictionary of thread-specific
  72 data.  Thread identifiers may be recycled when a thread exits and
  73 another thread is created.
  74 \end{funcdesc}
  75
  76
  77 Lock objects have the following methods:
  78
  79 \begin{methoddesc}[lock]{acquire}{\optional{waitflag}}
  80 Without the optional argument, this method acquires the lock
  81 unconditionally, if necessary waiting until it is released by another
  82 thread (only one thread at a time can acquire a lock --- that's their
  83 reason for existence), and returns \code{None}.  If the integer
  84 \var{waitflag} argument is present, the action depends on its
  85 value: if it is zero, the lock is only acquired if it can be acquired
  86 immediately without waiting, while if it is nonzero, the lock is
  87 acquired unconditionally as before.  If an argument is present, the
  88 return value is \code{True} if the lock is acquired successfully,
  89 \code{False} if not.
  90 \end{methoddesc}
  91
  92 \begin{methoddesc}[lock]{release}{}
  93 Releases the lock.  The lock must have been acquired earlier, but not
  94 necessarily by the same thread.
  95 \end{methoddesc}
  96
  97 \begin{methoddesc}[lock]{locked}{}
  98 Return the status of the lock:\ \code{True} if it has been acquired by
  99 some thread, \code{False} if not.
 100 \end{methoddesc}
 101
 102 \strong{Caveats:}
 103
 104 \begin{itemize}
 105 \item
 106 Threads interact strangely with interrupts: the
 107 \exception{KeyboardInterrupt} exception will be received by an
 108 arbitrary thread.  (When the \refmodule{signal}\refbimodindex{signal}
 109 module is available, interrupts always go to the main thread.)
 110
 111 \item
 112 Calling \function{sys.exit()} or raising the \exception{SystemExit}
 113 exception is equivalent to calling \function{exit()}.
 114
 115 \item
 116 Not all built-in functions that may block waiting for I/O allow other
 117 threads to run.  (The most popular ones (\function{time.sleep()},
 118 \method{\var{file}.read()}, \function{select.select()}) work as
 119 expected.)
 120
 121 \item
 122 It is not possible to interrupt the \method{acquire()} method on a lock
 123 --- the \exception{KeyboardInterrupt} exception will happen after the
 124 lock has been acquired.
 125
 126 \item
 127 When the main thread exits, it is system defined whether the other
 128 threads survive.  On SGI IRIX using the native thread implementation,
 129 they survive.  On most other systems, they are killed without
 130 executing \keyword{try} ... \keyword{finally} clauses or executing
 131 object destructors.
 132 \indexii{threads}{IRIX}
 133
 134 \item
 135 When the main thread exits, it does not do any of its usual cleanup
 136 (except that \keyword{try} ... \keyword{finally} clauses are honored),
 137 and the standard I/O files are not flushed.
 138
 139 \end{itemize}