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 NT and '95, SGI
  19 IRIX, Solaris 2.x, as well as on systems that have a \POSIX{} thread
  20 (a.k.a. ``pthread'') implementation.
  21 \index{pthreads}
  22 \indexii{threads}{\POSIX}
  23
  24 It defines the following constant and functions:
  25
  26 \begin{excdesc}{error}
  27 Raised on thread-specific errors.
  28 \end{excdesc}
  29
  30 \begin{datadesc}{LockType}
  31 This is the type of lock objects.
  32 \end{datadesc}
  33
  34 \begin{funcdesc}{start_new_thread}{function, args\optional{, kwargs}}
  35 Start a new thread and return its identifier.  The thread executes the function
  36 \var{function} with the argument list \var{args} (which must be a tuple).  The
  37 optional \var{kwargs} argument specifies a dictionary of keyword arguments.
  38 When the function returns, the thread silently exits.  When the function
  39 terminates with an unhandled exception, a stack trace is printed and
  40 then the thread exits (but other threads continue to run).
  41 \end{funcdesc}
  42
  43 \begin{funcdesc}{exit}{}
  44 Raise the \exception{SystemExit} exception.  When not caught, this
  45 will cause the thread to exit silently.
  46 \end{funcdesc}
  47
  48 \begin{funcdesc}{exit_thread}{}
  49 \deprecated{1.5.2}{Use \function{exit()}.}
  50 This is an obsolete synonym for \function{exit()}.
  51 \end{funcdesc}
  52
  53 %\begin{funcdesc}{exit_prog}{status}
  54 %Exit all threads and report the value of the integer argument
  55 %\var{status} as the exit status of the entire program.
  56 %\strong{Caveat:} code in pending \keyword{finally} clauses, in this thread
  57 %or in other threads, is not executed.
  58 %\end{funcdesc}
  59
  60 \begin{funcdesc}{allocate_lock}{}
  61 Return a new lock object.  Methods of locks are described below.  The
  62 lock is initially unlocked.
  63 \end{funcdesc}
  64
  65 \begin{funcdesc}{get_ident}{}
  66 Return the `thread identifier' of the current thread.  This is a
  67 nonzero integer.  Its value has no direct meaning; it is intended as a
  68 magic cookie to be used e.g. to index a dictionary of thread-specific
  69 data.  Thread identifiers may be recycled when a thread exits and
  70 another thread is created.
  71 \end{funcdesc}
  72
  73
  74 Lock objects have the following methods:
  75
  76 \begin{methoddesc}[lock]{acquire}{\optional{waitflag}}
  77 Without the optional argument, this method acquires the lock
  78 unconditionally, if necessary waiting until it is released by another
  79 thread (only one thread at a time can acquire a lock --- that's their
  80 reason for existence), and returns \code{None}.  If the integer
  81 \var{waitflag} argument is present, the action depends on its
  82 value: if it is zero, the lock is only acquired if it can be acquired
  83 immediately without waiting, while if it is nonzero, the lock is
  84 acquired unconditionally as before.  If an argument is present, the
  85 return value is \code{True} if the lock is acquired successfully,
  86 \code{False} if not.
  87 \end{methoddesc}
  88
  89 \begin{methoddesc}[lock]{release}{}
  90 Releases the lock.  The lock must have been acquired earlier, but not
  91 necessarily by the same thread.
  92 \end{methoddesc}
  93
  94 \begin{methoddesc}[lock]{locked}{}
  95 Return the status of the lock:\ \code{True} if it has been acquired by
  96 some thread, \code{False} if not.
  97 \end{methoddesc}
  98
  99 \strong{Caveats:}
 100
 101 \begin{itemize}
 102 \item
 103 Threads interact strangely with interrupts: the
 104 \exception{KeyboardInterrupt} exception will be received by an
 105 arbitrary thread.  (When the \refmodule{signal}\refbimodindex{signal}
 106 module is available, interrupts always go to the main thread.)
 107
 108 \item
 109 Calling \function{sys.exit()} or raising the \exception{SystemExit}
 110 exception is equivalent to calling \function{exit()}.
 111
 112 \item
 113 Not all built-in functions that may block waiting for I/O allow other
 114 threads to run.  (The most popular ones (\function{time.sleep()},
 115 \method{\var{file}.read()}, \function{select.select()}) work as
 116 expected.)
 117
 118 \item
 119 It is not possible to interrupt the \method{acquire()} method on a lock
 120 --- the \exception{KeyboardInterrupt} exception will happen after the
 121 lock has been acquired.
 122
 123 \item
 124 When the main thread exits, it is system defined whether the other
 125 threads survive.  On SGI IRIX using the native thread implementation,
 126 they survive.  On most other systems, they are killed without
 127 executing \keyword{try} ... \keyword{finally} clauses or executing
 128 object destructors.
 129 \indexii{threads}{IRIX}
 130
 131 \item
 132 When the main thread exits, it does not do any of its usual cleanup
 133 (except that \keyword{try} ... \keyword{finally} clauses are honored),
 134 and the standard I/O files are not flushed.
 135
 136 \end{itemize}