Doc/lib/libthread.tex

   1 \section{\module{thread} ---
   2          Multiple threads of control.}
   3 \declaremodule{builtin}{thread}
   4
   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.  The thread executes the function \var{function}
  36 with the argument list \var{args} (which must be a tuple).  The
  37 optional \var{kwargs} argument specifies a dictionary of keyword
  38 arguments.  When the
  39 function returns, the thread silently exits.  When the function
  40 terminates with an unhandled exception, a stack trace is printed and
  41 then the thread exits (but other threads continue to run).
  42 \end{funcdesc}
  43
  44 \begin{funcdesc}{exit}{}
  45 Raise the \exception{SystemExit} exception.  When not caught, this
  46 will cause the thread to exit silently.
  47 \end{funcdesc}
  48
  49 \begin{funcdesc}{exit_thread}{}
  50 \deprecated{1.5.2}{Use \function{exit()}.}
  51 This is an obsolete synonym for \function{exit()}.
  52 \end{funcdesc}
  53
  54 %\begin{funcdesc}{exit_prog}{status}
  55 %Exit all threads and report the value of the integer argument
  56 %\var{status} as the exit status of the entire program.
  57 %\strong{Caveat:} code in pending \code{finally} clauses, in this thread
  58 %or in other threads, is not executed.
  59 %\end{funcdesc}
  60
  61 \begin{funcdesc}{allocate_lock}{}
  62 Return a new lock object.  Methods of locks are described below.  The
  63 lock is initially unlocked.
  64 \end{funcdesc}
  65
  66 \begin{funcdesc}{get_ident}{}
  67 Return the `thread identifier' of the current thread.  This is a
  68 nonzero integer.  Its value has no direct meaning; it is intended as a
  69 magic cookie to be used e.g. to index a dictionary of thread-specific
  70 data.  Thread identifiers may be recycled when a thread exits and
  71 another thread is created.
  72 \end{funcdesc}
  73
  74
  75 Lock objects have the following methods:
  76
  77 \begin{methoddesc}[lock]{acquire}{\optional{waitflag}}
  78 Without the optional argument, this method acquires the lock
  79 unconditionally, if necessary waiting until it is released by another
  80 thread (only one thread at a time can acquire a lock --- that's their
  81 reason for existence), and returns \code{None}.  If the integer
  82 \var{waitflag} argument is present, the action depends on its
  83 value: if it is zero, the lock is only acquired if it can be acquired
  84 immediately without waiting, while if it is nonzero, the lock is
  85 acquired unconditionally as before.  If an argument is present, the
  86 return value is \code{1} if the lock is acquired successfully,
  87 \code{0} if not.
  88 \end{methoddesc}
  89
  90 \begin{methoddesc}[lock]{release}{}
  91 Releases the lock.  The lock must have been acquired earlier, but not
  92 necessarily by the same thread.
  93 \end{methoddesc}
  94
  95 \begin{methoddesc}[lock]{locked}{}
  96 Return the status of the lock:\ \code{1} if it has been acquired by
  97 some thread, \code{0} if not.
  98 \end{methoddesc}
  99
 100 \strong{Caveats:}
 101
 102 \begin{itemize}
 103 \item
 104 Threads interact strangely with interrupts: the
 105 \exception{KeyboardInterrupt} exception will be received by an
 106 arbitrary thread.  (When the \module{signal}\refbimodindex{signal}
 107 module is available, interrupts always go to the main thread.)
 108
 109 \item
 110 Calling \function{sys.exit()} or raising the \exception{SystemExit}
 111 exception is equivalent to calling \function{exit_thread()}.
 112
 113 \item
 114 Not all built-in functions that may block waiting for I/O allow other
 115 threads to run.  (The most popular ones (\function{time.sleep()},
 116 \method{\var{file}.read()}, \function{select.select()}) work as
 117 expected.)
 118
 119 \item
 120 It is not possible to interrupt the \method{acquire()} method on a lock
 121 --- the \exception{KeyboardInterrupt} exception will happen after the
 122 lock has been acquired.
 123
 124 \item
 125 When the main thread exits, it is system defined whether the other
 126 threads survive.  On SGI IRIX using the native thread implementation,
 127 they survive.  On most other systems, they are killed without
 128 executing \keyword{try} ... \keyword{finally} clauses or executing
 129 object destructors.
 130 \indexii{threads}{IRIX}
 131
 132 \item
 133 When the main thread exits, it does not do any of its usual cleanup
 134 (except that \keyword{try} ... \keyword{finally} clauses are honored),
 135 and the standard I/O files are not flushed.
 136
 137 \end{itemize}