Doc/lib/libthread.tex

   1 \section{Built-in Module \module{thread}}
   2 \label{module-thread}
   3 \bimodindex{thread}
   4
   5 This module provides low-level primitives for working with multiple
   6 threads (a.k.a.\ \dfn{light-weight processes} or \dfn{tasks}) --- multiple
   7 threads of control sharing their global data space.  For
   8 synchronization, simple locks (a.k.a.\ \dfn{mutexes} or \dfn{binary
   9 semaphores}) are provided.
  10 \index{light-weight processes}
  11 \index{processes, light-weight}
  12 \index{binary semaphores}
  13 \index{semaphores, binary}
  14
  15 The module is optional.  It is supported on Windows NT and '95, SGI
  16 IRIX, Solaris 2.x, as well as on systems that have a \POSIX{} thread
  17 (a.k.a. ``pthread'') implementation.
  18 \index{pthreads}
  19 \indexii{threads}{\POSIX{}}
  20
  21 It defines the following constant and functions:
  22
  23 \begin{excdesc}{error}
  24 Raised on thread-specific errors.
  25 \end{excdesc}
  26
  27 \begin{funcdesc}{start_new_thread}{func, arg}
  28 Start a new thread.  The thread executes the function \var{func}
  29 with the argument list \var{arg} (which must be a tuple).  When the
  30 function returns, the thread silently exits.  When the function
  31 terminates with an unhandled exception, a stack trace is printed and
  32 then the thread exits (but other threads continue to run).
  33 \end{funcdesc}
  34
  35 \begin{funcdesc}{exit}{}
  36 This is a shorthand for \function{exit_thread()}.
  37 \end{funcdesc}
  38
  39 \begin{funcdesc}{exit_thread}{}
  40 Raise the \exception{SystemExit} exception.  When not caught, this
  41 will cause the thread to exit silently.
  42 \end{funcdesc}
  43
  44 %\begin{funcdesc}{exit_prog}{status}
  45 %Exit all threads and report the value of the integer argument
  46 %\var{status} as the exit status of the entire program.
  47 %\strong{Caveat:} code in pending \code{finally} clauses, in this thread
  48 %or in other threads, is not executed.
  49 %\end{funcdesc}
  50
  51 \begin{funcdesc}{allocate_lock}{}
  52 Return a new lock object.  Methods of locks are described below.  The
  53 lock is initially unlocked.
  54 \end{funcdesc}
  55
  56 \begin{funcdesc}{get_ident}{}
  57 Return the `thread identifier' of the current thread.  This is a
  58 nonzero integer.  Its value has no direct meaning; it is intended as a
  59 magic cookie to be used e.g. to index a dictionary of thread-specific
  60 data.  Thread identifiers may be recycled when a thread exits and
  61 another thread is created.
  62 \end{funcdesc}
  63
  64
  65 Lock objects have the following methods:
  66
  67 \begin{methoddesc}[lock]{acquire}{\optional{waitflag}}
  68 Without the optional argument, this method acquires the lock
  69 unconditionally, if necessary waiting until it is released by another
  70 thread (only one thread at a time can acquire a lock --- that's their
  71 reason for existence), and returns \code{None}.  If the integer
  72 \var{waitflag} argument is present, the action depends on its value:\
  73 if it is zero, the lock is only acquired if it can be acquired
  74 immediately without waiting, while if it is nonzero, the lock is
  75 acquired unconditionally as before.  If an argument is present, the
  76 return value is \code{1} if the lock is acquired successfully,
  77 \code{0} if not.
  78 \end{methoddesc}
  79
  80 \begin{methoddesc}[lock]{release}{}
  81 Releases the lock.  The lock must have been acquired earlier, but not
  82 necessarily by the same thread.
  83 \end{methoddesc}
  84
  85 \begin{methoddesc}[lock]{locked}{}
  86 Return the status of the lock:\ \code{1} if it has been acquired by
  87 some thread, \code{0} if not.
  88 \end{methoddesc}
  89
  90 \strong{Caveats:}
  91
  92 \begin{itemize}
  93 \item
  94 Threads interact strangely with interrupts: the
  95 \exception{KeyboardInterrupt} exception will be received by an
  96 arbitrary thread.  (When the \module{signal}\refbimodindex{signal}
  97 module is available, interrupts always go to the main thread.)
  98
  99 \item
 100 Calling \function{sys.exit()} or raising the \exception{SystemExit}
 101 exception is equivalent to calling \function{exit_thread()}.
 102
 103 \item
 104 Not all built-in functions that may block waiting for I/O allow other
 105 threads to run.  (The most popular ones (\function{time.sleep()},
 106 \method{\var{file}.read()}, \function{select.select()}) work as
 107 expected.)
 108
 109 \item
 110 It is not possible to interrupt the \method{acquire()} method on a lock
 111 --- the \exception{KeyboardInterrupt} exception will happen after the
 112 lock has been acquired.
 113
 114 \item
 115 When the main thread exits, it is system defined whether the other
 116 threads survive.  On SGI IRIX using the native thread implementation,
 117 they survive.  On most other systems, they are killed without
 118 executing \keyword{try} ... \keyword{finally} clauses or executing
 119 object destructors.
 120 \indexii{threads}{IRIX}
 121
 122 \item
 123 When the main thread exits, it does not do any of its usual cleanup
 124 (except that \keyword{try} ... \keyword{finally} clauses are honored),
 125 and the standard I/O files are not flushed.
 126
 127 \end{itemize}