126 lines
4.4 KiB
TeX
126 lines
4.4 KiB
TeX
\section{Built-in Module \sectcode{thread}}
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\label{module-thread}
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\bimodindex{thread}
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This module provides low-level primitives for working with multiple
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threads (a.k.a.\ \dfn{light-weight processes} or \dfn{tasks}) --- multiple
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threads of control sharing their global data space. For
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synchronization, simple locks (a.k.a.\ \dfn{mutexes} or \dfn{binary
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semaphores}) are provided.
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\index{light-weight processes}
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\index{processes, light-weight}
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\index{binary semaphores}
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\index{semaphores, binary}
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The module is optional. It is supported on Windows NT and '95, SGI
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IRIX, Solaris 2.x, as well as on systems that have a \POSIX{} thread
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(a.k.a. ``pthread'') implementation.
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\index{pthreads}
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\indexii{threads}{\POSIX{}}
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It defines the following constant and functions:
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\setindexsubitem{(in module thread)}
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\begin{excdesc}{error}
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Raised on thread-specific errors.
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\end{excdesc}
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\begin{funcdesc}{start_new_thread}{func\, arg}
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Start a new thread. The thread executes the function \var{func}
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with the argument list \var{arg} (which must be a tuple). When the
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function returns, the thread silently exits. When the function
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terminates with an unhandled exception, a stack trace is printed and
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then the thread exits (but other threads continue to run).
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\end{funcdesc}
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\begin{funcdesc}{exit}{}
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This is a shorthand for \code{thread.exit_thread()}.
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\end{funcdesc}
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\begin{funcdesc}{exit_thread}{}
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Raise the \code{SystemExit} exception. When not caught, this will
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cause the thread to exit silently.
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\end{funcdesc}
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%\begin{funcdesc}{exit_prog}{status}
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%Exit all threads and report the value of the integer argument
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%\var{status} as the exit status of the entire program.
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%\strong{Caveat:} code in pending \code{finally} clauses, in this thread
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%or in other threads, is not executed.
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%\end{funcdesc}
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\begin{funcdesc}{allocate_lock}{}
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Return a new lock object. Methods of locks are described below. The
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lock is initially unlocked.
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\end{funcdesc}
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\begin{funcdesc}{get_ident}{}
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Return the `thread identifier' of the current thread. This is a
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nonzero integer. Its value has no direct meaning; it is intended as a
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magic cookie to be used e.g. to index a dictionary of thread-specific
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data. Thread identifiers may be recycled when a thread exits and
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another thread is created.
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\end{funcdesc}
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Lock objects have the following methods:
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\setindexsubitem{(lock method)}
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\begin{funcdesc}{acquire}{\optional{waitflag}}
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Without the optional argument, this method acquires the lock
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unconditionally, if necessary waiting until it is released by another
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thread (only one thread at a time can acquire a lock --- that's their
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reason for existence), and returns \code{None}. If the integer
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\var{waitflag} argument is present, the action depends on its value:\
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if it is zero, the lock is only acquired if it can be acquired
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immediately without waiting, while if it is nonzero, the lock is
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acquired unconditionally as before. If an argument is present, the
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return value is 1 if the lock is acquired successfully, 0 if not.
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\end{funcdesc}
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\begin{funcdesc}{release}{}
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Releases the lock. The lock must have been acquired earlier, but not
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necessarily by the same thread.
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\end{funcdesc}
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\begin{funcdesc}{locked}{}
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Return the status of the lock:\ 1 if it has been acquired by some
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thread, 0 if not.
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\end{funcdesc}
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\strong{Caveats:}
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\begin{itemize}
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\item
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Threads interact strangely with interrupts: the
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\code{KeyboardInterrupt} exception will be received by an arbitrary
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thread. (When the \code{signal}\refbimodindex{signal} module is
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available, interrupts always go to the main thread.)
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\item
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Calling \code{sys.exit()} or raising the \code{SystemExit} exception is
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equivalent to calling \code{thread.exit_thread()}.
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\item
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Not all built-in functions that may block waiting for I/O allow other
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threads to run. (The most popular ones (\code{sleep()}, \code{read()},
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\code{select()}) work as expected.)
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\item
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It is not possible to interrupt the \code{acquire()} method on a lock
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-- the \code{KeyboardInterrupt} exception will happen after the lock
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has been acquired.
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\item
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When the main thread exits, it is system defined whether the other
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threads survive. On SGI IRIX using the native thread implementation,
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they survive. On most other systems, they are killed without
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executing ``try-finally'' clauses or executing object destructors.
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\indexii{threads}{IRIX}
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\item
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When the main thread exits, it doesn't do any of its usual cleanup
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(except that ``try-finally'' clauses are honored), and the standard
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I/O files are not flushed.
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\end{itemize}
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