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90 lines
3.2 KiB
TeX
90 lines
3.2 KiB
TeX
\section{Built-in Module \sectcode{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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The module is optional and supported on SGI and Sun Sparc systems only.
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It defines the following constant and functions:
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\renewcommand{\indexsubitem}{(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 raises
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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_thread}{}
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Exit the current thread silently. Other threads continue to run.
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\strong{Caveat:} code in pending \code{finally} clauses is not executed.
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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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Lock objects have the following methods:
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\renewcommand{\indexsubitem}{(lock method)}
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\begin{funcdesc}{acquire}{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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{\bf 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.
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\item
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Calling \code{sys.exit(\var{status})} or executing
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\code{raise SystemExit, \var{status}} is almost equivalent to calling
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\code{thread.exit_prog(\var{status})}, except that the former ways of
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exiting the entire program do honor \code{finally} clauses in the
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current thread (but not in other threads).
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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, although the most popular ones (\code{sleep},
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\code{read}, \code{select}) work as expected.
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\end{itemize}
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