ref: 5b7d8e1f6fb4167e354545fef7e6dd6676986441
dir: /sys/src/cmd/python/Doc/lib/libmutex.tex/
\section{\module{mutex} ---
Mutual exclusion support}
\declaremodule{standard}{mutex}
\sectionauthor{Moshe Zadka}{moshez@zadka.site.co.il}
\modulesynopsis{Lock and queue for mutual exclusion.}
The \module{mutex} module defines a class that allows mutual-exclusion
via acquiring and releasing locks. It does not require (or imply)
threading or multi-tasking, though it could be useful for
those purposes.
The \module{mutex} module defines the following class:
\begin{classdesc}{mutex}{}
Create a new (unlocked) mutex.
A mutex has two pieces of state --- a ``locked'' bit and a queue.
When the mutex is not locked, the queue is empty.
Otherwise, the queue contains zero or more
\code{(\var{function}, \var{argument})} pairs
representing functions (or methods) waiting to acquire the lock.
When the mutex is unlocked while the queue is not empty,
the first queue entry is removed and its
\code{\var{function}(\var{argument})} pair called,
implying it now has the lock.
Of course, no multi-threading is implied -- hence the funny interface
for \method{lock()}, where a function is called once the lock is
acquired.
\end{classdesc}
\subsection{Mutex Objects \label{mutex-objects}}
\class{mutex} objects have following methods:
\begin{methoddesc}{test}{}
Check whether the mutex is locked.
\end{methoddesc}
\begin{methoddesc}{testandset}{}
``Atomic'' test-and-set, grab the lock if it is not set,
and return \code{True}, otherwise, return \code{False}.
\end{methoddesc}
\begin{methoddesc}{lock}{function, argument}
Execute \code{\var{function}(\var{argument})}, unless the mutex is locked.
In the case it is locked, place the function and argument on the queue.
See \method{unlock} for explanation of when
\code{\var{function}(\var{argument})} is executed in that case.
\end{methoddesc}
\begin{methoddesc}{unlock}{}
Unlock the mutex if queue is empty, otherwise execute the first element
in the queue.
\end{methoddesc}