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<html><!-- #BeginTemplate "/Templates/tmpl.dwt" --><head><!-- #BeginEditable "doctitle" --> <title>PTypes: multithreading: mutex</title><!-- #EndEditable --> <meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1"><link rel="stylesheet" href="styles.css"></head><body bgcolor="#FFFFFF" leftmargin="40" marginwidth="40"><p><a href="../index.html"><img src="title-1.7.gif" width="213" height="34" alt="C++ Portable Types Library (PTypes) Version 1.7" border="0"></a> <hr noshade><!-- #BeginEditable "body" --> <p class="hpath"><a href="index.html">Top</a>: <a href="async.html">Multithreading</a>: mutex </p><blockquote> <pre class="lang">#include <pasync.h>mutex::mutex();void mutex::enter();void mutex::leave();void mutex::lock(); <span class="comment">// alias for enter()</span>void mutex::unlock(); <span class="comment">// alias for leave()</span></pre></blockquote><p><span class="lang">Mutex</span> (mutual exclusion) is another synchronization object which helps to protect shared data structures from being concurrently accessed and modified.</p><p>Accessing and changing simple variables like <span class="lang">int</span> concurrently can be considered safe provided that the variable is aligned on a boundary "native" to the given CPU (32 bits on most systems). More often, however, applications use more complex shared data structures which can not be modified and accessed at the same time. Otherwise, the logical integrity of the structure might be corrupt from the "reader's" point of view when some other process sequentially modifies the fields of a shared structure.</p><p>To logically lock the part of code which modifies such complex structures the thread creates a mutex object and embraces the critical code with calls to <span class="lang">enter()</span> and <span class="lang">leave()</span>. Reading threads should also mark their transactions with <span class="lang">enter()</span> and <span class="lang">leave()</span>. When either a reader or a writer enters the critical section, any attempt to enter the same section concurrently causes the thread to "hang" until the first thread leaves the critical section.</p><p>If more than two threads are trying to lock the same critical section, <span class="lang">mutex</span> builds a queue for them and allows threads to enter the section only one by one. The order of entering the section is platform dependent.</p><p>To avoid infinite locks on a mutex object, applications usually put the critical section into <span class="lang">try {}</span> block and call <span class="lang">leave()</span> from within <span class="lang">catch {}</span> in case an exception is raised during the transaction.</p><p>More often applications use a smarter mutual exclusion object called read/write lock -- <a href="async.rwlock.html">rwlock</a>. In some situations it is also possible to define the order of modification of logically dependent data such that synchronization is not necessary at all. This technique should be used with great care.</p><p>PTypes' <span class="lang">mutex</span> object encapsulates either Windows CRITICAL_SECTION structure or POSIX mutex object and implements the minimal set of features common to both platforms. <span class="lang">Mutex</span> can not be shared between processes. <b>Note</b>: mutex may not be reentrant on POSIX systems, i.e. a recursive lock from one thread may cause deadlock.</p><p><span class="def">mutex::mutex()</span> creates a mutex object.</p><p><span class="def">void mutex::enter()</span> marks the start of an indivisible transaction.</p><p><span class="def">void mutex::leave()</span> marks the end of an indivisible transaction.</p><p><span class="def">void mutex::lock()</span> is an alias for <span class="lang">enter()</span>.</p><p><span class="def">void mutex::unlock()</span> is an alias for <span class="lang">leave()</span>.</p><p class="seealso">See also: <a href="async.thread.html">thread</a>, <a href="async.rwlock.html">rwlock</a>, <a href="async.trigger.html">trigger</a>, <a href="async.semaphore.html">semaphore</a>, <a href="async.examples.html">Examples</a></p><!-- #EndEditable --><hr size="1"><a href="../index.html" class="ns">PTypes home</a></body><!-- #EndTemplate --></html>⌨️ 快捷键说明
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