<html>
<head>
<title>PTypes: multithreading: rwlock</title>
<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-21.png" width="253" height="39" alt="C++ Portable Types Library (PTypes) Version 2.1" border="0"></a>
<hr size="1" noshade>
<p class="hpath"><a href="index.html">Top</a>: <a href="async.html">Multithreading</a>:
rwlock</p>
<blockquote>
<pre class="lang">#include <pasync.h>
class rwlock {
rwlock();
void rdlock();
void wrlock();
void unlock();
}
class scoperead {
scoperead(rwlock&);
~scoperead();
}
class scopewrite {
scopewrite(rwlock&);
~scopewrite();
}</pre>
</blockquote>
<p><span class="lang">Rwlock</span> (read/write lock) is a variation of <span class="lang">mutex</span>
with a possibility to control critical sections more efficiently. Unlike the simple
mutex, <span class="lang">rwlock</span> allows multiple <i>reader</i> threads
to enter the critical section, and only one <i>writer</i> thread at a time. Reading
and writing in this context means access to a resource or compound data shared
between threads. Reader threads must lock the critical section with <span class="lang">rdlock()</span>
and the writers must lock it with <span class="lang">wrlock()</span>. Both leave
the critical section with <span class="lang">unlock()</span>.</p>
<p>This class incorporates POSIX rwlock interface on UNIX and library's own implementation
on Windows and MacOS X. When using the <span class="lang">rwlock</span> class
on Linux, you need to define a symbol <span class="lang">_GNU_SOURCE</span> either
in the command line, or anywhere in your source before including any system headers.</p>
<p>Analogously to <span class="lang">scopelock</span> (see <a href="async.mutex.html">mutex</a>),
<span class="lang">scoperead</span> and <span class="lang">scopewrite</span> are
fully-inline'd utility classes provided for exception-safe locking of operator
blocks.</p>
<p>Please, see <a href="portability.html">Portability and performance issues</a>
for additional notes on rwlock implementation. </p>
<p><span class="def">rwlock::rwlock()</span> creates a <span class="lang">rwlock</span>
object.</p>
<p><span class="def">void rwlock::rdlock()</span> locks the object for reading.
Multiple threads can enter the critical section through <span class="lang">rdlock()</span>,
however, if the object is already locked for writing, all reader threads wait
until the writer leaves the critical section.</p>
<p><span class="def">void rwlock::wrlock()</span> locks the object for writing.
If there are readers inside the critical section, the writer waits until all threads
leave and unlock the object. Only one writer at a time can enter the critical
section.</p>
<p><span class="def">void rwlock::unlock()</span> unlocks the object. Both readers
and writers must use this method when leaving the critical section.</p>
<p><span class="def">scoperead::scoperead(rwlock& rw)</span> creates a <span class="lang">scoperead</span>
object and calls <span class="lang">rdlock()</span> for the object <span class="lang">rw</span>.</p>
<p><span class="def">scoperead::~scoperead()</span> calls <span class="lang">unlock()</span>
for the <span class="lang">rwlock</span> object specified during construction
and destroys the <span class="lang">scoperead</span> object.</p>
<p><span class="def">scopewrite::scopewrite(rwlock& rw)</span> creates a <span class="def">scopewrite</span>
object and calls <span class="lang">wrlock()</span> for the object <span class="lang">rw</span>.</p>
<p><span class="def">scopewrite::~scopewrite()</span> calls <span class="lang">unlock()</span>
for the <span class="lang">rwlock</span> object specified during construction
and destroys the <span class="def">scopewrite</span> object.</p>
<p class="seealso">See also: <a href="async.thread.html">thread</a>, <a href="async.mutex.html">mutex</a>,
<a href="async.trigger.html">trigger</a>, <a href="async.semaphore.html">semaphore</a>,
<a href="async.examples.html">Examples</a></p>
<hr size="1">
<a href="../index.html" class="ns">PTypes home</a>
</body>
</html>
