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<title>PTypes: multithreading: utils</title>
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<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>
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<p class="hpath"><a href="index.html">Top</a>: <a href="async.html">Multithreading</a>:
Atomic functions, utilities</p>
<blockquote>
<pre class="lang">#include <pasync.h>
int pexchange(int* target, int value);
void* pexchange(void** target, void* value);
int pincrement(int* target);
int pdecrement(int* target);
template <class T> T* tpexchange(T** target, T* value);
void psleep(unsigned milliseconds);
pthread_id_t pthrself();
bool pthrequal(pthread_id_t id);
</pre>
</blockquote>
<p>The atomic functions <span class="lang">pexchange()</span>, <span class="lang">pincrement()</span>
and <span class="lang">pdecrement()</span> can be used in place of mutex locking
in some simple situations. A typical usage of <span class="lang">pexchange()</span>
in a multithreaded environment could be, for example, freeing a dynamic object
and assigning NULL to the pointer <b>atomically</b> to prevent concurrent threads
from freeing the same object more than once.</p>
<p>It is sometimes necessary to increment or decrement some shared counter and
atomically compare it with some value. For example, you have a shared resource
and you keep track of its usage by maintaining a reference counter. When this
counter reaches 0 you want to free the shared resource. To avoid conflicts between
concurrent threads you need to decrement the counter and atomically compare it
with 0. In this situation you can use <span class="lang">pdecrement()</span> instead
of time-consuming mutex locking. (For additional notes, see <a href="portability.html">Portability
and performance issues</a>.)</p>
<p><span class="def">int pexchange(int* target, int value)</span> -- atomically
exchanges two int values.</p>
<p><span class="def">void* pexchange(void** target, void* value)</span> -- atomically
exchanges two pointers.</p>
<p><span class="def">int pincrement(int* target)</span> -- atomically increments
the value of <span class="lang">*target</span> and returns the new value.</p>
<p><span class="def">int pdecrement(int* target)</span> -- atomically decrements
the value of <span class="lang">*target</span> and returns the new value.</p>
<p><span class="def">template <class T> T* tpexchange(T** target, T* value)</span>
-- is equivalent to <span class="lang">pexchange()</span> that adds compile-time
type checking. Use this template to check the correspondence of pointer types
of <span class="lang">target</span> and the returning value.</p>
<p><span class="def">void psleep(unsigned milliseconds)</span> -- suspends execution
for the specified amount of time in milliseconds.</p>
<p><span class="def">pthread_id_t pthrself()</span> -- returns the thread ID of
the calling thread.</p>
<p><span class="def">bool pthrequal(pthread_id_t id)</span> -- checks whether
the calling thread has the given thread <span class="lang">id</span>.</p>
<p class="seealso">See also: <a href="async.semaphore.html">semaphore</a>, <a href="async.examples.html">mutex</a>,
<a href="async.thread.html">thread</a></p>
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