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| <div class="titlepage"><div><div><h4 class="title"> |
| <a name="boost_asio.overview.core.allocation"></a><a class="link" href="allocation.html" title="Custom Memory Allocation">Custom Memory |
| Allocation</a> |
| </h4></div></div></div> |
| <p> |
| Many asynchronous operations need to allocate an object to store state |
| associated with the operation. For example, a Win32 implementation needs |
| <code class="computeroutput"><span class="identifier">OVERLAPPED</span></code>-derived objects |
| to pass to Win32 API functions. |
| </p> |
| <p> |
| Furthermore, programs typically contain easily identifiable chains of asynchronous |
| operations. A half duplex protocol implementation (e.g. an HTTP server) |
| would have a single chain of operations per client (receives followed by |
| sends). A full duplex protocol implementation would have two chains executing |
| in parallel. Programs should be able to leverage this knowledge to reuse |
| memory for all asynchronous operations in a chain. |
| </p> |
| <p> |
| Given a copy of a user-defined <code class="computeroutput"><span class="identifier">Handler</span></code> |
| object <code class="computeroutput"><span class="identifier">h</span></code>, if the implementation |
| needs to allocate memory associated with that handler it will execute the |
| code: |
| </p> |
| <pre class="programlisting"><span class="keyword">void</span><span class="special">*</span> <span class="identifier">pointer</span> <span class="special">=</span> <span class="identifier">asio_handler_allocate</span><span class="special">(</span><span class="identifier">size</span><span class="special">,</span> <span class="special">&</span><span class="identifier">h</span><span class="special">);</span> |
| </pre> |
| <p> |
| Similarly, to deallocate the memory it will execute: |
| </p> |
| <pre class="programlisting"><span class="identifier">asio_handler_deallocate</span><span class="special">(</span><span class="identifier">pointer</span><span class="special">,</span> <span class="identifier">size</span><span class="special">,</span> <span class="special">&</span><span class="identifier">h</span><span class="special">);</span> |
| </pre> |
| <p> |
| These functions are located using argument-dependent lookup. The implementation |
| provides default implementations of the above functions in the <code class="computeroutput"><span class="identifier">asio</span></code> namespace: |
| </p> |
| <pre class="programlisting"><span class="keyword">void</span><span class="special">*</span> <span class="identifier">asio_handler_allocate</span><span class="special">(</span><span class="identifier">size_t</span><span class="special">,</span> <span class="special">...);</span> |
| <span class="keyword">void</span> <span class="identifier">asio_handler_deallocate</span><span class="special">(</span><span class="keyword">void</span><span class="special">*,</span> <span class="identifier">size_t</span><span class="special">,</span> <span class="special">...);</span> |
| </pre> |
| <p> |
| which are implemented in terms of <code class="computeroutput"><span class="special">::</span><span class="keyword">operator</span> <span class="keyword">new</span><span class="special">()</span></code> and <code class="computeroutput"><span class="special">::</span><span class="keyword">operator</span> <span class="keyword">delete</span><span class="special">()</span></code> respectively. |
| </p> |
| <p> |
| The implementation guarantees that the deallocation will occur before the |
| associated handler is invoked, which means the memory is ready to be reused |
| for any new asynchronous operations started by the handler. |
| </p> |
| <p> |
| The custom memory allocation functions may be called from any user-created |
| thread that is calling a library function. The implementation guarantees |
| that, for the asynchronous operations included the library, the implementation |
| will not make concurrent calls to the memory allocation functions for that |
| handler. The implementation will insert appropriate memory barriers to |
| ensure correct memory visibility should allocation functions need to be |
| called from different threads. |
| </p> |
| <p> |
| Custom memory allocation support is currently implemented for all asynchronous |
| operations with the following exceptions: |
| </p> |
| <div class="itemizedlist"><ul class="itemizedlist" type="disc"><li class="listitem"> |
| Asynchronous SSL operations. |
| </li></ul></div> |
| <a name="boost_asio.overview.core.allocation.see_also"></a><h6> |
| <a name="id767080"></a> |
| <a class="link" href="allocation.html#boost_asio.overview.core.allocation.see_also">See Also</a> |
| </h6> |
| <p> |
| <a class="link" href="../../reference/asio_handler_allocate.html" title="asio_handler_allocate">asio_handler_allocate</a>, |
| <a class="link" href="../../reference/asio_handler_deallocate.html" title="asio_handler_deallocate">asio_handler_deallocate</a>, |
| <a class="link" href="../../examples.html#boost_asio.examples.allocation">custom memory allocation |
| example</a>. |
| </p> |
| </div> |
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| <td align="left"></td> |
| <td align="right"><div class="copyright-footer">Copyright © 2003 - 2010 Christopher M. Kohlhoff<p> |
| Distributed under the Boost Software License, Version 1.0. (See accompanying |
| file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>) |
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