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<title>Random-Access HANDLEs</title>
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<div class="titlepage"><div><div><h4 class="title">
<a name="boost_asio.overview.windows.random_access_handle"></a><a class="link" href="random_access_handle.html" title="Random-Access HANDLEs">Random-Access
HANDLEs</a>
</h4></div></div></div>
<p>
Boost.Asio provides Windows-specific classes that permit asynchronous read
and write operations to be performed on HANDLEs that refer to regular files.
</p>
<p>
For example, to perform asynchronous operations on a file the following
object may be created:
</p>
<pre class="programlisting">HANDLE handle = ::CreateFile(...);
windows::random_access_handle file(my_io_context, handle);
</pre>
<p>
Data may be read from or written to the handle using one of the <code class="computeroutput">read_some_at()</code>,
<code class="computeroutput">async_read_some_at()</code>, <code class="computeroutput">write_some_at()</code> or <code class="computeroutput">async_write_some_at()</code>
member functions. However, like the equivalent functions (<code class="computeroutput">read_some()</code>,
etc.) on streams, these functions are only required to transfer one or
more bytes in a single operation. Therefore free functions called <a class="link" href="../../reference/read_at.html" title="read_at">read_at()</a>, <a class="link" href="../../reference/async_read_at.html" title="async_read_at">async_read_at()</a>,
<a class="link" href="../../reference/write_at.html" title="write_at">write_at()</a> and <a class="link" href="../../reference/async_write_at.html" title="async_write_at">async_write_at()</a> have
been created to repeatedly call the corresponding <code class="literal"><span class="bold"><strong>*</strong></span>_some_at()</code>
function until all data has been transferred.
</p>
<h6>
<a name="boost_asio.overview.windows.random_access_handle.h0"></a>
<span class="phrase"><a name="boost_asio.overview.windows.random_access_handle.see_also"></a></span><a class="link" href="random_access_handle.html#boost_asio.overview.windows.random_access_handle.see_also">See
Also</a>
</h6>
<p>
<a class="link" href="../../reference/windows__random_access_handle.html" title="windows::random_access_handle">windows::random_access_handle</a>.
</p>
<h6>
<a name="boost_asio.overview.windows.random_access_handle.h1"></a>
<span class="phrase"><a name="boost_asio.overview.windows.random_access_handle.notes"></a></span><a class="link" href="random_access_handle.html#boost_asio.overview.windows.random_access_handle.notes">Notes</a>
</h6>
<p>
Windows random-access <code class="computeroutput">HANDLE</code>s are only available at compile
time when targeting Windows and only when the I/O completion port backend
is used (which is the default). A program may test for the macro <code class="computeroutput">BOOST_ASIO_HAS_WINDOWS_RANDOM_ACCESS_HANDLE</code>
to determine whether they are supported.
</p>
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<td align="right"><div class="copyright-footer">Copyright © 2003-2018 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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