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<title>async_read_at (4 of 4 overloads)</title>
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<div class="titlepage"><div><div><h4 class="title">
<a name="asio.reference.async_read_at.overload4"></a><a class="link" href="overload4.html" title="async_read_at (4 of 4 overloads)">async_read_at
(4 of 4 overloads)</a>
</h4></div></div></div>
<p>
Start an asynchronous operation to read a certain amount of data at the
specified offset.
</p>
<pre class="programlisting">template<
typename <a class="link" href="../AsyncRandomAccessReadDevice.html" title="Buffer-oriented asynchronous random-access read device requirements">AsyncRandomAccessReadDevice</a>,
typename Allocator,
typename <a class="link" href="../CompletionCondition.html" title="Completion condition requirements">CompletionCondition</a>,
typename <a class="link" href="../ReadHandler.html" title="Read handler requirements">ReadHandler</a> = <a class="link" href="../asynchronous_operations.html#asio.reference.asynchronous_operations.default_completion_tokens"><span class="emphasis"><em>DEFAULT</em></span></a>>
<a class="link" href="../asynchronous_operations.html#asio.reference.asynchronous_operations.automatic_deduction_of_initiating_function_return_type"><span class="emphasis"><em>DEDUCED</em></span></a> async_read_at(
AsyncRandomAccessReadDevice & d,
uint64_t offset,
basic_streambuf< Allocator > & b,
CompletionCondition completion_condition,
ReadHandler && handler = <a class="link" href="../asynchronous_operations.html#asio.reference.asynchronous_operations.default_completion_tokens"><span class="emphasis"><em>DEFAULT</em></span></a>);
</pre>
<p>
This function is used to asynchronously read a certain number of bytes
of data from a random access device at the specified offset. The function
call always returns immediately. The asynchronous operation will continue
until one of the following conditions is true:
</p>
<div class="itemizedlist"><ul class="itemizedlist" type="disc"><li class="listitem">
The completion_condition function object returns 0.
</li></ul></div>
<p>
This operation is implemented in terms of zero or more calls to the device's
async_read_some_at function.
</p>
<h6>
<a name="asio.reference.async_read_at.overload4.h0"></a>
<span><a name="asio.reference.async_read_at.overload4.parameters"></a></span><a class="link" href="overload4.html#asio.reference.async_read_at.overload4.parameters">Parameters</a>
</h6>
<div class="variablelist">
<p class="title"><b></b></p>
<dl>
<dt><span class="term">d</span></dt>
<dd><p>
The device from which the data is to be read. The type must support
the AsyncRandomAccessReadDevice concept.
</p></dd>
<dt><span class="term">offset</span></dt>
<dd><p>
The offset at which the data will be read.
</p></dd>
<dt><span class="term">b</span></dt>
<dd><p>
A <a class="link" href="../basic_streambuf.html" title="basic_streambuf"><code class="computeroutput">basic_streambuf</code></a>
object into which the data will be read. Ownership of the streambuf
is retained by the caller, which must guarantee that it remains valid
until the handler is called.
</p></dd>
<dt><span class="term">completion_condition</span></dt>
<dd>
<p>
The function object to be called to determine whether the read operation
is complete. The signature of the function object must be:
</p>
<pre class="programlisting">std::size_t completion_condition(
// Result of latest async_read_some_at operation.
const asio::error_code& error,
// Number of bytes transferred so far.
std::size_t bytes_transferred
);
</pre>
<p>
A return value of 0 indicates that the read operation is complete.
A non-zero return value indicates the maximum number of bytes to
be read on the next call to the device's async_read_some_at function.
</p>
</dd>
<dt><span class="term">handler</span></dt>
<dd>
<p>
The handler to be called when the read operation completes. Copies
will be made of the handler as required. The function signature of
the handler must be:
</p>
<pre class="programlisting">void handler(
// Result of operation.
const asio::error_code& error,
// Number of bytes copied into the buffers. If an error
// occurred, this will be the number of bytes successfully
// transferred prior to the error.
std::size_t bytes_transferred
);
</pre>
<p>
Regardless of whether the asynchronous operation completes immediately
or not, the handler will not be invoked from within this function.
On immediate completion, invocation of the handler will be performed
in a manner equivalent to using <a class="link" href="../post.html" title="post"><code class="computeroutput">post</code></a>.
</p>
</dd>
</dl>
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<td align="right"><div class="copyright-footer">Copyright © 2003-2020 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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