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<title>async_read (6 of 8 overloads)</title>
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<div class="titlepage"><div><div><h4 class="title">
<a name="boost_asio.reference.async_read.overload6"></a><a class="link" href="overload6.html" title="async_read (6 of 8 overloads)">async_read
(6 of 8 overloads)</a>
</h4></div></div></div>
<p>
Start an asynchronous operation to read a certain amount of data from a
stream.
</p>
<pre class="programlisting">template<
typename <a class="link" href="../AsyncReadStream.html" title="Buffer-oriented asynchronous read stream requirements">AsyncReadStream</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#boost_asio.reference.asynchronous_operations.automatic_deduction_of_initiating_function_return_type"><span class="emphasis"><em>DEDUCED</em></span></a> async_read(
AsyncReadStream & s,
basic_streambuf< Allocator > & b,
CompletionCondition completion_condition,
ReadHandler && handler);
</pre>
<p>
This function is used to asynchronously read a certain number of bytes
of data from a stream. 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" style="list-style-type: disc; ">
<li class="listitem">
The supplied buffer is full (that is, it has reached maximum size).
</li>
<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 stream's
async_read_some function, and is known as a <span class="emphasis"><em>composed operation</em></span>.
The program must ensure that the stream performs no other read operations
(such as async_read, the stream's async_read_some function, or any other
composed operations that perform reads) until this operation completes.
</p>
<h6>
<a name="boost_asio.reference.async_read.overload6.h0"></a>
<span class="phrase"><a name="boost_asio.reference.async_read.overload6.parameters"></a></span><a class="link" href="overload6.html#boost_asio.reference.async_read.overload6.parameters">Parameters</a>
</h6>
<div class="variablelist">
<p class="title"><b></b></p>
<dl class="variablelist">
<dt><span class="term">s</span></dt>
<dd><p>
The stream from which the data is to be read. The type must support
the AsyncReadStream concept.
</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 operation.
const boost::system::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 stream's async_read_some 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(
const boost::system::error_code& error, // Result of operation.
std::size_t bytes_transferred // Number of bytes copied into the
// buffers. If an error occurred,
// this will be the number of
// bytes successfully transferred
// prior to the error.
);
</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 © 2003-2019 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>)
</p>
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