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<title>async_read (1 of 8 overloads)</title>
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<div class="titlepage"><div><div><h4 class="title">
<a name="asio.reference.async_read.overload1"></a><a class="link" href="overload1.html" title="async_read (1 of 8 overloads)">async_read (1
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 <a class="link" href="../MutableBufferSequence.html" title="Mutable buffer sequence requirements">MutableBufferSequence</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(
AsyncReadStream & s,
const MutableBufferSequence & buffers,
ReadHandler && handler = <a class="link" href="../asynchronous_operations.html#asio.reference.asynchronous_operations.default_completion_tokens"><span class="emphasis"><em>DEFAULT</em></span></a>,
typename enable_if< is_mutable_buffer_sequence< MutableBufferSequence >::value >::type * = 0);
</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" type="disc">
<li class="listitem">
The supplied buffers are full. That is, the bytes transferred is equal
to the sum of the buffer sizes.
</li>
<li class="listitem">
An error occurred.
</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="asio.reference.async_read.overload1.h0"></a>
<span><a name="asio.reference.async_read.overload1.parameters"></a></span><a class="link" href="overload1.html#asio.reference.async_read.overload1.parameters">Parameters</a>
</h6>
<div class="variablelist">
<p class="title"><b></b></p>
<dl>
<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">buffers</span></dt>
<dd><p>
One or more buffers into which the data will be read. The sum of
the buffer sizes indicates the maximum number of bytes to read from
the stream. Although the buffers object may be copied as necessary,
ownership of the underlying memory blocks is retained by the caller,
which must guarantee that they remain valid until the handler is
called.
</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 asio::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>
</div>
<h6>
<a name="asio.reference.async_read.overload1.h1"></a>
<span><a name="asio.reference.async_read.overload1.example"></a></span><a class="link" href="overload1.html#asio.reference.async_read.overload1.example">Example</a>
</h6>
<p>
To read into a single data buffer use the <a class="link" href="../buffer.html" title="buffer"><code class="computeroutput">buffer</code></a>
function as follows:
</p>
<pre class="programlisting">asio::async_read(s, asio::buffer(data, size), handler);
</pre>
<p>
See the <a class="link" href="../buffer.html" title="buffer"><code class="computeroutput">buffer</code></a>
documentation for information on reading into multiple buffers in one go,
and how to use it with arrays, boost::array or std::vector.
</p>
<h6>
<a name="asio.reference.async_read.overload1.h2"></a>
<span><a name="asio.reference.async_read.overload1.remarks"></a></span><a class="link" href="overload1.html#asio.reference.async_read.overload1.remarks">Remarks</a>
</h6>
<p>
This overload is equivalent to calling:
</p>
<pre class="programlisting">asio::async_read(
s, buffers,
asio::transfer_all(),
handler);
</pre>
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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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