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<title>async_read_until (2 of 12 overloads)</title>
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<div class="section">
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<div class="titlepage"><div><div><h4 class="title">
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<a name="boost_asio.reference.async_read_until.overload2"></a><a class="link" href="overload2.html" title="async_read_until (2 of 12 overloads)">async_read_until
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(2 of 12 overloads)</a>
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</h4></div></div></div>
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<p>
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Start an asynchronous operation to read data into a dynamic buffer sequence
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until it contains a specified delimiter.
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</p>
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<pre class="programlisting">template<
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typename <a class="link" href="../AsyncReadStream.html" title="Buffer-oriented asynchronous read stream requirements">AsyncReadStream</a>,
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typename <a class="link" href="../DynamicBuffer_v1.html" title="Dynamic buffer requirements (version 1)">DynamicBuffer_v1</a>,
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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.default_completion_tokens"><span class="emphasis"><em>DEFAULT</em></span></a>>
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<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_until(
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AsyncReadStream & s,
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DynamicBuffer_v1 && buffers,
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string_view delim,
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ReadHandler && handler = <a class="link" href="../asynchronous_operations.html#boost_asio.reference.asynchronous_operations.default_completion_tokens"><span class="emphasis"><em>DEFAULT</em></span></a>,
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typename constraint< is_dynamic_buffer_v1< typename decay< DynamicBuffer_v1 >::type >::value >::type = 0,
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typename constraint< !is_dynamic_buffer_v2< typename decay< DynamicBuffer_v1 >::type >::value >::type = 0);
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</pre>
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<p>
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This function is used to asynchronously read data into the specified dynamic
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buffer sequence until the dynamic buffer sequence's get area contains the
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specified delimiter. The function call always returns immediately. The
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asynchronous operation will continue until one of the following conditions
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is true:
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</p>
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<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
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<li class="listitem">
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The get area of the dynamic buffer sequence contains the specified
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delimiter.
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</li>
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<li class="listitem">
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An error occurred.
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</li>
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</ul></div>
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<p>
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This operation is implemented in terms of zero or more calls to the stream's
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async_read_some function, and is known as a <span class="emphasis"><em>composed operation</em></span>.
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If the dynamic buffer sequence's get area already contains the delimiter,
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this asynchronous operation completes immediately. The program must ensure
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that the stream performs no other read operations (such as async_read,
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async_read_until, the stream's async_read_some function, or any other composed
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operations that perform reads) until this operation completes.
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</p>
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<h6>
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<a name="boost_asio.reference.async_read_until.overload2.h0"></a>
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<span class="phrase"><a name="boost_asio.reference.async_read_until.overload2.parameters"></a></span><a class="link" href="overload2.html#boost_asio.reference.async_read_until.overload2.parameters">Parameters</a>
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</h6>
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<div class="variablelist">
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<p class="title"><b></b></p>
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<dl class="variablelist">
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<dt><span class="term">s</span></dt>
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<dd><p>
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The stream from which the data is to be read. The type must support
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the AsyncReadStream concept.
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</p></dd>
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<dt><span class="term">buffers</span></dt>
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<dd><p>
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The dynamic buffer sequence into which the data will be read. Although
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the buffers object may be copied as necessary, ownership of the underlying
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memory blocks is retained by the caller, which must guarantee that
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they remain valid until the handler is called.
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</p></dd>
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<dt><span class="term">delim</span></dt>
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<dd><p>
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The delimiter string.
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</p></dd>
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<dt><span class="term">handler</span></dt>
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<dd>
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<p>
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The handler to be called when the read operation completes. Copies
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will be made of the handler as required. The function signature of
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the handler must be:
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</p>
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<pre class="programlisting">void handler(
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// Result of operation.
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const boost::system::error_code& error,
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// The number of bytes in the dynamic buffer sequence's
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// get area up to and including the delimiter.
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// 0 if an error occurred.
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std::size_t bytes_transferred
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);
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</pre>
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<p>
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Regardless of whether the asynchronous operation completes immediately
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or not, the handler will not be invoked from within this function.
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On immediate completion, invocation of the handler will be performed
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in a manner equivalent to using <a class="link" href="../post.html" title="post"><code class="computeroutput">post</code></a>.
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</p>
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</dd>
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</dl>
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</div>
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<h6>
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<a name="boost_asio.reference.async_read_until.overload2.h1"></a>
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<span class="phrase"><a name="boost_asio.reference.async_read_until.overload2.remarks"></a></span><a class="link" href="overload2.html#boost_asio.reference.async_read_until.overload2.remarks">Remarks</a>
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</h6>
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<p>
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After a successful async_read_until operation, the dynamic buffer sequence
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may contain additional data beyond the delimiter. An application will typically
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leave that data in the dynamic buffer sequence for a subsequent async_read_until
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operation to examine.
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</p>
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<h6>
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<a name="boost_asio.reference.async_read_until.overload2.h2"></a>
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<span class="phrase"><a name="boost_asio.reference.async_read_until.overload2.example"></a></span><a class="link" href="overload2.html#boost_asio.reference.async_read_until.overload2.example">Example</a>
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</h6>
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<p>
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To asynchronously read data into a <code class="computeroutput">std::string</code> until a CR-LF
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sequence is encountered:
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</p>
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<pre class="programlisting">std::string data;
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...
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void handler(const boost::system::error_code& e, std::size_t size)
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{
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if (!e)
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{
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std::string line = data.substr(0, n);
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data.erase(0, n);
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...
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}
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}
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...
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boost::asio::async_read_until(s, data, "\r\n", handler);
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</pre>
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<p>
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After the <code class="computeroutput">async_read_until</code> operation completes successfully,
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the string <code class="computeroutput">data</code> contains the delimiter:
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</p>
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<pre class="programlisting">{ 'a', 'b', ..., 'c', '\r', '\n', 'd', 'e', ... }
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</pre>
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<p>
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The call to <code class="computeroutput">substr</code> then extracts the data up to and including
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the delimiter, so that the string <code class="computeroutput">line</code> contains:
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</p>
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<pre class="programlisting">{ 'a', 'b', ..., 'c', '\r', '\n' }
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</pre>
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<p>
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After the call to <code class="computeroutput">erase</code>, the remaining data is left in the
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string <code class="computeroutput">data</code> as follows:
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</p>
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<pre class="programlisting">{ 'd', 'e', ... }
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</pre>
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<p>
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This data may be the start of a new line, to be extracted by a subsequent
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<code class="computeroutput">async_read_until</code> operation.
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</p>
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</div>
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<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
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<td align="left"></td>
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<td align="right"><div class="copyright-footer">Copyright © 2003-2021 Christopher
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M. Kohlhoff<p>
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Distributed under the Boost Software License, Version 1.0. (See accompanying
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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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</p>
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</div></td>
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</tr></table>
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