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  25 <div class="section">
  26 <div class="titlepage"><div><div><h4 class="title">
  27 <a name="beast.ref.boost__beast__http__async_read_header"></a><a class="link" href="boost__beast__http__async_read_header.html" title="http::async_read_header">http::async_read_header</a>
  28 </h4></div></div></div>
  29 <p>
  30         <a class="indexterm" name="idm45873506918000"></a>
  31 Read a complete message header asynchronously from
  32         a stream using a parser.
  33       </p>
  34 <h5>
  35 <a name="beast.ref.boost__beast__http__async_read_header.h0"></a>
  36         <span class="phrase"><a name="beast.ref.boost__beast__http__async_read_header.synopsis"></a></span><a class="link" href="boost__beast__http__async_read_header.html#beast.ref.boost__beast__http__async_read_header.synopsis">Synopsis</a>
  37       </h5>
  38 <p>
  39         Defined in header <code class="literal">&lt;<a href="../../../../../../boost/beast/http/read.hpp" target="_top">boost/beast/http/read.hpp</a>&gt;</code>
  40       </p>
  41 <pre class="programlisting"><span class="keyword">template</span><span class="special">&lt;</span>
  42     <span class="keyword">class</span> <a href="../../../../../../doc/html/boost_asio/reference/AsyncReadStream.html" target="_top"><span class="emphasis"><em>AsyncReadStream</em></span></a><span class="special">,</span>
  43     <span class="keyword">class</span> <a class="link" href="../concepts/DynamicBuffer.html" title="DynamicBuffer"><span class="emphasis"><em>DynamicBuffer</em></span></a><span class="special">,</span>
  44     <span class="keyword">bool</span> <span class="identifier">isRequest</span><span class="special">,</span>
  45     <span class="keyword">class</span> <a href="../../../../../../doc/html/boost_asio/reference/ReadHandler.html" target="_top"><span class="emphasis"><em>ReadHandler</em></span></a> <span class="special">=</span> <span class="identifier">net</span><span class="special">::</span><span class="identifier">default_completion_token_t</span><span class="special">&lt;</span>            <span class="identifier">executor_type</span><span class="special">&lt;</span><span class="identifier">AsyncReadStream</span><span class="special">&gt;&gt;&gt;</span>
  46 <a href="../../../../../../doc/html/boost_asio/reference/asynchronous_operations.html#boost_asio.reference.asynchronous_operations.automatic_deduction_of_initiating_function_return_type" target="_top"><span class="emphasis"><em>DEDUCED</em></span></a>
  47 <span class="identifier">async_read_header</span><span class="special">(</span>
  48     <span class="identifier">AsyncReadStream</span><span class="special">&amp;</span> <span class="identifier">stream</span><span class="special">,</span>
  49     <span class="identifier">DynamicBuffer</span><span class="special">&amp;</span> <span class="identifier">buffer</span><span class="special">,</span>
  50     <span class="identifier">basic_parser</span><span class="special">&lt;</span> <span class="identifier">isRequest</span> <span class="special">&gt;&amp;</span> <span class="identifier">parser</span><span class="special">,</span>
  51     <span class="identifier">ReadHandler</span><span class="special">&amp;&amp;</span> <span class="identifier">handler</span> <span class="special">=</span> <span class="identifier">net</span><span class="special">::</span><span class="identifier">default_completion_token_t</span><span class="special">&lt;</span> <span class="identifier">executor_type</span><span class="special">&lt;</span> <span class="identifier">AsyncReadStream</span> <span class="special">&gt;&gt;{});</span>
  52 </pre>
  53 <h5>
  54 <a name="beast.ref.boost__beast__http__async_read_header.h1"></a>
  55         <span class="phrase"><a name="beast.ref.boost__beast__http__async_read_header.description"></a></span><a class="link" href="boost__beast__http__async_read_header.html#beast.ref.boost__beast__http__async_read_header.description">Description</a>
  56       </h5>
  57 <p>
  58         This function is used to asynchronously read a complete message header from
  59         a stream into an instance of <a class="link" href="boost__beast__http__basic_parser.html" title="http::basic_parser"><code class="computeroutput"><span class="identifier">http</span><span class="special">::</span><span class="identifier">basic_parser</span></code></a>. The function call
  60         always returns immediately. The asynchronous operation will continue until
  61         one of the following conditions is true:
  62       </p>
  63 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
  64 <li class="listitem">
  65             <a class="link" href="boost__beast__http__basic_parser/is_header_done.html" title="http::basic_parser::is_header_done"><code class="computeroutput"><span class="identifier">http</span><span class="special">::</span><span class="identifier">basic_parser</span><span class="special">::</span><span class="identifier">is_header_done</span></code></a> returns <code class="computeroutput"><span class="keyword">true</span></code>
  66           </li>
  67 <li class="listitem">
  68             An error occurs.
  69           </li>
  70 </ul></div>
  71 <p>
  72         This operation is implemented in terms of zero or more calls to the next
  73         layer's <code class="computeroutput"><span class="identifier">async_read_some</span></code> function,
  74         and is known as a <span class="emphasis"><em>composed operation</em></span>. The program must
  75         ensure that the stream performs no other reads until this operation completes.
  76         The implementation may read additional bytes from the stream that lie past
  77         the end of the message being read. These additional bytes are stored in the
  78         dynamic buffer, which must be preserved for subsequent reads. If the end
  79         of file error is received while reading from the stream, then the error returned
  80         from this function will be:
  81       </p>
  82 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
  83 <li class="listitem">
  84             <a class="link" href="boost__beast__http__error.html" title="http::error"><code class="computeroutput"><span class="identifier">http</span><span class="special">::</span><span class="identifier">end_of_stream</span></code></a>
  85             if no bytes were parsed, or
  86           </li>
  87 <li class="listitem">
  88             <a class="link" href="boost__beast__http__error.html" title="http::error"><code class="computeroutput"><span class="identifier">http</span><span class="special">::</span><span class="identifier">partial_message</span></code></a>
  89             if any bytes were parsed but the message was incomplete, otherwise:
  90           </li>
  91 <li class="listitem">
  92             A successful result. The next attempt to read will return <a class="link" href="boost__beast__http__error.html" title="http::error"><code class="computeroutput"><span class="identifier">http</span><span class="special">::</span><span class="identifier">end_of_stream</span></code></a>
  93           </li>
  94 </ul></div>
  95 <h5>
  96 <a name="beast.ref.boost__beast__http__async_read_header.h2"></a>
  97         <span class="phrase"><a name="beast.ref.boost__beast__http__async_read_header.parameters"></a></span><a class="link" href="boost__beast__http__async_read_header.html#beast.ref.boost__beast__http__async_read_header.parameters">Parameters</a>
  98       </h5>
  99 <div class="informaltable"><table class="table">
 100 <colgroup>
 101 <col>
 102 <col>
 103 </colgroup>
 104 <thead><tr>
 105 <th>
 106                 <p>
 107                   Name
 108                 </p>
 109               </th>
 110 <th>
 111                 <p>
 112                   Description
 113                 </p>
 114               </th>
 115 </tr></thead>
 116 <tbody>
 117 <tr>
 118 <td>
 119                 <p>
 120                   <code class="computeroutput"><span class="identifier">stream</span></code>
 121                 </p>
 122               </td>
 123 <td>
 124                 <p>
 125                   The stream from which the data is to be read. The type must meet
 126                   the <span class="emphasis"><em>AsyncReadStream</em></span> requirements.
 127                 </p>
 128               </td>
 129 </tr>
 130 <tr>
 131 <td>
 132                 <p>
 133                   <code class="computeroutput"><span class="identifier">buffer</span></code>
 134                 </p>
 135               </td>
 136 <td>
 137                 <p>
 138                   Storage for additional bytes read by the implementation from the
 139                   stream. This is both an input and an output parameter; on entry,
 140                   the parser will be presented with any remaining data in the dynamic
 141                   buffer's readable bytes sequence first. The type must meet the
 142                   <span class="emphasis"><em>DynamicBuffer</em></span> requirements. The object must
 143                   remain valid at least until the handler is called; ownership is
 144                   not transferred.
 145                 </p>
 146               </td>
 147 </tr>
 148 <tr>
 149 <td>
 150                 <p>
 151                   <code class="computeroutput"><span class="identifier">parser</span></code>
 152                 </p>
 153               </td>
 154 <td>
 155                 <p>
 156                   The parser to use. The object must remain valid at least until
 157                   the handler is called; ownership is not transferred.
 158                 </p>
 159               </td>
 160 </tr>
 161 <tr>
 162 <td>
 163                 <p>
 164                   <code class="computeroutput"><span class="identifier">handler</span></code>
 165                 </p>
 166               </td>
 167 <td>
 168                 <p>
 169                   The completion handler to invoke when the operation completes.
 170                   The implementation takes ownership of the handler by performing
 171                   a decay-copy. The equivalent function signature of the handler
 172                   must be:
 173                 </p>
 174 <pre xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" class="table-programlisting"><span class="keyword">void</span> <span class="identifier">handler</span><span class="special">(</span>
 175     <span class="identifier">error_code</span> <span class="keyword">const</span><span class="special">&amp;</span> <span class="identifier">error</span><span class="special">,</span>        <span class="comment">// result of operation</span>
 176     <span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span> <span class="identifier">bytes_transferred</span>   <span class="comment">// the total number of bytes transferred from the stream</span>
 177 <span class="special">);</span>
 178 </pre>
 179                 <p>
 180                   Regardless of whether the asynchronous operation completes immediately
 181                   or not, the handler will not be invoked from within this function.
 182                   Invocation of the handler will be performed in a manner equivalent
 183                   to using <code class="computeroutput"><span class="identifier">net</span><span class="special">::</span><span class="identifier">post</span></code>.
 184                 </p>
 185               </td>
 186 </tr>
 187 </tbody>
 188 </table></div>
 189 <h5>
 190 <a name="beast.ref.boost__beast__http__async_read_header.h3"></a>
 191         <span class="phrase"><a name="beast.ref.boost__beast__http__async_read_header.remarks"></a></span><a class="link" href="boost__beast__http__async_read_header.html#beast.ref.boost__beast__http__async_read_header.remarks">Remarks</a>
 192       </h5>
 193 <p>
 194         The completion handler will receive as a parameter the total number of bytes
 195         transferred from the stream. This may be zero for the case where there is
 196         sufficient pre-existing message data in the dynamic buffer. The implementation
 197         will call <a class="link" href="boost__beast__http__basic_parser/eager.html" title="http::basic_parser::eager"><code class="computeroutput"><span class="identifier">http</span><span class="special">::</span><span class="identifier">basic_parser</span><span class="special">::</span><span class="identifier">eager</span></code></a> with the value <code class="computeroutput"><span class="keyword">false</span></code> on the parser passed in.
 198       </p>
 199 <p>
 200         Convenience header <code class="literal">&lt;<a href="../../../../../../boost/beast/http.hpp" target="_top">boost/beast/http.hpp</a>&gt;</code>
 201       </p>
 202 </div>
 203 <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
 204 <td align="left"></td>
 205 <td align="right"><div class="copyright-footer">Copyright &#169; 2016-2019 Vinnie
 206       Falco<p>
 207         Distributed under the Boost Software License, Version 1.0. (See accompanying
 208         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>)
 209       </p>
 210 </div></td>
 211 </tr></table>
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