1<html> 2<head> 3<meta http-equiv="Content-Type" content="text/html; charset=UTF-8"> 4<title>async_read (4 of 8 overloads)</title> 5<link rel="stylesheet" href="../../../../../doc/src/boostbook.css" type="text/css"> 6<meta name="generator" content="DocBook XSL Stylesheets V1.79.1"> 7<link rel="home" href="../../../boost_asio.html" title="Boost.Asio"> 8<link rel="up" href="../async_read.html" title="async_read"> 9<link rel="prev" href="overload3.html" title="async_read (3 of 8 overloads)"> 10<link rel="next" href="overload5.html" title="async_read (5 of 8 overloads)"> 11</head> 12<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"> 13<table cellpadding="2" width="100%"><tr> 14<td valign="top"><img alt="Boost C++ Libraries" width="277" height="86" src="../../../../../boost.png"></td> 15<td align="center"><a href="../../../../../index.html">Home</a></td> 16<td align="center"><a href="../../../../../libs/libraries.htm">Libraries</a></td> 17<td align="center"><a href="http://www.boost.org/users/people.html">People</a></td> 18<td align="center"><a href="http://www.boost.org/users/faq.html">FAQ</a></td> 19<td align="center"><a href="../../../../../more/index.htm">More</a></td> 20</tr></table> 21<hr> 22<div class="spirit-nav"> 23<a accesskey="p" href="overload3.html"><img src="../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../async_read.html"><img src="../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../../../boost_asio.html"><img src="../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="overload5.html"><img src="../../../../../doc/src/images/next.png" alt="Next"></a> 24</div> 25<div class="section"> 26<div class="titlepage"><div><div><h4 class="title"> 27<a name="boost_asio.reference.async_read.overload4"></a><a class="link" href="overload4.html" title="async_read (4 of 8 overloads)">async_read 28 (4 of 8 overloads)</a> 29</h4></div></div></div> 30<p> 31 Start an asynchronous operation to read a certain amount of data from a 32 stream. 33 </p> 34<pre class="programlisting">template< 35 typename <a class="link" href="../AsyncReadStream.html" title="Buffer-oriented asynchronous read stream requirements">AsyncReadStream</a>, 36 typename <a class="link" href="../DynamicBuffer_v1.html" title="Dynamic buffer requirements (version 1)">DynamicBuffer_v1</a>, 37 typename <a class="link" href="../CompletionCondition.html" title="Completion condition requirements">CompletionCondition</a>, 38 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>> 39<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( 40 AsyncReadStream & s, 41 DynamicBuffer_v1 && buffers, 42 CompletionCondition completion_condition, 43 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>, 44 typename enable_if< is_dynamic_buffer_v1< typename decay< DynamicBuffer_v1 >::type >::value &&!is_dynamic_buffer_v2< typename decay< DynamicBuffer_v1 >::type >::value >::type * = 0); 45</pre> 46<p> 47 This function is used to asynchronously read a certain number of bytes 48 of data from a stream. The function call always returns immediately. The 49 asynchronous operation will continue until one of the following conditions 50 is true: 51 </p> 52<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "> 53<li class="listitem"> 54 The specified dynamic buffer sequence is full (that is, it has reached 55 maximum size). 56 </li> 57<li class="listitem"> 58 The completion_condition function object returns 0. 59 </li> 60</ul></div> 61<p> 62 This operation is implemented in terms of zero or more calls to the stream's 63 async_read_some function, and is known as a <span class="emphasis"><em>composed operation</em></span>. 64 The program must ensure that the stream performs no other read operations 65 (such as async_read, the stream's async_read_some function, or any other 66 composed operations that perform reads) until this operation completes. 67 </p> 68<h6> 69<a name="boost_asio.reference.async_read.overload4.h0"></a> 70 <span class="phrase"><a name="boost_asio.reference.async_read.overload4.parameters"></a></span><a class="link" href="overload4.html#boost_asio.reference.async_read.overload4.parameters">Parameters</a> 71 </h6> 72<div class="variablelist"> 73<p class="title"><b></b></p> 74<dl class="variablelist"> 75<dt><span class="term">s</span></dt> 76<dd><p> 77 The stream from which the data is to be read. The type must support 78 the AsyncReadStream concept. 79 </p></dd> 80<dt><span class="term">buffers</span></dt> 81<dd><p> 82 The dynamic buffer sequence into which the data will be read. Although 83 the buffers object may be copied as necessary, ownership of the underlying 84 memory blocks is retained by the caller, which must guarantee that 85 they remain valid until the handler is called. 86 </p></dd> 87<dt><span class="term">completion_condition</span></dt> 88<dd> 89<p> 90 The function object to be called to determine whether the read operation 91 is complete. The signature of the function object must be: 92</p> 93<pre class="programlisting">std::size_t completion_condition( 94 // Result of latest async_read_some operation. 95 const boost::system::error_code& error, 96 97 // Number of bytes transferred so far. 98 std::size_t bytes_transferred 99); 100</pre> 101<p> 102 A return value of 0 indicates that the read operation is complete. 103 A non-zero return value indicates the maximum number of bytes to 104 be read on the next call to the stream's async_read_some function. 105 </p> 106</dd> 107<dt><span class="term">handler</span></dt> 108<dd> 109<p> 110 The handler to be called when the read operation completes. Copies 111 will be made of the handler as required. The function signature of 112 the handler must be: 113</p> 114<pre class="programlisting">void handler( 115 const boost::system::error_code& error, // Result of operation. 116 117 std::size_t bytes_transferred // Number of bytes copied into the 118 // buffers. If an error occurred, 119 // this will be the number of 120 // bytes successfully transferred 121 // prior to the error. 122); 123</pre> 124<p> 125 Regardless of whether the asynchronous operation completes immediately 126 or not, the handler will not be invoked from within this function. 127 On immediate completion, invocation of the handler will be performed 128 in a manner equivalent to using <a class="link" href="../post.html" title="post"><code class="computeroutput">post</code></a>. 129 </p> 130</dd> 131</dl> 132</div> 133</div> 134<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> 135<td align="left"></td> 136<td align="right"><div class="copyright-footer">Copyright © 2003-2020 Christopher M. 137 Kohlhoff<p> 138 Distributed under the Boost Software License, Version 1.0. (See accompanying 139 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>) 140 </p> 141</div></td> 142</tr></table> 143<hr> 144<div class="spirit-nav"> 145<a accesskey="p" href="overload3.html"><img src="../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../async_read.html"><img src="../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../../../boost_asio.html"><img src="../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="overload5.html"><img src="../../../../../doc/src/images/next.png" alt="Next"></a> 146</div> 147</body> 148</html> 149
