1<html> 2<head> 3<meta http-equiv="Content-Type" content="text/html; charset=UTF-8"> 4<title>async_read (2 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="overload1.html" title="async_read (1 of 8 overloads)"> 10<link rel="next" href="overload3.html" title="async_read (3 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="overload1.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="overload3.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.overload2"></a><a class="link" href="overload2.html" title="async_read (2 of 8 overloads)">async_read 28 (2 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="../MutableBufferSequence.html" title="Mutable buffer sequence requirements">MutableBufferSequence</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 const MutableBufferSequence & 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_mutable_buffer_sequence< MutableBufferSequence >::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 supplied buffers are full. That is, the bytes transferred is equal 55 to the sum of the buffer sizes. 56 </li> 57<li class="listitem"> 58 The completion_condition function object returns 0. 59 </li> 60</ul></div> 61<h6> 62<a name="boost_asio.reference.async_read.overload2.h0"></a> 63 <span class="phrase"><a name="boost_asio.reference.async_read.overload2.parameters"></a></span><a class="link" href="overload2.html#boost_asio.reference.async_read.overload2.parameters">Parameters</a> 64 </h6> 65<div class="variablelist"> 66<p class="title"><b></b></p> 67<dl class="variablelist"> 68<dt><span class="term">s</span></dt> 69<dd><p> 70 The stream from which the data is to be read. The type must support 71 the AsyncReadStream concept. 72 </p></dd> 73<dt><span class="term">buffers</span></dt> 74<dd><p> 75 One or more buffers into which the data will be read. The sum of 76 the buffer sizes indicates the maximum number of bytes to read from 77 the stream. Although the buffers object may be copied as necessary, 78 ownership of the underlying memory blocks is retained by the caller, 79 which must guarantee that they remain valid until the handler is 80 called. 81 </p></dd> 82<dt><span class="term">completion_condition</span></dt> 83<dd> 84<p> 85 The function object to be called to determine whether the read operation 86 is complete. The signature of the function object must be: 87</p> 88<pre class="programlisting">std::size_t completion_condition( 89 // Result of latest async_read_some operation. 90 const boost::system::error_code& error, 91 92 // Number of bytes transferred so far. 93 std::size_t bytes_transferred 94); 95</pre> 96<p> 97 A return value of 0 indicates that the read operation is complete. 98 A non-zero return value indicates the maximum number of bytes to 99 be read on the next call to the stream's async_read_some function. 100 </p> 101</dd> 102<dt><span class="term">handler</span></dt> 103<dd> 104<p> 105 The handler to be called when the read operation completes. Copies 106 will be made of the handler as required. The function signature of 107 the handler must be: 108</p> 109<pre class="programlisting">void handler( 110 const boost::system::error_code& error, // Result of operation. 111 112 std::size_t bytes_transferred // Number of bytes copied into the 113 // buffers. If an error occurred, 114 // this will be the number of 115 // bytes successfully transferred 116 // prior to the error. 117); 118</pre> 119<p> 120 Regardless of whether the asynchronous operation completes immediately 121 or not, the handler will not be invoked from within this function. 122 On immediate completion, invocation of the handler will be performed 123 in a manner equivalent to using <a class="link" href="../post.html" title="post"><code class="computeroutput">post</code></a>. 124 </p> 125</dd> 126</dl> 127</div> 128<h6> 129<a name="boost_asio.reference.async_read.overload2.h1"></a> 130 <span class="phrase"><a name="boost_asio.reference.async_read.overload2.example"></a></span><a class="link" href="overload2.html#boost_asio.reference.async_read.overload2.example">Example</a> 131 </h6> 132<p> 133 To read into a single data buffer use the <a class="link" href="../buffer.html" title="buffer"><code class="computeroutput">buffer</code></a> 134 function as follows: 135 </p> 136<pre class="programlisting">boost::asio::async_read(s, 137 boost::asio::buffer(data, size), 138 boost::asio::transfer_at_least(32), 139 handler); 140</pre> 141<p> 142 See the <a class="link" href="../buffer.html" title="buffer"><code class="computeroutput">buffer</code></a> 143 documentation for information on reading into multiple buffers in one go, 144 and how to use it with arrays, boost::array or std::vector. 145 </p> 146</div> 147<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> 148<td align="left"></td> 149<td align="right"><div class="copyright-footer">Copyright © 2003-2020 Christopher M. 150 Kohlhoff<p> 151 Distributed under the Boost Software License, Version 1.0. (See accompanying 152 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>) 153 </p> 154</div></td> 155</tr></table> 156<hr> 157<div class="spirit-nav"> 158<a accesskey="p" href="overload1.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="overload3.html"><img src="../../../../../doc/src/images/next.png" alt="Next"></a> 159</div> 160</body> 161</html> 162
