1<html> 2<head> 3<meta http-equiv="Content-Type" content="text/html; charset=UTF-8"> 4<title>async_read (6 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="overload5.html" title="async_read (5 of 8 overloads)"> 10<link rel="next" href="overload7.html" title="async_read (7 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="overload5.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="overload7.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.overload6"></a><a class="link" href="overload6.html" title="async_read (6 of 8 overloads)">async_read 28 (6 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 Allocator, 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 basic_streambuf< Allocator > & b, 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</pre> 45<p> 46 This function is used to asynchronously read a certain number of bytes 47 of data from a stream. The function call always returns immediately. The 48 asynchronous operation will continue until one of the following conditions 49 is true: 50 </p> 51<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "> 52<li class="listitem"> 53 The supplied buffer is full (that is, it has reached maximum size). 54 </li> 55<li class="listitem"> 56 The completion_condition function object returns 0. 57 </li> 58</ul></div> 59<p> 60 This operation is implemented in terms of zero or more calls to the stream's 61 async_read_some function, and is known as a <span class="emphasis"><em>composed operation</em></span>. 62 The program must ensure that the stream performs no other read operations 63 (such as async_read, the stream's async_read_some function, or any other 64 composed operations that perform reads) until this operation completes. 65 </p> 66<h6> 67<a name="boost_asio.reference.async_read.overload6.h0"></a> 68 <span class="phrase"><a name="boost_asio.reference.async_read.overload6.parameters"></a></span><a class="link" href="overload6.html#boost_asio.reference.async_read.overload6.parameters">Parameters</a> 69 </h6> 70<div class="variablelist"> 71<p class="title"><b></b></p> 72<dl class="variablelist"> 73<dt><span class="term">s</span></dt> 74<dd><p> 75 The stream from which the data is to be read. The type must support 76 the AsyncReadStream concept. 77 </p></dd> 78<dt><span class="term">b</span></dt> 79<dd><p> 80 A <a class="link" href="../basic_streambuf.html" title="basic_streambuf"><code class="computeroutput">basic_streambuf</code></a> 81 object into which the data will be read. Ownership of the streambuf 82 is retained by the caller, which must guarantee that it remains valid 83 until the handler is called. 84 </p></dd> 85<dt><span class="term">completion_condition</span></dt> 86<dd> 87<p> 88 The function object to be called to determine whether the read operation 89 is complete. The signature of the function object must be: 90</p> 91<pre class="programlisting">std::size_t completion_condition( 92 // Result of latest async_read_some operation. 93 const boost::system::error_code& error, 94 95 // Number of bytes transferred so far. 96 std::size_t bytes_transferred 97); 98</pre> 99<p> 100 A return value of 0 indicates that the read operation is complete. 101 A non-zero return value indicates the maximum number of bytes to 102 be read on the next call to the stream's async_read_some function. 103 </p> 104</dd> 105<dt><span class="term">handler</span></dt> 106<dd> 107<p> 108 The handler to be called when the read operation completes. Copies 109 will be made of the handler as required. The function signature of 110 the handler must be: 111</p> 112<pre class="programlisting">void handler( 113 const boost::system::error_code& error, // Result of operation. 114 115 std::size_t bytes_transferred // Number of bytes copied into the 116 // buffers. If an error occurred, 117 // this will be the number of 118 // bytes successfully transferred 119 // prior to the error. 120); 121</pre> 122<p> 123 Regardless of whether the asynchronous operation completes immediately 124 or not, the handler will not be invoked from within this function. 125 On immediate completion, invocation of the handler will be performed 126 in a manner equivalent to using <a class="link" href="../post.html" title="post"><code class="computeroutput">post</code></a>. 127 </p> 128</dd> 129</dl> 130</div> 131</div> 132<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> 133<td align="left"></td> 134<td align="right"><div class="copyright-footer">Copyright © 2003-2020 Christopher M. 135 Kohlhoff<p> 136 Distributed under the Boost Software License, Version 1.0. (See accompanying 137 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>) 138 </p> 139</div></td> 140</tr></table> 141<hr> 142<div class="spirit-nav"> 143<a accesskey="p" href="overload5.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="overload7.html"><img src="../../../../../doc/src/images/next.png" alt="Next"></a> 144</div> 145</body> 146</html> 147
