1<html> 2<head> 3<meta http-equiv="Content-Type" content="text/html; charset=UTF-8"> 4<title>read_at (3 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="../read_at.html" title="read_at"> 9<link rel="prev" href="overload2.html" title="read_at (2 of 8 overloads)"> 10<link rel="next" href="overload4.html" title="read_at (4 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="overload2.html"><img src="../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../read_at.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="overload4.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.read_at.overload3"></a><a class="link" href="overload3.html" title="read_at (3 of 8 overloads)">read_at (3 28 of 8 overloads)</a> 29</h4></div></div></div> 30<p> 31 Attempt to read a certain amount of data at the specified offset before 32 returning. 33 </p> 34<pre class="programlisting">template< 35 typename <a class="link" href="../SyncRandomAccessReadDevice.html" title="Buffer-oriented synchronous random-access read device requirements">SyncRandomAccessReadDevice</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>> 38std::size_t read_at( 39 SyncRandomAccessReadDevice & d, 40 uint64_t offset, 41 const MutableBufferSequence & buffers, 42 CompletionCondition completion_condition); 43</pre> 44<p> 45 This function is used to read a certain number of bytes of data from a 46 random access device at the specified offset. The call will block until 47 one of the following conditions is true: 48 </p> 49<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "> 50<li class="listitem"> 51 The supplied buffers are full. That is, the bytes transferred is equal 52 to the sum of the buffer sizes. 53 </li> 54<li class="listitem"> 55 The completion_condition function object returns 0. 56 </li> 57</ul></div> 58<p> 59 This operation is implemented in terms of zero or more calls to the device's 60 read_some_at function. 61 </p> 62<h6> 63<a name="boost_asio.reference.read_at.overload3.h0"></a> 64 <span class="phrase"><a name="boost_asio.reference.read_at.overload3.parameters"></a></span><a class="link" href="overload3.html#boost_asio.reference.read_at.overload3.parameters">Parameters</a> 65 </h6> 66<div class="variablelist"> 67<p class="title"><b></b></p> 68<dl class="variablelist"> 69<dt><span class="term">d</span></dt> 70<dd><p> 71 The device from which the data is to be read. The type must support 72 the SyncRandomAccessReadDevice concept. 73 </p></dd> 74<dt><span class="term">offset</span></dt> 75<dd><p> 76 The offset at which the data will be read. 77 </p></dd> 78<dt><span class="term">buffers</span></dt> 79<dd><p> 80 One or more buffers into which the data will be read. The sum of 81 the buffer sizes indicates the maximum number of bytes to read from 82 the device. 83 </p></dd> 84<dt><span class="term">completion_condition</span></dt> 85<dd> 86<p> 87 The function object to be called to determine whether the read operation 88 is complete. The signature of the function object must be: 89</p> 90<pre class="programlisting">std::size_t completion_condition( 91 // Result of latest read_some_at operation. 92 const boost::system::error_code& error, 93 94 // Number of bytes transferred so far. 95 std::size_t bytes_transferred 96); 97</pre> 98<p> 99 A return value of 0 indicates that the read operation is complete. 100 A non-zero return value indicates the maximum number of bytes to 101 be read on the next call to the device's read_some_at function. 102 </p> 103</dd> 104</dl> 105</div> 106<h6> 107<a name="boost_asio.reference.read_at.overload3.h1"></a> 108 <span class="phrase"><a name="boost_asio.reference.read_at.overload3.return_value"></a></span><a class="link" href="overload3.html#boost_asio.reference.read_at.overload3.return_value">Return Value</a> 109 </h6> 110<p> 111 The number of bytes transferred. 112 </p> 113<h6> 114<a name="boost_asio.reference.read_at.overload3.h2"></a> 115 <span class="phrase"><a name="boost_asio.reference.read_at.overload3.exceptions"></a></span><a class="link" href="overload3.html#boost_asio.reference.read_at.overload3.exceptions">Exceptions</a> 116 </h6> 117<div class="variablelist"> 118<p class="title"><b></b></p> 119<dl class="variablelist"> 120<dt><span class="term">boost::system::system_error</span></dt> 121<dd><p> 122 Thrown on failure. 123 </p></dd> 124</dl> 125</div> 126<h6> 127<a name="boost_asio.reference.read_at.overload3.h3"></a> 128 <span class="phrase"><a name="boost_asio.reference.read_at.overload3.example"></a></span><a class="link" href="overload3.html#boost_asio.reference.read_at.overload3.example">Example</a> 129 </h6> 130<p> 131 To read into a single data buffer use the <a class="link" href="../buffer.html" title="buffer"><code class="computeroutput">buffer</code></a> 132 function as follows: 133 </p> 134<pre class="programlisting">boost::asio::read_at(d, 42, boost::asio::buffer(data, size), 135 boost::asio::transfer_at_least(32)); 136</pre> 137<p> 138 See the <a class="link" href="../buffer.html" title="buffer"><code class="computeroutput">buffer</code></a> 139 documentation for information on reading into multiple buffers in one go, 140 and how to use it with arrays, boost::array or std::vector. 141 </p> 142</div> 143<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> 144<td align="left"></td> 145<td align="right"><div class="copyright-footer">Copyright © 2003-2020 Christopher M. 146 Kohlhoff<p> 147 Distributed under the Boost Software License, Version 1.0. (See accompanying 148 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>) 149 </p> 150</div></td> 151</tr></table> 152<hr> 153<div class="spirit-nav"> 154<a accesskey="p" href="overload2.html"><img src="../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../read_at.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="overload4.html"><img src="../../../../../doc/src/images/next.png" alt="Next"></a> 155</div> 156</body> 157</html> 158
