1<html> 2<head> 3<meta http-equiv="Content-Type" content="text/html; charset=UTF-8"> 4<title>read_at (7 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="overload6.html" title="read_at (6 of 8 overloads)"> 10<link rel="next" href="overload8.html" title="read_at (8 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="overload6.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="overload8.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.overload7"></a><a class="link" href="overload7.html" title="read_at (7 of 8 overloads)">read_at (7 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 Allocator, 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 basic_streambuf< Allocator > & b, 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; "><li class="listitem"> 50 The completion_condition function object returns 0. 51 </li></ul></div> 52<p> 53 This operation is implemented in terms of zero or more calls to the device's 54 read_some_at function. 55 </p> 56<h6> 57<a name="boost_asio.reference.read_at.overload7.h0"></a> 58 <span class="phrase"><a name="boost_asio.reference.read_at.overload7.parameters"></a></span><a class="link" href="overload7.html#boost_asio.reference.read_at.overload7.parameters">Parameters</a> 59 </h6> 60<div class="variablelist"> 61<p class="title"><b></b></p> 62<dl class="variablelist"> 63<dt><span class="term">d</span></dt> 64<dd><p> 65 The device from which the data is to be read. The type must support 66 the SyncRandomAccessReadDevice concept. 67 </p></dd> 68<dt><span class="term">offset</span></dt> 69<dd><p> 70 The offset at which the data will be read. 71 </p></dd> 72<dt><span class="term">b</span></dt> 73<dd><p> 74 The <a class="link" href="../basic_streambuf.html" title="basic_streambuf"><code class="computeroutput">basic_streambuf</code></a> 75 object into which the data will be read. 76 </p></dd> 77<dt><span class="term">completion_condition</span></dt> 78<dd> 79<p> 80 The function object to be called to determine whether the read operation 81 is complete. The signature of the function object must be: 82</p> 83<pre class="programlisting">std::size_t completion_condition( 84 // Result of latest read_some_at operation. 85 const boost::system::error_code& error, 86 87 // Number of bytes transferred so far. 88 std::size_t bytes_transferred 89); 90</pre> 91<p> 92 A return value of 0 indicates that the read operation is complete. 93 A non-zero return value indicates the maximum number of bytes to 94 be read on the next call to the device's read_some_at function. 95 </p> 96</dd> 97</dl> 98</div> 99<h6> 100<a name="boost_asio.reference.read_at.overload7.h1"></a> 101 <span class="phrase"><a name="boost_asio.reference.read_at.overload7.return_value"></a></span><a class="link" href="overload7.html#boost_asio.reference.read_at.overload7.return_value">Return Value</a> 102 </h6> 103<p> 104 The number of bytes transferred. 105 </p> 106<h6> 107<a name="boost_asio.reference.read_at.overload7.h2"></a> 108 <span class="phrase"><a name="boost_asio.reference.read_at.overload7.exceptions"></a></span><a class="link" href="overload7.html#boost_asio.reference.read_at.overload7.exceptions">Exceptions</a> 109 </h6> 110<div class="variablelist"> 111<p class="title"><b></b></p> 112<dl class="variablelist"> 113<dt><span class="term">boost::system::system_error</span></dt> 114<dd><p> 115 Thrown on failure. 116 </p></dd> 117</dl> 118</div> 119</div> 120<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> 121<td align="left"></td> 122<td align="right"><div class="copyright-footer">Copyright © 2003-2020 Christopher M. 123 Kohlhoff<p> 124 Distributed under the Boost Software License, Version 1.0. (See accompanying 125 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>) 126 </p> 127</div></td> 128</tr></table> 129<hr> 130<div class="spirit-nav"> 131<a accesskey="p" href="overload6.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="overload8.html"><img src="../../../../../doc/src/images/next.png" alt="Next"></a> 132</div> 133</body> 134</html> 135