1<html> 2<head> 3<meta http-equiv="Content-Type" content="text/html; charset=UTF-8"> 4<title>read_at (8 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="overload7.html" title="read_at (7 of 8 overloads)"> 10<link rel="next" href="../read_until.html" title="read_until"> 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="overload7.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="../read_until.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.overload8"></a><a class="link" href="overload8.html" title="read_at (8 of 8 overloads)">read_at (8 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 boost::system::error_code & ec); 44</pre> 45<p> 46 This function is used to read a certain number of bytes of data from a 47 random access device at the specified offset. The call will block until 48 one of the following conditions is true: 49 </p> 50<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem"> 51 The completion_condition function object returns 0. 52 </li></ul></div> 53<p> 54 This operation is implemented in terms of zero or more calls to the device's 55 read_some_at function. 56 </p> 57<h6> 58<a name="boost_asio.reference.read_at.overload8.h0"></a> 59 <span class="phrase"><a name="boost_asio.reference.read_at.overload8.parameters"></a></span><a class="link" href="overload8.html#boost_asio.reference.read_at.overload8.parameters">Parameters</a> 60 </h6> 61<div class="variablelist"> 62<p class="title"><b></b></p> 63<dl class="variablelist"> 64<dt><span class="term">d</span></dt> 65<dd><p> 66 The device from which the data is to be read. The type must support 67 the SyncRandomAccessReadDevice concept. 68 </p></dd> 69<dt><span class="term">offset</span></dt> 70<dd><p> 71 The offset at which the data will be read. 72 </p></dd> 73<dt><span class="term">b</span></dt> 74<dd><p> 75 The <a class="link" href="../basic_streambuf.html" title="basic_streambuf"><code class="computeroutput">basic_streambuf</code></a> 76 object into which the data will be read. 77 </p></dd> 78<dt><span class="term">completion_condition</span></dt> 79<dd> 80<p> 81 The function object to be called to determine whether the read operation 82 is complete. The signature of the function object must be: 83</p> 84<pre class="programlisting">std::size_t completion_condition( 85 // Result of latest read_some_at operation. 86 const boost::system::error_code& error, 87 88 // Number of bytes transferred so far. 89 std::size_t bytes_transferred 90); 91</pre> 92<p> 93 A return value of 0 indicates that the read operation is complete. 94 A non-zero return value indicates the maximum number of bytes to 95 be read on the next call to the device's read_some_at function. 96 </p> 97</dd> 98<dt><span class="term">ec</span></dt> 99<dd><p> 100 Set to indicate what error occurred, if any. 101 </p></dd> 102</dl> 103</div> 104<h6> 105<a name="boost_asio.reference.read_at.overload8.h1"></a> 106 <span class="phrase"><a name="boost_asio.reference.read_at.overload8.return_value"></a></span><a class="link" href="overload8.html#boost_asio.reference.read_at.overload8.return_value">Return Value</a> 107 </h6> 108<p> 109 The number of bytes read. If an error occurs, returns the total number 110 of bytes successfully transferred prior to the error. 111 </p> 112</div> 113<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> 114<td align="left"></td> 115<td align="right"><div class="copyright-footer">Copyright © 2003-2020 Christopher M. 116 Kohlhoff<p> 117 Distributed under the Boost Software License, Version 1.0. (See accompanying 118 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>) 119 </p> 120</div></td> 121</tr></table> 122<hr> 123<div class="spirit-nav"> 124<a accesskey="p" href="overload7.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="../read_until.html"><img src="../../../../../doc/src/images/next.png" alt="Next"></a> 125</div> 126</body> 127</html> 128
