1<html> 2<head> 3<meta http-equiv="Content-Type" content="text/html; charset=UTF-8"> 4<title>async_read_at (1 of 4 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_at.html" title="async_read_at"> 9<link rel="prev" href="../async_read_at.html" title="async_read_at"> 10<link rel="next" href="overload2.html" title="async_read_at (2 of 4 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="../async_read_at.html"><img src="../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../async_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="overload2.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_at.overload1"></a><a class="link" href="overload1.html" title="async_read_at (1 of 4 overloads)">async_read_at 28 (1 of 4 overloads)</a> 29</h4></div></div></div> 30<p> 31 Start an asynchronous operation to read a certain amount of data at the 32 specified offset. 33 </p> 34<pre class="programlisting">template< 35 typename <a class="link" href="../AsyncRandomAccessReadDevice.html" title="Buffer-oriented asynchronous random-access read device requirements">AsyncRandomAccessReadDevice</a>, 36 typename <a class="link" href="../MutableBufferSequence.html" title="Mutable buffer sequence requirements">MutableBufferSequence</a>, 37 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>> 38<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_at( 39 AsyncRandomAccessReadDevice & d, 40 uint64_t offset, 41 const MutableBufferSequence & buffers, 42 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>); 43</pre> 44<p> 45 This function is used to asynchronously read a certain number of bytes 46 of data from a random access device at the specified offset. The function 47 call always returns immediately. The asynchronous operation will continue 48 until one of the following conditions is true: 49 </p> 50<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "> 51<li class="listitem"> 52 The supplied buffers are full. That is, the bytes transferred is equal 53 to the sum of the buffer sizes. 54 </li> 55<li class="listitem"> 56 An error occurred. 57 </li> 58</ul></div> 59<p> 60 This operation is implemented in terms of zero or more calls to the device's 61 async_read_some_at function. 62 </p> 63<h6> 64<a name="boost_asio.reference.async_read_at.overload1.h0"></a> 65 <span class="phrase"><a name="boost_asio.reference.async_read_at.overload1.parameters"></a></span><a class="link" href="overload1.html#boost_asio.reference.async_read_at.overload1.parameters">Parameters</a> 66 </h6> 67<div class="variablelist"> 68<p class="title"><b></b></p> 69<dl class="variablelist"> 70<dt><span class="term">d</span></dt> 71<dd><p> 72 The device from which the data is to be read. The type must support 73 the AsyncRandomAccessReadDevice concept. 74 </p></dd> 75<dt><span class="term">offset</span></dt> 76<dd><p> 77 The offset at which the data will be read. 78 </p></dd> 79<dt><span class="term">buffers</span></dt> 80<dd><p> 81 One or more buffers into which the data will be read. The sum of 82 the buffer sizes indicates the maximum number of bytes to read from 83 the device. Although the buffers object may be copied as necessary, 84 ownership of the underlying memory blocks is retained by the caller, 85 which must guarantee that they remain valid until the handler is 86 called. 87 </p></dd> 88<dt><span class="term">handler</span></dt> 89<dd> 90<p> 91 The handler to be called when the read operation completes. Copies 92 will be made of the handler as required. The function signature of 93 the handler must be: 94</p> 95<pre class="programlisting">void handler( 96 // Result of operation. 97 const boost::system::error_code& error, 98 99 // Number of bytes copied into the buffers. If an error 100 // occurred, this will be the number of bytes successfully 101 // transferred prior to the error. 102 std::size_t bytes_transferred 103); 104</pre> 105<p> 106 Regardless of whether the asynchronous operation completes immediately 107 or not, the handler will not be invoked from within this function. 108 On immediate completion, invocation of the handler will be performed 109 in a manner equivalent to using <a class="link" href="../post.html" title="post"><code class="computeroutput">post</code></a>. 110 </p> 111</dd> 112</dl> 113</div> 114<h6> 115<a name="boost_asio.reference.async_read_at.overload1.h1"></a> 116 <span class="phrase"><a name="boost_asio.reference.async_read_at.overload1.example"></a></span><a class="link" href="overload1.html#boost_asio.reference.async_read_at.overload1.example">Example</a> 117 </h6> 118<p> 119 To read into a single data buffer use the <a class="link" href="../buffer.html" title="buffer"><code class="computeroutput">buffer</code></a> 120 function as follows: 121 </p> 122<pre class="programlisting">boost::asio::async_read_at(d, 42, boost::asio::buffer(data, size), handler); 123</pre> 124<p> 125 See the <a class="link" href="../buffer.html" title="buffer"><code class="computeroutput">buffer</code></a> 126 documentation for information on reading into multiple buffers in one go, 127 and how to use it with arrays, boost::array or std::vector. 128 </p> 129<h6> 130<a name="boost_asio.reference.async_read_at.overload1.h2"></a> 131 <span class="phrase"><a name="boost_asio.reference.async_read_at.overload1.remarks"></a></span><a class="link" href="overload1.html#boost_asio.reference.async_read_at.overload1.remarks">Remarks</a> 132 </h6> 133<p> 134 This overload is equivalent to calling: 135 </p> 136<pre class="programlisting">boost::asio::async_read_at( 137 d, 42, buffers, 138 boost::asio::transfer_all(), 139 handler); 140</pre> 141</div> 142<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> 143<td align="left"></td> 144<td align="right"><div class="copyright-footer">Copyright © 2003-2020 Christopher M. 145 Kohlhoff<p> 146 Distributed under the Boost Software License, Version 1.0. (See accompanying 147 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>) 148 </p> 149</div></td> 150</tr></table> 151<hr> 152<div class="spirit-nav"> 153<a accesskey="p" href="../async_read_at.html"><img src="../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../async_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="overload2.html"><img src="../../../../../doc/src/images/next.png" alt="Next"></a> 154</div> 155</body> 156</html> 157
