1<html> 2<head> 3<meta http-equiv="Content-Type" content="text/html; charset=UTF-8"> 4<title>async_read_at (3 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="overload2.html" title="async_read_at (2 of 4 overloads)"> 10<link rel="next" href="overload4.html" title="async_read_at (4 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="overload2.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="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.async_read_at.overload3"></a><a class="link" href="overload3.html" title="async_read_at (3 of 4 overloads)">async_read_at 28 (3 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 Allocator, 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 basic_streambuf< Allocator > & b, 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; "><li class="listitem"> 51 An error occurred. 52 </li></ul></div> 53<p> 54 This operation is implemented in terms of zero or more calls to the device's 55 async_read_some_at function. 56 </p> 57<h6> 58<a name="boost_asio.reference.async_read_at.overload3.h0"></a> 59 <span class="phrase"><a name="boost_asio.reference.async_read_at.overload3.parameters"></a></span><a class="link" href="overload3.html#boost_asio.reference.async_read_at.overload3.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 AsyncRandomAccessReadDevice 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 A <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. Ownership of the streambuf 77 is retained by the caller, which must guarantee that it remains valid 78 until the handler is called. 79 </p></dd> 80<dt><span class="term">handler</span></dt> 81<dd> 82<p> 83 The handler to be called when the read operation completes. Copies 84 will be made of the handler as required. The function signature of 85 the handler must be: 86</p> 87<pre class="programlisting">void handler( 88 // Result of operation. 89 const boost::system::error_code& error, 90 91 // Number of bytes copied into the buffers. If an error 92 // occurred, this will be the number of bytes successfully 93 // transferred prior to the error. 94 std::size_t bytes_transferred 95); 96</pre> 97<p> 98 Regardless of whether the asynchronous operation completes immediately 99 or not, the handler will not be invoked from within this function. 100 On immediate completion, invocation of the handler will be performed 101 in a manner equivalent to using <a class="link" href="../post.html" title="post"><code class="computeroutput">post</code></a>. 102 </p> 103</dd> 104</dl> 105</div> 106<h6> 107<a name="boost_asio.reference.async_read_at.overload3.h1"></a> 108 <span class="phrase"><a name="boost_asio.reference.async_read_at.overload3.remarks"></a></span><a class="link" href="overload3.html#boost_asio.reference.async_read_at.overload3.remarks">Remarks</a> 109 </h6> 110<p> 111 This overload is equivalent to calling: 112 </p> 113<pre class="programlisting">boost::asio::async_read_at( 114 d, 42, b, 115 boost::asio::transfer_all(), 116 handler); 117</pre> 118</div> 119<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> 120<td align="left"></td> 121<td align="right"><div class="copyright-footer">Copyright © 2003-2020 Christopher M. 122 Kohlhoff<p> 123 Distributed under the Boost Software License, Version 1.0. (See accompanying 124 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>) 125 </p> 126</div></td> 127</tr></table> 128<hr> 129<div class="spirit-nav"> 130<a accesskey="p" href="overload2.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="overload4.html"><img src="../../../../../doc/src/images/next.png" alt="Next"></a> 131</div> 132</body> 133</html> 134
