1<html> 2<head> 3<meta http-equiv="Content-Type" content="text/html; charset=UTF-8"> 4<title>async_read_at (4 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="overload3.html" title="async_read_at (3 of 4 overloads)"> 10<link rel="next" href="../async_read_until.html" title="async_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="overload3.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="../async_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.async_read_at.overload4"></a><a class="link" href="overload4.html" title="async_read_at (4 of 4 overloads)">async_read_at 28 (4 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="../CompletionCondition.html" title="Completion condition requirements">CompletionCondition</a>, 38 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>> 39<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( 40 AsyncRandomAccessReadDevice & d, 41 uint64_t offset, 42 basic_streambuf< Allocator > & b, 43 CompletionCondition completion_condition, 44 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>); 45</pre> 46<p> 47 This function is used to asynchronously read a certain number of bytes 48 of data from a random access device at the specified offset. The function 49 call always returns immediately. The asynchronous operation will continue 50 until one of the following conditions is true: 51 </p> 52<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem"> 53 The completion_condition function object returns 0. 54 </li></ul></div> 55<p> 56 This operation is implemented in terms of zero or more calls to the device's 57 async_read_some_at function. 58 </p> 59<h6> 60<a name="boost_asio.reference.async_read_at.overload4.h0"></a> 61 <span class="phrase"><a name="boost_asio.reference.async_read_at.overload4.parameters"></a></span><a class="link" href="overload4.html#boost_asio.reference.async_read_at.overload4.parameters">Parameters</a> 62 </h6> 63<div class="variablelist"> 64<p class="title"><b></b></p> 65<dl class="variablelist"> 66<dt><span class="term">d</span></dt> 67<dd><p> 68 The device from which the data is to be read. The type must support 69 the AsyncRandomAccessReadDevice concept. 70 </p></dd> 71<dt><span class="term">offset</span></dt> 72<dd><p> 73 The offset at which the data will be read. 74 </p></dd> 75<dt><span class="term">b</span></dt> 76<dd><p> 77 A <a class="link" href="../basic_streambuf.html" title="basic_streambuf"><code class="computeroutput">basic_streambuf</code></a> 78 object into which the data will be read. Ownership of the streambuf 79 is retained by the caller, which must guarantee that it remains valid 80 until the handler is called. 81 </p></dd> 82<dt><span class="term">completion_condition</span></dt> 83<dd> 84<p> 85 The function object to be called to determine whether the read operation 86 is complete. The signature of the function object must be: 87</p> 88<pre class="programlisting">std::size_t completion_condition( 89 // Result of latest async_read_some_at operation. 90 const boost::system::error_code& error, 91 92 // Number of bytes transferred so far. 93 std::size_t bytes_transferred 94); 95</pre> 96<p> 97 A return value of 0 indicates that the read operation is complete. 98 A non-zero return value indicates the maximum number of bytes to 99 be read on the next call to the device's async_read_some_at function. 100 </p> 101</dd> 102<dt><span class="term">handler</span></dt> 103<dd> 104<p> 105 The handler to be called when the read operation completes. Copies 106 will be made of the handler as required. The function signature of 107 the handler must be: 108</p> 109<pre class="programlisting">void handler( 110 // Result of operation. 111 const boost::system::error_code& error, 112 113 // Number of bytes copied into the buffers. If an error 114 // occurred, this will be the number of bytes successfully 115 // transferred prior to the error. 116 std::size_t bytes_transferred 117); 118</pre> 119<p> 120 Regardless of whether the asynchronous operation completes immediately 121 or not, the handler will not be invoked from within this function. 122 On immediate completion, invocation of the handler will be performed 123 in a manner equivalent to using <a class="link" href="../post.html" title="post"><code class="computeroutput">post</code></a>. 124 </p> 125</dd> 126</dl> 127</div> 128</div> 129<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> 130<td align="left"></td> 131<td align="right"><div class="copyright-footer">Copyright © 2003-2020 Christopher M. 132 Kohlhoff<p> 133 Distributed under the Boost Software License, Version 1.0. (See accompanying 134 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>) 135 </p> 136</div></td> 137</tr></table> 138<hr> 139<div class="spirit-nav"> 140<a accesskey="p" href="overload3.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="../async_read_until.html"><img src="../../../../../doc/src/images/next.png" alt="Next"></a> 141</div> 142</body> 143</html> 144
