1<html> 2<head> 3<meta http-equiv="Content-Type" content="text/html; charset=UTF-8"> 4<title>async_write_at (2 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_write_at.html" title="async_write_at"> 9<link rel="prev" href="overload1.html" title="async_write_at (1 of 4 overloads)"> 10<link rel="next" href="overload3.html" title="async_write_at (3 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="overload1.html"><img src="../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../async_write_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="overload3.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_write_at.overload2"></a><a class="link" href="overload2.html" title="async_write_at (2 of 4 overloads)">async_write_at 28 (2 of 4 overloads)</a> 29</h4></div></div></div> 30<p> 31 Start an asynchronous operation to write a certain amount of data at the 32 specified offset. 33 </p> 34<pre class="programlisting">template< 35 typename <a class="link" href="../AsyncRandomAccessWriteDevice.html" title="Buffer-oriented asynchronous random-access write device requirements">AsyncRandomAccessWriteDevice</a>, 36 typename <a class="link" href="../ConstBufferSequence.html" title="Constant buffer sequence requirements">ConstBufferSequence</a>, 37 typename <a class="link" href="../CompletionCondition.html" title="Completion condition requirements">CompletionCondition</a>, 38 typename <a class="link" href="../WriteHandler.html" title="Write handler requirements">WriteHandler</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_write_at( 40 AsyncRandomAccessWriteDevice & d, 41 uint64_t offset, 42 const ConstBufferSequence & buffers, 43 CompletionCondition completion_condition, 44 WriteHandler && 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 write a certain number of bytes 48 of data to a random access device at a specified offset. The function call 49 always returns immediately. The asynchronous operation will continue until 50 one of the following conditions is true: 51 </p> 52<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "> 53<li class="listitem"> 54 All of the data in the supplied buffers has been written. That is, 55 the bytes transferred is equal to the sum of the buffer sizes. 56 </li> 57<li class="listitem"> 58 The completion_condition function object returns 0. 59 </li> 60</ul></div> 61<p> 62 This operation is implemented in terms of zero or more calls to the device's 63 async_write_some_at function, and is known as a <span class="emphasis"><em>composed operation</em></span>. 64 The program must ensure that the device performs no <span class="emphasis"><em>overlapping</em></span> 65 write operations (such as async_write_at, the device's async_write_some_at 66 function, or any other composed operations that perform writes) until this 67 operation completes. Operations are overlapping if the regions defined 68 by their offsets, and the numbers of bytes to write, intersect. 69 </p> 70<h6> 71<a name="boost_asio.reference.async_write_at.overload2.h0"></a> 72 <span class="phrase"><a name="boost_asio.reference.async_write_at.overload2.parameters"></a></span><a class="link" href="overload2.html#boost_asio.reference.async_write_at.overload2.parameters">Parameters</a> 73 </h6> 74<div class="variablelist"> 75<p class="title"><b></b></p> 76<dl class="variablelist"> 77<dt><span class="term">d</span></dt> 78<dd><p> 79 The device to which the data is to be written. The type must support 80 the AsyncRandomAccessWriteDevice concept. 81 </p></dd> 82<dt><span class="term">offset</span></dt> 83<dd><p> 84 The offset at which the data will be written. 85 </p></dd> 86<dt><span class="term">buffers</span></dt> 87<dd><p> 88 One or more buffers containing the data to be written. Although the 89 buffers object may be copied as necessary, ownership of the underlying 90 memory blocks is retained by the caller, which must guarantee that 91 they remain valid until the handler is called. 92 </p></dd> 93<dt><span class="term">completion_condition</span></dt> 94<dd> 95<p> 96 The function object to be called to determine whether the write operation 97 is complete. The signature of the function object must be: 98</p> 99<pre class="programlisting">std::size_t completion_condition( 100 // Result of latest async_write_some_at operation. 101 const boost::system::error_code& error, 102 103 // Number of bytes transferred so far. 104 std::size_t bytes_transferred 105); 106</pre> 107<p> 108 A return value of 0 indicates that the write operation is complete. 109 A non-zero return value indicates the maximum number of bytes to 110 be written on the next call to the device's async_write_some_at function. 111 </p> 112</dd> 113<dt><span class="term">handler</span></dt> 114<dd> 115<p> 116 The handler to be called when the write operation completes. Copies 117 will be made of the handler as required. The function signature of 118 the handler must be: 119</p> 120<pre class="programlisting">void handler( 121 // Result of operation. 122 const boost::system::error_code& error, 123 124 // Number of bytes written from the buffers. If an error 125 // occurred, this will be less than the sum of the buffer sizes. 126 std::size_t bytes_transferred 127); 128</pre> 129<p> 130 Regardless of whether the asynchronous operation completes immediately 131 or not, the handler will not be invoked from within this function. 132 On immediate completion, invocation of the handler will be performed 133 in a manner equivalent to using <a class="link" href="../post.html" title="post"><code class="computeroutput">post</code></a>. 134 </p> 135</dd> 136</dl> 137</div> 138<h6> 139<a name="boost_asio.reference.async_write_at.overload2.h1"></a> 140 <span class="phrase"><a name="boost_asio.reference.async_write_at.overload2.example"></a></span><a class="link" href="overload2.html#boost_asio.reference.async_write_at.overload2.example">Example</a> 141 </h6> 142<p> 143 To write a single data buffer use the <a class="link" href="../buffer.html" title="buffer"><code class="computeroutput">buffer</code></a> 144 function as follows: 145 </p> 146<pre class="programlisting">boost::asio::async_write_at(d, 42, 147 boost::asio::buffer(data, size), 148 boost::asio::transfer_at_least(32), 149 handler); 150</pre> 151<p> 152 See the <a class="link" href="../buffer.html" title="buffer"><code class="computeroutput">buffer</code></a> 153 documentation for information on writing multiple buffers in one go, and 154 how to use it with arrays, boost::array or std::vector. 155 </p> 156</div> 157<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> 158<td align="left"></td> 159<td align="right"><div class="copyright-footer">Copyright © 2003-2020 Christopher M. 160 Kohlhoff<p> 161 Distributed under the Boost Software License, Version 1.0. (See accompanying 162 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>) 163 </p> 164</div></td> 165</tr></table> 166<hr> 167<div class="spirit-nav"> 168<a accesskey="p" href="overload1.html"><img src="../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../async_write_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="overload3.html"><img src="../../../../../doc/src/images/next.png" alt="Next"></a> 169</div> 170</body> 171</html> 172
