1<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> 2<html> 3<head> 4<meta http-equiv="Content-Type" content="text/html; charset=UTF-8"> 5<title>Mapping Address Independent Pointer: offset_ptr</title> 6<link rel="stylesheet" href="../../../doc/src/boostbook.css" type="text/css"> 7<meta name="generator" content="DocBook XSL Stylesheets V1.79.1"> 8<link rel="home" href="../index.html" title="The Boost C++ Libraries BoostBook Documentation Subset"> 9<link rel="up" href="../interprocess.html" title="Chapter 18. Boost.Interprocess"> 10<link rel="prev" href="sharedmemorybetweenprocesses.html" title="Sharing memory between processes"> 11<link rel="next" href="synchronization_mechanisms.html" title="Synchronization mechanisms"> 12</head> 13<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"> 14<table cellpadding="2" width="100%"><tr> 15<td valign="top"><img alt="Boost C++ Libraries" width="277" height="86" src="../../../boost.png"></td> 16<td align="center"><a href="../../../index.html">Home</a></td> 17<td align="center"><a href="../../../libs/libraries.htm">Libraries</a></td> 18<td align="center"><a href="http://www.boost.org/users/people.html">People</a></td> 19<td align="center"><a href="http://www.boost.org/users/faq.html">FAQ</a></td> 20<td align="center"><a href="../../../more/index.htm">More</a></td> 21</tr></table> 22<hr> 23<div class="spirit-nav"> 24<a accesskey="p" href="sharedmemorybetweenprocesses.html"><img src="../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../interprocess.html"><img src="../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../index.html"><img src="../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="synchronization_mechanisms.html"><img src="../../../doc/src/images/next.png" alt="Next"></a> 25</div> 26<div class="section"> 27<div class="titlepage"><div><div><h2 class="title" style="clear: both"> 28<a name="interprocess.offset_ptr"></a><a class="link" href="offset_ptr.html" title="Mapping Address Independent Pointer: offset_ptr">Mapping Address Independent Pointer: 29 offset_ptr</a> 30</h2></div></div></div> 31<p> 32 When creating shared memory and memory mapped files to communicate two processes 33 the memory segment can be mapped in a different address in each process: 34 </p> 35<pre class="programlisting"><span class="preprocessor">#include</span><span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">interprocess</span><span class="special">/</span><span class="identifier">shared_memory_object</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> 36 37<span class="comment">// ...</span> 38 39<span class="keyword">using</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">interprocess</span><span class="special">;</span> 40 41<span class="comment">//Open a shared memory segment</span> 42<span class="identifier">shared_memory_object</span> <span class="identifier">shm_obj</span> 43 <span class="special">(</span><span class="identifier">open_only</span> <span class="comment">//open or create</span> 44 <span class="special">,</span><span class="string">"shared_memory"</span> <span class="comment">//name</span> 45 <span class="special">,</span><span class="identifier">read_only</span> <span class="comment">//read-only mode</span> 46 <span class="special">);</span> 47 48<span class="comment">//Map the whole shared memory</span> 49<span class="identifier">mapped_region</span> <span class="identifier">region</span> 50 <span class="special">(</span> <span class="identifier">shm</span> <span class="comment">//Memory-mappable object</span> 51 <span class="special">,</span> <span class="identifier">read_write</span> <span class="comment">//Access mode</span> 52 <span class="special">);</span> 53 54<span class="comment">//This address can be different in each process</span> 55<span class="keyword">void</span> <span class="special">*</span><span class="identifier">addr</span> <span class="special">=</span> <span class="identifier">region</span><span class="special">.</span><span class="identifier">get_address</span><span class="special">();</span> 56</pre> 57<p> 58 This makes the creation of complex objects in mapped regions difficult: a C++ 59 class instance placed in a mapped region might have a pointer pointing to another 60 object also placed in the mapped region. Since the pointer stores an absolute 61 address, that address is only valid for the process that placed the object 62 there unless all processes map the mapped region in the same address. 63 </p> 64<p> 65 To be able to simulate pointers in mapped regions, users must use <span class="bold"><strong>offsets</strong></span> (distance between objects) instead of absolute 66 addresses. The offset between two objects in a mapped region is the same for 67 any process that maps the mapped region, even if that region is placed in different 68 base addresses. To facilitate the use of offsets, <span class="bold"><strong>Boost.Interprocess</strong></span> 69 offers <code class="computeroutput"><a class="link" href="../boost/interprocess/offset_ptr.html" title="Class template offset_ptr">offset_ptr</a></code>. 70 </p> 71<p> 72 <code class="computeroutput"><a class="link" href="../boost/interprocess/offset_ptr.html" title="Class template offset_ptr">offset_ptr</a></code> wraps 73 all the background operations needed to offer a pointer-like interface. The 74 class interface is inspired in Boost Smart Pointers and this smart pointer 75 stores the offset (distance in bytes) between the pointee's address and it's 76 own <code class="computeroutput"><span class="keyword">this</span></code> pointer. Imagine a structure 77 in a common 32 bit processor: 78 </p> 79<pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">structure</span> 80<span class="special">{</span> 81 <span class="keyword">int</span> <span class="identifier">integer1</span><span class="special">;</span> <span class="comment">//The compiler places this at offset 0 in the structure</span> 82 <span class="identifier">offset_ptr</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span> <span class="identifier">ptr</span><span class="special">;</span> <span class="comment">//The compiler places this at offset 4 in the structure</span> 83 <span class="keyword">int</span> <span class="identifier">integer2</span><span class="special">;</span> <span class="comment">//The compiler places this at offset 8 in the structure</span> 84<span class="special">};</span> 85 86<span class="comment">//...</span> 87 88<span class="identifier">structure</span> <span class="identifier">s</span><span class="special">;</span> 89 90<span class="comment">//Assign the address of "integer1" to "ptr".</span> 91<span class="comment">//"ptr" will store internally "-4":</span> 92<span class="comment">// (char*)&s.integer1 - (char*)&s.ptr;</span> 93<span class="identifier">s</span><span class="special">.</span><span class="identifier">ptr</span> <span class="special">=</span> <span class="special">&</span><span class="identifier">s</span><span class="special">.</span><span class="identifier">integer1</span><span class="special">;</span> 94 95<span class="comment">//Assign the address of "integer2" to "ptr".</span> 96<span class="comment">//"ptr" will store internally "4":</span> 97<span class="comment">// (char*)&s.integer2 - (char*)&s.ptr;</span> 98<span class="identifier">s</span><span class="special">.</span><span class="identifier">ptr</span> <span class="special">=</span> <span class="special">&</span><span class="identifier">s</span><span class="special">.</span><span class="identifier">integer2</span><span class="special">;</span> 99</pre> 100<p> 101 One of the big problems of <code class="computeroutput"><span class="identifier">offset_ptr</span></code> 102 is the representation of the null pointer. The null pointer can't be safely 103 represented like an offset, since the absolute address 0 is always outside 104 of the mapped region. Due to the fact that the segment can be mapped in a different 105 base address in each process the distance between the address 0 and <code class="computeroutput"><span class="identifier">offset_ptr</span></code> is different for every process. 106 </p> 107<p> 108 Some implementations choose the offset 0 (that is, an <code class="computeroutput"><span class="identifier">offset_ptr</span></code> 109 pointing to itself) as the null pointer pointer representation but this is 110 not valid for many use cases since many times structures like linked lists 111 or nodes from STL containers point to themselves (the end node in an empty 112 container, for example) and 0 offset value is needed. An alternative is to 113 store, in addition to the offset, a boolean to indicate if the pointer is null. 114 However, this increments the size of the pointer and hurts performance. 115 </p> 116<p> 117 In consequence, <code class="computeroutput"><a class="link" href="../boost/interprocess/offset_ptr.html" title="Class template offset_ptr">offset_ptr</a></code> 118 defines offset 1 as the null pointer, meaning that this class <span class="bold"><strong>can't</strong></span> 119 point to the byte after its own <span class="emphasis"><em>this</em></span> pointer: 120 </p> 121<pre class="programlisting"><span class="keyword">using</span> <span class="keyword">namespace</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">interprocess</span><span class="special">;</span> 122 123<span class="identifier">offset_ptr</span><span class="special"><</span><span class="keyword">char</span><span class="special">></span> <span class="identifier">ptr</span><span class="special">;</span> 124 125<span class="comment">//Pointing to the next byte of it's own address</span> 126<span class="comment">//marks the smart pointer as null.</span> 127<span class="identifier">ptr</span> <span class="special">=</span> <span class="special">(</span><span class="keyword">char</span><span class="special">*)&</span><span class="identifier">ptr</span> <span class="special">+</span> <span class="number">1</span><span class="special">;</span> 128 129<span class="comment">//ptr is equal to null</span> 130<span class="identifier">assert</span><span class="special">(!</span><span class="identifier">ptr</span><span class="special">);</span> 131 132<span class="comment">//This is the same as assigning the null value...</span> 133<span class="identifier">ptr</span> <span class="special">=</span> <span class="number">0</span><span class="special">;</span> 134 135<span class="comment">//ptr is also equal to null</span> 136<span class="identifier">assert</span><span class="special">(!</span><span class="identifier">ptr</span><span class="special">);</span> 137</pre> 138<p> 139 In practice, this limitation is not important, since a user almost never wants 140 to point to this address. 141 </p> 142<p> 143 <code class="computeroutput"><a class="link" href="../boost/interprocess/offset_ptr.html" title="Class template offset_ptr">offset_ptr</a></code> offers 144 all pointer-like operations and random_access_iterator typedefs, so it can 145 be used in STL algorithms requiring random access iterators and detected via 146 traits. For more information about the members and operations of the class, 147 see <code class="computeroutput"><a class="link" href="../boost/interprocess/offset_ptr.html" title="Class template offset_ptr">offset_ptr reference</a></code>. 148 </p> 149</div> 150<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> 151<td align="left"></td> 152<td align="right"><div class="copyright-footer">Copyright © 2005-2015 Ion Gaztanaga<p> 153 Distributed under the Boost Software License, Version 1.0. (See accompanying 154 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>) 155 </p> 156</div></td> 157</tr></table> 158<hr> 159<div class="spirit-nav"> 160<a accesskey="p" href="sharedmemorybetweenprocesses.html"><img src="../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../interprocess.html"><img src="../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../index.html"><img src="../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="synchronization_mechanisms.html"><img src="../../../doc/src/images/next.png" alt="Next"></a> 161</div> 162</body> 163</html> 164
