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<div class="titlepage"><div><div><h2 class="title" style="clear: both">
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<a name="interprocess.offset_ptr"></a><a class="link" href="offset_ptr.html" title="Mapping Address Independent Pointer: offset_ptr">Mapping Address Independent Pointer:
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offset_ptr</a>
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<p>
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When creating shared memory and memory mapped files to communicate two processes
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the memory segment can be mapped in a different address in each process:
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</p>
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<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>
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<span class="comment">// ...</span>
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<span class="keyword">using</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">interprocess</span><span class="special">;</span>
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<span class="comment">//Open a shared memory segment</span>
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<span class="identifier">shared_memory_object</span> <span class="identifier">shm_obj</span>
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<span class="special">(</span><span class="identifier">open_only</span> <span class="comment">//open or create</span>
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<span class="special">,</span><span class="string">"shared_memory"</span> <span class="comment">//name</span>
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<span class="special">,</span><span class="identifier">read_only</span> <span class="comment">//read-only mode</span>
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<span class="special">);</span>
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<span class="comment">//Map the whole shared memory</span>
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<span class="identifier">mapped_region</span> <span class="identifier">region</span>
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<span class="special">(</span> <span class="identifier">shm</span> <span class="comment">//Memory-mappable object</span>
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<span class="special">,</span> <span class="identifier">read_write</span> <span class="comment">//Access mode</span>
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<span class="special">);</span>
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<span class="comment">//This address can be different in each process</span>
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<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>
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</pre>
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<p>
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This makes the creation of complex objects in mapped regions difficult: a C++
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class instance placed in a mapped region might have a pointer pointing to another
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object also placed in the mapped region. Since the pointer stores an absolute
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address, that address is only valid for the process that placed the object
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there unless all processes map the mapped region in the same address.
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</p>
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<p>
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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
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addresses. The offset between two objects in a mapped region is the same for
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any process that maps the mapped region, even if that region is placed in different
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base addresses. To facilitate the use of offsets, <span class="bold"><strong>Boost.Interprocess</strong></span>
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offers <code class="computeroutput"><a class="link" href="../boost/interprocess/offset_ptr.html" title="Class template offset_ptr">offset_ptr</a></code>.
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</p>
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<p>
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<code class="computeroutput"><a class="link" href="../boost/interprocess/offset_ptr.html" title="Class template offset_ptr">offset_ptr</a></code> wraps
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all the background operations needed to offer a pointer-like interface. The
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class interface is inspired in Boost Smart Pointers and this smart pointer
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stores the offset (distance in bytes) between the pointee's address and it's
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own <code class="computeroutput"><span class="keyword">this</span></code> pointer. Imagine a structure
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in a common 32 bit processor:
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</p>
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<pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">structure</span>
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<span class="special">{</span>
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<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>
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<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>
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<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>
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<span class="special">};</span>
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<span class="comment">//...</span>
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<span class="identifier">structure</span> <span class="identifier">s</span><span class="special">;</span>
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<span class="comment">//Assign the address of "integer1" to "ptr".</span>
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<span class="comment">//"ptr" will store internally "-4":</span>
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<span class="comment">// (char*)&s.integer1 - (char*)&s.ptr;</span>
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<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>
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<span class="comment">//Assign the address of "integer2" to "ptr".</span>
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<span class="comment">//"ptr" will store internally "4":</span>
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<span class="comment">// (char*)&s.integer2 - (char*)&s.ptr;</span>
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<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>
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</pre>
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<p>
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One of the big problems of <code class="computeroutput"><span class="identifier">offset_ptr</span></code>
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is the representation of the null pointer. The null pointer can't be safely
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represented like an offset, since the absolute address 0 is always outside
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of the mapped region. Due to the fact that the segment can be mapped in a different
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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.
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</p>
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<p>
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Some implementations choose the offset 0 (that is, an <code class="computeroutput"><span class="identifier">offset_ptr</span></code>
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pointing to itself) as the null pointer pointer representation but this is
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not valid for many use cases since many times structures like linked lists
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or nodes from STL containers point to themselves (the end node in an empty
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container, for example) and 0 offset value is needed. An alternative is to
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store, in addition to the offset, a boolean to indicate if the pointer is null.
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However, this increments the size of the pointer and hurts performance.
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</p>
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<p>
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In consequence, <code class="computeroutput"><a class="link" href="../boost/interprocess/offset_ptr.html" title="Class template offset_ptr">offset_ptr</a></code>
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defines offset 1 as the null pointer, meaning that this class <span class="bold"><strong>can't</strong></span>
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point to the byte after its own <span class="emphasis"><em>this</em></span> pointer:
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</p>
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<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>
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<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>
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<span class="comment">//Pointing to the next byte of it's own address</span>
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<span class="comment">//marks the smart pointer as null.</span>
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<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>
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<span class="comment">//ptr is equal to null</span>
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<span class="identifier">assert</span><span class="special">(!</span><span class="identifier">ptr</span><span class="special">);</span>
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<span class="comment">//This is the same as assigning the null value...</span>
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<span class="identifier">ptr</span> <span class="special">=</span> <span class="number">0</span><span class="special">;</span>
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<span class="comment">//ptr is also equal to null</span>
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<span class="identifier">assert</span><span class="special">(!</span><span class="identifier">ptr</span><span class="special">);</span>
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</pre>
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<p>
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In practice, this limitation is not important, since a user almost never wants
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to point to this address.
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</p>
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<p>
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<code class="computeroutput"><a class="link" href="../boost/interprocess/offset_ptr.html" title="Class template offset_ptr">offset_ptr</a></code> offers
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all pointer-like operations and random_access_iterator typedefs, so it can
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be used in STL algorithms requiring random access iterators and detected via
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traits. For more information about the members and operations of the class,
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see <code class="computeroutput"><a class="link" href="../boost/interprocess/offset_ptr.html" title="Class template offset_ptr">offset_ptr reference</a></code>.
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</p>
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<td align="left"></td>
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<td align="right"><div class="copyright-footer">Copyright © 2005-2015 Ion Gaztanaga<p>
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Distributed under the Boost Software License, Version 1.0. (See accompanying
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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>)
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