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| <div class="titlepage"><div><div><h2 class="title" style="clear: both"> |
| <a name="interprocess.offset_ptr"></a><a class="link" href="offset_ptr.html" title="Mapping Address Independent Pointer: offset_ptr">Mapping Address Independent Pointer: |
| offset_ptr</a> |
| </h2></div></div></div> |
| <p> |
| When creating shared memory and memory mapped files to communicate two processes |
| the memory segment can be mapped in a different address in each process: |
| </p> |
| <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> |
| |
| <span class="comment">// ...</span> |
| |
| <span class="keyword">using</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">interprocess</span><span class="special">;</span> |
| |
| <span class="comment">//Open a shared memory segment</span> |
| <span class="identifier">shared_memory_object</span> <span class="identifier">shm_obj</span> |
| <span class="special">(</span><span class="identifier">open_only</span> <span class="comment">//open or create</span> |
| <span class="special">,</span><span class="string">"shared_memory"</span> <span class="comment">//name</span> |
| <span class="special">,</span><span class="identifier">read_only</span> <span class="comment">//read-only mode</span> |
| <span class="special">);</span> |
| |
| <span class="comment">//Map the whole shared memory</span> |
| <span class="identifier">mapped_region</span> <span class="identifier">region</span> |
| <span class="special">(</span> <span class="identifier">shm</span> <span class="comment">//Memory-mappable object</span> |
| <span class="special">,</span> <span class="identifier">read_write</span> <span class="comment">//Access mode</span> |
| <span class="special">);</span> |
| |
| <span class="comment">//This address can be different in each process</span> |
| <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> |
| </pre> |
| <p> |
| This makes the creation of complex objects in mapped regions difficult: a C++ |
| class instance placed in a mapped region might have a pointer pointing to another |
| object also placed in the mapped region. Since the pointer stores an absolute |
| address, that address is only valid for the process that placed the object |
| there unless all processes map the mapped region in the same address. |
| </p> |
| <p> |
| 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 |
| addresses. The offset between two objects in a mapped region is the same for |
| any process that maps the mapped region, even if that region is placed in different |
| base addresses. To facilitate the use of offsets, <span class="bold"><strong>Boost.Interprocess</strong></span> |
| offers <code class="computeroutput"><a class="link" href="../boost/interprocess/offset_ptr.html" title="Class template offset_ptr">offset_ptr</a></code>. |
| </p> |
| <p> |
| <code class="computeroutput"><a class="link" href="../boost/interprocess/offset_ptr.html" title="Class template offset_ptr">offset_ptr</a></code> wraps |
| all the background operations needed to offer a pointer-like interface. The |
| class interface is inspired in Boost Smart Pointers and this smart pointer |
| stores the offset (distance in bytes) between the pointee's address and it's |
| own <code class="computeroutput"><span class="keyword">this</span></code> pointer. Imagine a structure |
| in a common 32 bit processor: |
| </p> |
| <pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">structure</span> |
| <span class="special">{</span> |
| <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> |
| <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> |
| <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> |
| <span class="special">};</span> |
| |
| <span class="comment">//...</span> |
| |
| <span class="identifier">structure</span> <span class="identifier">s</span><span class="special">;</span> |
| |
| <span class="comment">//Assign the address of "integer1" to "ptr".</span> |
| <span class="comment">//"ptr" will store internally "-4":</span> |
| <span class="comment">// (char*)&s.integer1 - (char*)&s.ptr;</span> |
| <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> |
| |
| <span class="comment">//Assign the address of "integer2" to "ptr".</span> |
| <span class="comment">//"ptr" will store internally "4":</span> |
| <span class="comment">// (char*)&s.integer2 - (char*)&s.ptr;</span> |
| <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> |
| </pre> |
| <p> |
| One of the big problems of <code class="computeroutput"><span class="identifier">offset_ptr</span></code> |
| is the representation of the null pointer. The null pointer can't be safely |
| represented like an offset, since the absolute address 0 is always outside |
| of the mapped region. Due to the fact that the segment can be mapped in a different |
| 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. |
| </p> |
| <p> |
| Some implementations choose the offset 0 (that is, an <code class="computeroutput"><span class="identifier">offset_ptr</span></code> |
| pointing to itself) as the null pointer pointer representation but this is |
| not valid for many use cases since many times structures like linked lists |
| or nodes from STL containers point to themselves (the end node in an empty |
| container, for example) and 0 offset value is needed. An alternative is to |
| store, in addition to the offset, a boolean to indicate if the pointer is null. |
| However, this increments the size of the pointer and hurts performance. |
| </p> |
| <p> |
| In consequence, <code class="computeroutput"><a class="link" href="../boost/interprocess/offset_ptr.html" title="Class template offset_ptr">offset_ptr</a></code> |
| defines offset 1 as the null pointer, meaning that this class <span class="bold"><strong>can't</strong></span> |
| point to the byte after its own <span class="emphasis"><em>this</em></span> pointer: |
| </p> |
| <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> |
| |
| <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> |
| |
| <span class="comment">//Pointing to the next byte of it's own address</span> |
| <span class="comment">//marks the smart pointer as null.</span> |
| <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> |
| |
| <span class="comment">//ptr is equal to null</span> |
| <span class="identifier">assert</span><span class="special">(!</span><span class="identifier">ptr</span><span class="special">);</span> |
| |
| <span class="comment">//This is the same as assigning the null value...</span> |
| <span class="identifier">ptr</span> <span class="special">=</span> <span class="number">0</span><span class="special">;</span> |
| |
| <span class="comment">//ptr is also equal to null</span> |
| <span class="identifier">assert</span><span class="special">(!</span><span class="identifier">ptr</span><span class="special">);</span> |
| </pre> |
| <p> |
| In practice, this limitation is not important, since a user almost never wants |
| to point to this address. |
| </p> |
| <p> |
| <code class="computeroutput"><a class="link" href="../boost/interprocess/offset_ptr.html" title="Class template offset_ptr">offset_ptr</a></code> offers |
| all pointer-like operations and random_access_iterator typedefs, so it can |
| be used in STL algorithms requiring random access iterators and detected via |
| traits. For more information about the members and operations of the class, |
| see <code class="computeroutput"><a class="link" href="../boost/interprocess/offset_ptr.html" title="Class template offset_ptr">offset_ptr reference</a></code>. |
| </p> |
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| <td align="right"><div class="copyright-footer">Copyright © 2005-2012 Ion Gaztanaga<p> |
| Distributed under the Boost Software License, Version 1.0. (See accompanying |
| 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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