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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> |
| </div> |
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| <td align="right"><div class="copyright-footer">Copyright © 2005 - 2010 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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