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<title>Mapping Address Independent Pointer: offset_ptr</title>
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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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