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<a name="interprocess.managed_memory_segments"></a><a class="link" href="managed_memory_segments.html" title="Managed Memory Segments"> Managed Memory Segments</a>
</h2></div></div></div>
<div class="toc"><dl>
<dt><span class="section"><a href="managed_memory_segments.html#interprocess.managed_memory_segments.making_ipc_easy">
Making Interprocess Data Communication Easy</a></span></dt>
<dt><span class="section"><a href="managed_memory_segments.html#interprocess.managed_memory_segments.managed_shared_memory">
Managed Shared Memory</a></span></dt>
<dt><span class="section"><a href="managed_memory_segments.html#interprocess.managed_memory_segments.managed_mapped_files">
Managed Mapped File</a></span></dt>
<dt><span class="section"><a href="managed_memory_segments.html#interprocess.managed_memory_segments.managed_memory_segment_features">
Managed Memory Segment Features</a></span></dt>
<dt><span class="section"><a href="managed_memory_segments.html#interprocess.managed_memory_segments.managed_memory_segment_advanced_features">
Managed Memory Segment Advanced Features</a></span></dt>
<dt><span class="section"><a href="managed_memory_segments.html#interprocess.managed_memory_segments.managed_heap_memory_external_buffer">
Managed Heap Memory And Managed External Buffer</a></span></dt>
</dl></div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="interprocess.managed_memory_segments.making_ipc_easy"></a><a class="link" href="managed_memory_segments.html#interprocess.managed_memory_segments.making_ipc_easy" title="Making Interprocess Data Communication Easy">
Making Interprocess Data Communication Easy</a>
</h3></div></div></div>
<div class="toc"><dl>
<dt><span class="section"><a href="managed_memory_segments.html#interprocess.managed_memory_segments.making_ipc_easy.managed_memory_segments_intro">
Introduction</a></span></dt>
<dt><span class="section"><a href="managed_memory_segments.html#interprocess.managed_memory_segments.making_ipc_easy.managed_memory_segment_int">
Declaration of managed memory segment classes</a></span></dt>
</dl></div>
<div class="section">
<div class="titlepage"><div><div><h4 class="title">
<a name="interprocess.managed_memory_segments.making_ipc_easy.managed_memory_segments_intro"></a><a class="link" href="managed_memory_segments.html#interprocess.managed_memory_segments.making_ipc_easy.managed_memory_segments_intro" title="Introduction">
Introduction</a>
</h4></div></div></div>
<p>
As we have seen, <span class="bold"><strong>Boost.Interprocess</strong></span> offers
some basic classes to create shared memory objects and file mappings and
map those mappable classes to the process' address space.
</p>
<p>
However, managing those memory segments is not not easy for non-trivial
tasks. A mapped region is a fixed-length memory buffer and creating and
destroying objects of any type dynamically, requires a lot of work, since
it would require programming a memory management algorithm to allocate
portions of that segment. Many times, we also want to associate names to
objects created in shared memory, so all the processes can find the object
using the name.
</p>
<p>
<span class="bold"><strong>Boost.Interprocess</strong></span> offers 4 managed memory
segment classes:
</p>
<div class="itemizedlist"><ul class="itemizedlist" type="disc">
<li class="listitem">
To manage a shared memory mapped region (<span class="bold"><strong>basic_managed_shared_memory</strong></span>
class).
</li>
<li class="listitem">
To manage a memory mapped file (<span class="bold"><strong>basic_managed_mapped_file</strong></span>).
</li>
<li class="listitem">
To manage a heap allocated (<code class="computeroutput"><span class="keyword">operator</span>
<span class="keyword">new</span></code>) memory buffer (<span class="bold"><strong>basic_managed_heap_memory</strong></span> class).
</li>
<li class="listitem">
To manage a user provided fixed size buffer (<span class="bold"><strong>basic_managed_external_buffer</strong></span>
class).
</li>
</ul></div>
<p>
The first two classes manage memory segments that can be shared between
processes. The third is useful to create complex data-bases to be sent
though other mechanisms like message queues to other processes. The fourth
class can manage any fixed size memory buffer. The first two classes will
be explained in the next two sections. <span class="bold"><strong>basic_managed_heap_memory</strong></span>
and <span class="bold"><strong>basic_managed_external_buffer</strong></span> will
be explained later.
</p>
<p>
The most important services of a managed memory segment are:
</p>
<div class="itemizedlist"><ul class="itemizedlist" type="disc">
<li class="listitem">
Dynamic allocation of portions of a memory the segment.
</li>
<li class="listitem">
Construction of C++ objects in the memory segment. These objects can
be anonymous or we can associate a name to them.
</li>
<li class="listitem">
Searching capabilities for named objects.
</li>
<li class="listitem">
Customization of many features: memory allocation algorithm, index
types or character types.
</li>
<li class="listitem">
Atomic constructions and destructions so that if the segment is shared
between two processes it's impossible to create two objects associated
with the same name, simplifying synchronization.
</li>
</ul></div>
</div>
<div class="section">
<div class="titlepage"><div><div><h4 class="title">
<a name="interprocess.managed_memory_segments.making_ipc_easy.managed_memory_segment_int"></a><a class="link" href="managed_memory_segments.html#interprocess.managed_memory_segments.making_ipc_easy.managed_memory_segment_int" title="Declaration of managed memory segment classes">
Declaration of managed memory segment classes</a>
</h4></div></div></div>
<p>
All <span class="bold"><strong>Boost.Interprocess</strong></span> managed memory
segment classes are templatized classes that can be customized by the user:
</p>
<p>
</p>
<pre class="programlisting"><span class="keyword">template</span>
<span class="special"><</span>
<span class="keyword">class</span> <span class="identifier">CharType</span><span class="special">,</span>
<span class="keyword">class</span> <span class="identifier">MemoryAlgorithm</span><span class="special">,</span>
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">IndexConfig</span><span class="special">></span> <span class="keyword">class</span> <span class="identifier">IndexType</span>
<span class="special">></span>
<span class="keyword">class</span> <span class="identifier">basic_managed_shared_memory</span> <span class="special">/</span> <span class="identifier">basic_managed_mapped_file</span> <span class="special">/</span>
<span class="identifier">basic_managed_heap_memory</span> <span class="special">/</span> <span class="identifier">basic_external_buffer</span><span class="special">;</span>
</pre>
<p>
These classes can be customized with the following template parameters:
</p>
<div class="itemizedlist"><ul class="itemizedlist" type="disc">
<li class="listitem">
<span class="bold"><strong>CharType</strong></span> is the type of the character
that will be used to identify the created named objects (for example,
<span class="bold"><strong>char</strong></span> or <span class="bold"><strong>wchar_t</strong></span>)
</li>
<li class="listitem">
<span class="bold"><strong>MemoryAlgorithm</strong></span> is the memory algorithm
used to allocate portions of the segment (for example, rbtree_best_fit
). The internal typedefs of the memory algorithm also define:
<div class="itemizedlist"><ul class="itemizedlist" type="circle">
<li class="listitem">
The synchronization type (<code class="computeroutput"><span class="identifier">MemoryAlgorithm</span><span class="special">::</span><span class="identifier">mutex_family</span></code>)
to be used in all allocation operations. This allows the use
of user-defined mutexes or avoiding internal locking (maybe code
will be externally synchronized by the user).
</li>
<li class="listitem">
The Pointer type (<code class="computeroutput"><span class="identifier">MemoryAlgorithm</span><span class="special">::</span><span class="identifier">void_pointer</span></code>)
to be used by the memory allocation algorithm or additional helper
structures (like a map to maintain object/name associations).
All STL compatible allocators and containers to be used with
this managed memory segment will use this pointer type. The pointer
type will define if the managed memory segment can be mapped
between several processes. For example, if <code class="computeroutput"><span class="identifier">void_pointer</span></code>
is <code class="computeroutput"><span class="identifier">offset_ptr</span><span class="special"><</span><span class="keyword">void</span><span class="special">></span></code> we will be able to map the
managed segment in different base addresses in each process.
If <code class="computeroutput"><span class="identifier">void_pointer</span></code>
is <code class="computeroutput"><span class="keyword">void</span><span class="special">*</span></code>
only fixed address mapping could be used.
</li>
<li class="listitem">
See <a class="link" href="customizing_interprocess.html#interprocess.customizing_interprocess.custom_interprocess_alloc" title="Writing a new shared memory allocation algorithm">Writing
a new memory allocation algorithm</a> for more details about
memory algorithms.
</li>
</ul></div>
</li>
<li class="listitem">
<span class="bold"><strong>IndexType</strong></span> is the type of index that
will be used to store the name-object association (for example, a map,
a hash-map, or an ordered vector).
</li>
</ul></div>
<p>
This way, we can use <code class="computeroutput"><span class="keyword">char</span></code>
or <code class="computeroutput"><span class="keyword">wchar_t</span></code> strings to identify
created C++ objects in the memory segment, we can plug new shared memory
allocation algorithms, and use the index type that is best suited to our
needs.
</p>
</div>
</div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="interprocess.managed_memory_segments.managed_shared_memory"></a><a class="link" href="managed_memory_segments.html#interprocess.managed_memory_segments.managed_shared_memory" title="Managed Shared Memory">
Managed Shared Memory</a>
</h3></div></div></div>
<div class="toc"><dl>
<dt><span class="section"><a href="managed_memory_segments.html#interprocess.managed_memory_segments.managed_shared_memory.managed_memory_common_shm">
Common Managed Shared Memory Classes</a></span></dt>
<dt><span class="section"><a href="managed_memory_segments.html#interprocess.managed_memory_segments.managed_shared_memory.constructing_managed_shared_memories">
Constructing Managed Shared Memory</a></span></dt>
<dt><span class="section"><a href="managed_memory_segments.html#interprocess.managed_memory_segments.managed_shared_memory.windows_managed_memory_common_shm">
Using native windows shared memory</a></span></dt>
</dl></div>
<div class="section">
<div class="titlepage"><div><div><h4 class="title">
<a name="interprocess.managed_memory_segments.managed_shared_memory.managed_memory_common_shm"></a><a class="link" href="managed_memory_segments.html#interprocess.managed_memory_segments.managed_shared_memory.managed_memory_common_shm" title="Common Managed Shared Memory Classes">
Common Managed Shared Memory Classes</a>
</h4></div></div></div>
<p>
As seen, <span class="bold"><strong>basic_managed_shared_memory</strong></span> offers
a great variety of customization. But for the average user, a common, default
shared memory named object creation is needed. Because of this, <span class="bold"><strong>Boost.Interprocess</strong></span> defines the most common managed
shared memory specializations:
</p>
<p>
</p>
<pre class="programlisting"><span class="comment">//!Defines a managed shared memory with c-strings as keys for named objects,
</span><span class="comment">//!the default memory algorithm (with process-shared mutexes,
</span><span class="comment">//!and offset_ptr as internal pointers) as memory allocation algorithm
</span><span class="comment">//!and the default index type as the index.
</span><span class="comment">//!This class allows the shared memory to be mapped in different base
</span><span class="comment">//!in different processes
</span><span class="keyword">typedef</span>
<span class="identifier">basic_managed_shared_memory</span><span class="special"><</span><span class="keyword">char</span>
<span class="special">,/*</span><span class="identifier">Default</span> <span class="identifier">memory</span> <span class="identifier">algorithm</span> <span class="identifier">defining</span> <span class="identifier">offset_ptr</span><span class="special"><</span><span class="keyword">void</span><span class="special">></span> <span class="identifier">as</span> <span class="identifier">void_pointer</span><span class="special">*/</span>
<span class="special">,/*</span><span class="identifier">Default</span> <span class="identifier">index</span> <span class="identifier">type</span><span class="special">*/></span>
<span class="identifier">managed_shared_memory</span><span class="special">;</span>
<span class="comment">//!Defines a managed shared memory with wide strings as keys for named objects,
</span><span class="comment">//!the default memory algorithm (with process-shared mutexes,
</span><span class="comment">//!and offset_ptr as internal pointers) as memory allocation algorithm
</span><span class="comment">//!and the default index type as the index.
</span><span class="comment">//!This class allows the shared memory to be mapped in different base
</span><span class="comment">//!in different processes
</span><span class="keyword">typedef</span>
<span class="identifier">basic_managed_shared_memory</span><span class="special"><</span><span class="keyword">wchar_t</span>
<span class="special">,/*</span><span class="identifier">Default</span> <span class="identifier">memory</span> <span class="identifier">algorithm</span> <span class="identifier">defining</span> <span class="identifier">offset_ptr</span><span class="special"><</span><span class="keyword">void</span><span class="special">></span> <span class="identifier">as</span> <span class="identifier">void_pointer</span><span class="special">*/</span>
<span class="special">,/*</span><span class="identifier">Default</span> <span class="identifier">index</span> <span class="identifier">type</span><span class="special">*/></span>
<span class="identifier">wmanaged_shared_memory</span><span class="special">;</span>
</pre>
<p>
<code class="computeroutput"><span class="identifier">managed_shared_memory</span></code> allocates
objects in shared memory associated with a c-string and <code class="computeroutput"><span class="identifier">wmanaged_shared_memory</span></code>
allocates objects in shared memory associated with a wchar_t null terminated
string. Both define the pointer type as <code class="computeroutput"><span class="identifier">offset_ptr</span><span class="special"><</span><span class="keyword">void</span><span class="special">></span></code> so they can be used to map the shared
memory at different base addresses in different processes.
</p>
<p>
If the user wants to map the shared memory in the same address in all processes
and want to use raw pointers internally instead of offset pointers, <span class="bold"><strong>Boost.Interprocess</strong></span> defines the following types:
</p>
<p>
</p>
<pre class="programlisting"><span class="comment">//!Defines a managed shared memory with c-strings as keys for named objects,
</span><span class="comment">//!the default memory algorithm (with process-shared mutexes,
</span><span class="comment">//!and offset_ptr as internal pointers) as memory allocation algorithm
</span><span class="comment">//!and the default index type as the index.
</span><span class="comment">//!This class allows the shared memory to be mapped in different base
</span><span class="comment">//!in different processes*/
</span><span class="keyword">typedef</span> <span class="identifier">basic_managed_shared_memory</span>
<span class="special"><</span><span class="keyword">char</span>
<span class="special">,/*</span><span class="identifier">Default</span> <span class="identifier">memory</span> <span class="identifier">algorithm</span> <span class="identifier">defining</span> <span class="keyword">void</span> <span class="special">*</span> <span class="identifier">as</span> <span class="identifier">void_pointer</span><span class="special">*/</span>
<span class="special">,/*</span><span class="identifier">Default</span> <span class="identifier">index</span> <span class="identifier">type</span><span class="special">*/></span>
<span class="identifier">fixed_managed_shared_memory</span><span class="special">;</span>
<span class="comment">//!Defines a managed shared memory with wide strings as keys for named objects,
</span><span class="comment">//!the default memory algorithm (with process-shared mutexes,
</span><span class="comment">//!and offset_ptr as internal pointers) as memory allocation algorithm
</span><span class="comment">//!and the default index type as the index.
</span><span class="comment">//!This class allows the shared memory to be mapped in different base
</span><span class="comment">//!in different processes
</span><span class="keyword">typedef</span> <span class="identifier">basic_managed_shared_memory</span>
<span class="special"><</span><span class="keyword">wchar_t</span>
<span class="special">,/*</span><span class="identifier">Default</span> <span class="identifier">memory</span> <span class="identifier">algorithm</span> <span class="identifier">defining</span> <span class="keyword">void</span> <span class="special">*</span> <span class="identifier">as</span> <span class="identifier">void_pointer</span><span class="special">*/</span>
<span class="special">,/*</span><span class="identifier">Default</span> <span class="identifier">index</span> <span class="identifier">type</span><span class="special">*/></span>
<span class="identifier">wfixed_managed_shared_memory</span><span class="special">;</span>
</pre>
</div>
<div class="section">
<div class="titlepage"><div><div><h4 class="title">
<a name="interprocess.managed_memory_segments.managed_shared_memory.constructing_managed_shared_memories"></a><a class="link" href="managed_memory_segments.html#interprocess.managed_memory_segments.managed_shared_memory.constructing_managed_shared_memories" title="Constructing Managed Shared Memory">
Constructing Managed Shared Memory</a>
</h4></div></div></div>
<p>
Managed shared memory is an advanced class that combines a shared memory
object and a mapped region that covers all the shared memory object. That
means that when we <span class="bold"><strong>create</strong></span> a new managed
shared memory:
</p>
<div class="itemizedlist"><ul class="itemizedlist" type="disc">
<li class="listitem">
A new shared memory object is created.
</li>
<li class="listitem">
The whole shared memory object is mapped in the process' address space.
</li>
<li class="listitem">
Some helper objects are constructed (name-object index, internal synchronization
objects, internal variables...) in the mapped region to implement managed
memory segment features.
</li>
</ul></div>
<p>
When we <span class="bold"><strong>open</strong></span> a managed shared memory
</p>
<div class="itemizedlist"><ul class="itemizedlist" type="disc">
<li class="listitem">
A shared memory object is opened.
</li>
<li class="listitem">
The whole shared memory object is mapped in the process' address space.
</li>
</ul></div>
<p>
To use a managed shared memory, you must include the following header:
</p>
<p>
</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">managed_shared_memory</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span>
</pre>
<p>
</p>
<pre class="programlisting"><span class="comment">//1. Creates a new shared memory object
</span><span class="comment">// called "MySharedMemory".
</span><span class="comment">//2. Maps the whole object to this
</span><span class="comment">// process' address space.
</span><span class="comment">//3. Constructs some objects in shared memory
</span><span class="comment">// to implement managed features.
</span><span class="comment">//!! If anything fails, throws interprocess_exception
</span><span class="comment">//
</span><span class="identifier">managed_shared_memory</span> <span class="identifier">segment</span> <span class="special">(</span> <span class="identifier">create_only</span>
<span class="special">,</span> <span class="string">"MySharedMemory"</span> <span class="comment">//Shared memory object name
</span> <span class="special">,</span> <span class="number">65536</span><span class="special">);</span> <span class="comment">//Shared memory object size in bytes
</span></pre>
<p>
</p>
<pre class="programlisting"><span class="comment">//1. Opens a shared memory object
</span><span class="comment">// called "MySharedMemory".
</span><span class="comment">//2. Maps the whole object to this
</span><span class="comment">// process' address space.
</span><span class="comment">//3. Obtains pointers to constructed internal objects
</span><span class="comment">// to implement managed features.
</span><span class="comment">//!! If anything fails, throws interprocess_exception
</span><span class="comment">//
</span><span class="identifier">managed_shared_memory</span> <span class="identifier">segment</span> <span class="special">(</span><span class="identifier">open_only</span><span class="special">,</span> <span class="string">"MySharedMemory"</span><span class="special">);//</span><span class="identifier">Shared</span> <span class="identifier">memory</span> <span class="identifier">object</span> <span class="identifier">name</span>
</pre>
<p>
</p>
<pre class="programlisting"><span class="comment">//1. If the segment was previously created
</span><span class="comment">// equivalent to "open_only".
</span><span class="comment">//2. Otherwise, equivalent to "open_only" (size is ignored)
</span><span class="comment">//!! If anything fails, throws interprocess_exception
</span><span class="comment">//
</span><span class="identifier">managed_shared_memory</span> <span class="identifier">segment</span> <span class="special">(</span> <span class="identifier">open_or_create</span>
<span class="special">,</span> <span class="string">"MySharedMemory"</span> <span class="comment">//Shared memory object name
</span> <span class="special">,</span> <span class="number">65536</span><span class="special">);</span> <span class="comment">//Shared memory object size in bytes
</span></pre>
<p>
When the <code class="computeroutput"><span class="identifier">managed_shared_memory</span></code>
object is destroyed, the shared memory object is automatically unmapped,
and all the resources are freed. To remove the shared memory object from
the system you must use the <code class="computeroutput"><span class="identifier">shared_memory_object</span><span class="special">::</span><span class="identifier">remove</span></code>
function. Shared memory object removing might fail if any process still
has the shared memory object mapped.
</p>
<p>
The user can also map the managed shared memory in a fixed address. This
option is essential when using using <code class="computeroutput"><span class="identifier">fixed_managed_shared_memory</span></code>.
To do this, just add the mapping address as an extra parameter:
</p>
<p>
</p>
<pre class="programlisting"><span class="identifier">fixed_managed_shared_memory</span> <span class="identifier">segment</span> <span class="special">(</span><span class="identifier">open_only</span> <span class="special">,</span><span class="string">"MyFixedAddressSharedMemory"</span> <span class="comment">//Shared memory object name
</span> <span class="special">,(</span><span class="keyword">void</span><span class="special">*)</span><span class="number">0x30000000</span> <span class="comment">//Mapping address
</span></pre>
</div>
<div class="section">
<div class="titlepage"><div><div><h4 class="title">
<a name="interprocess.managed_memory_segments.managed_shared_memory.windows_managed_memory_common_shm"></a><a class="link" href="managed_memory_segments.html#interprocess.managed_memory_segments.managed_shared_memory.windows_managed_memory_common_shm" title="Using native windows shared memory">
Using native windows shared memory</a>
</h4></div></div></div>
<p>
Windows users might also want to use native windows shared memory instead
of the portable <code class="computeroutput"><a class="link" href="../boost/interprocess/shared_memory_object.html" title="Class shared_memory_object">shared_memory_object</a></code>
based managed memory. This is achieved through the <code class="computeroutput"><a class="link" href="../boost/interprocess/basic_managed_windows_s_id820482.html" title="Class template basic_managed_windows_shared_memory">basic_managed_windows_shared_memory</a></code>
class. To use it just include:
</p>
<p>
</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">managed_windows_shared_memory</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span>
</pre>
<p>
This class has the same interface as <code class="computeroutput"><a class="link" href="../boost/interprocess/basic_managed_shared_me_id820177.html" title="Class template basic_managed_shared_memory">basic_managed_shared_memory</a></code>
but uses native windows shared memory. Note that this managed class has
the same lifetime issues as the windows shared memory: when the last process
attached to the windows shared memory is detached from the memory (or ends/crashes)
the memory is destroyed. So there is no persistence support for windows
shared memory.
</p>
</div>
<p>
For more information about managed shared memory capabilities, see <code class="computeroutput"><a class="link" href="../boost/interprocess/basic_managed_shared_me_id820177.html" title="Class template basic_managed_shared_memory">basic_managed_shared_memory</a></code>
class reference.
</p>
</div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="interprocess.managed_memory_segments.managed_mapped_files"></a><a class="link" href="managed_memory_segments.html#interprocess.managed_memory_segments.managed_mapped_files" title="Managed Mapped File">
Managed Mapped File</a>
</h3></div></div></div>
<div class="toc"><dl>
<dt><span class="section"><a href="managed_memory_segments.html#interprocess.managed_memory_segments.managed_mapped_files.managed_memory_common_mfile">
Common Managed Mapped Files</a></span></dt>
<dt><span class="section"><a href="managed_memory_segments.html#interprocess.managed_memory_segments.managed_mapped_files.constructing_managed_mapped_files">
Constructing Managed Mapped Files</a></span></dt>
</dl></div>
<div class="section">
<div class="titlepage"><div><div><h4 class="title">
<a name="interprocess.managed_memory_segments.managed_mapped_files.managed_memory_common_mfile"></a><a class="link" href="managed_memory_segments.html#interprocess.managed_memory_segments.managed_mapped_files.managed_memory_common_mfile" title="Common Managed Mapped Files">
Common Managed Mapped Files</a>
</h4></div></div></div>
<p>
As seen, <span class="bold"><strong>basic_managed_mapped_file</strong></span> offers
a great variety of customization. But for the average user, a common, default
shared memory named object creation is needed. Because of this, <span class="bold"><strong>Boost.Interprocess</strong></span> defines the most common managed
mapped file specializations:
</p>
<p>
</p>
<pre class="programlisting"><span class="comment">//Named object creation managed memory segment
</span><span class="comment">//All objects are constructed in the memory-mapped file
</span><span class="comment">// Names are c-strings,
</span><span class="comment">// Default memory management algorithm(rbtree_best_fit with no mutexes)
</span><span class="comment">// Name-object mappings are stored in the default index type (flat_map)
</span><span class="keyword">typedef</span> <span class="identifier">basic_managed_mapped_file</span> <span class="special"><</span>
<span class="keyword">char</span><span class="special">,</span>
<span class="identifier">rbtree_best_fit</span><span class="special"><</span><span class="identifier">mutex_family</span><span class="special">,</span> <span class="identifier">offset_ptr</span><span class="special"><</span><span class="keyword">void</span><span class="special">></span> <span class="special">>,</span>
<span class="identifier">flat_map_index</span>
<span class="special">></span> <span class="identifier">managed_mapped_file</span><span class="special">;</span>
<span class="comment">//Named object creation managed memory segment
</span><span class="comment">//All objects are constructed in the memory-mapped file
</span><span class="comment">// Names are wide-strings,
</span><span class="comment">// Default memory management algorithm(rbtree_best_fit with no mutexes)
</span><span class="comment">// Name-object mappings are stored in the default index type (flat_map)
</span><span class="keyword">typedef</span> <span class="identifier">basic_managed_mapped_file</span><span class="special"><</span>
<span class="keyword">wchar_t</span><span class="special">,</span>
<span class="identifier">rbtree_best_fit</span><span class="special"><</span><span class="identifier">mutex_family</span><span class="special">,</span> <span class="identifier">offset_ptr</span><span class="special"><</span><span class="keyword">void</span><span class="special">></span> <span class="special">>,</span>
<span class="identifier">flat_map_index</span>
<span class="special">></span> <span class="identifier">wmanaged_mapped_file</span><span class="special">;</span>
</pre>
<p>
<code class="computeroutput"><span class="identifier">managed_mapped_file</span></code> allocates
objects in a memory mapped files associated with a c-string and <code class="computeroutput"><span class="identifier">wmanaged_mapped_file</span></code> allocates objects
in a memory mapped file associated with a wchar_t null terminated string.
Both define the pointer type as <code class="computeroutput"><span class="identifier">offset_ptr</span><span class="special"><</span><span class="keyword">void</span><span class="special">></span></code> so they can be used to map the file
at different base addresses in different processes.
</p>
</div>
<div class="section">
<div class="titlepage"><div><div><h4 class="title">
<a name="interprocess.managed_memory_segments.managed_mapped_files.constructing_managed_mapped_files"></a><a class="link" href="managed_memory_segments.html#interprocess.managed_memory_segments.managed_mapped_files.constructing_managed_mapped_files" title="Constructing Managed Mapped Files">
Constructing Managed Mapped Files</a>
</h4></div></div></div>
<p>
Managed mapped file is an advanced class that combines a file and a mapped
region that covers all the file. That means that when we <span class="bold"><strong>create</strong></span>
a new managed mapped file:
</p>
<div class="itemizedlist"><ul class="itemizedlist" type="disc">
<li class="listitem">
A new file is created.
</li>
<li class="listitem">
The whole file is mapped in the process' address space.
</li>
<li class="listitem">
Some helper objects are constructed (name-object index, internal synchronization
objects, internal variables...) in the mapped region to implement managed
memory segment features.
</li>
</ul></div>
<p>
When we <span class="bold"><strong>open</strong></span> a managed mapped file
</p>
<div class="itemizedlist"><ul class="itemizedlist" type="disc">
<li class="listitem">
A file is opened.
</li>
<li class="listitem">
The whole file is mapped in the process' address space.
</li>
</ul></div>
<p>
To use a managed mapped file, you must include the following header:
</p>
<p>
</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">managed_mapped_file</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span>
</pre>
<p>
</p>
<pre class="programlisting"><span class="comment">//1. Creates a new file
</span><span class="comment">// called "MyMappedFile".
</span><span class="comment">//2. Maps the whole file to this
</span><span class="comment">// process' address space.
</span><span class="comment">//3. Constructs some objects in the memory mapped
</span><span class="comment">// file to implement managed features.
</span><span class="comment">//!! If anything fails, throws interprocess_exception
</span><span class="comment">//
</span><span class="identifier">managed_mapped_file</span> <span class="identifier">mfile</span> <span class="special">(</span><span class="identifier">create_only</span><span class="special">,</span> <span class="string">"MyMappedFile"</span><span class="special">,</span> <span class="comment">//Mapped file name 65536); //Mapped file size
</span></pre>
<p>
<span class="emphasis"><em>/1. Creates a new file /</em></span> called "MyMappedFile".
<span class="emphasis"><em>/2. Maps the whole file to this /</em></span> process' address
space. <span class="emphasis"><em>/3. Constructs some objects in the memory mapped /</em></span>
file to implement managed features. <span class="emphasis"><em>/!! If anything fails, throws
interprocess_exception /</em></span> managed_mapped_file mfile (create_only,
"MyMappedFile", //Mapped file name 65536); //Mapped file size
</p>
<pre class="programlisting"><span class="comment">//1. Opens a file
</span><span class="comment">// called "MyMappedFile".
</span><span class="comment">//2. Maps the whole file to this
</span><span class="comment">// process' address space.
</span><span class="comment">//3. Obtains pointers to constructed internal objects
</span><span class="comment">// to implement managed features.
</span><span class="comment">//!! If anything fails, throws interprocess_exception
</span><span class="comment">//
</span><span class="identifier">managed_mapped_file</span> <span class="identifier">mfile</span> <span class="special">(</span><span class="identifier">open_only</span><span class="special">,</span> <span class="string">"MyMappedFile"</span><span class="special">);</span> <span class="comment">//Mapped file name[c++]
</span>
<span class="comment">//1. If the file was previously created
</span><span class="comment">// equivalent to "open_only".
</span><span class="comment">//2. Otherwise, equivalent to "open_only" (size is ignored)
</span><span class="comment">//
</span><span class="comment">//!! If anything fails, throws interprocess_exception
</span><span class="comment">//
</span><span class="identifier">managed_mapped_file</span> <span class="identifier">mfile</span> <span class="special">(</span><span class="identifier">open_or_create</span><span class="special">,</span> <span class="string">"MyMappedFile"</span><span class="special">,</span> <span class="comment">//Mapped file name 65536); //Mapped file size
</span></pre>
<p>
<span class="emphasis"><em>/1. Opens a file /</em></span> called "MyMappedFile".
<span class="emphasis"><em>/2. Maps the whole file to this /</em></span> process' address
space. <span class="emphasis"><em>/3. Obtains pointers to constructed internal objects /</em></span>
to implement managed features. <span class="emphasis"><em>/!! If anything fails, throws
interprocess_exception /</em></span> managed_mapped_file mfile (open_only,
"MyMappedFile"); //Mapped file name
</p>
<pre class="programlisting"><span class="comment">//1. If the file was previously created
</span><span class="comment">// equivalent to "open_only".
</span><span class="comment">//2. Otherwise, equivalent to "open_only" (size is ignored)
</span><span class="comment">//
</span><span class="comment">//!! If anything fails, throws interprocess_exception
</span><span class="comment">//
</span><span class="identifier">managed_mapped_file</span> <span class="identifier">mfile</span> <span class="special">(</span><span class="identifier">open_or_create</span><span class="special">,</span> <span class="string">"MyMappedFile"</span><span class="special">,</span> <span class="comment">//Mapped file name 65536); //Mapped file size
</span></pre>
<p>
<span class="emphasis"><em>/1. If the file was previously created /</em></span> equivalent
to "open_only". <span class="emphasis"><em>/2. Otherwise, equivalent to "open_only"
(size is ignored) /</em></span> <span class="emphasis"><em>/!! If anything fails, throws
interprocess_exception /</em></span> managed_mapped_file mfile (open_or_create,
"MyMappedFile", //Mapped file name 65536); //Mapped file size
When the <code class="computeroutput"><span class="identifier">managed_mapped_file</span></code>
object is destroyed, the file is automatically unmapped, and all the resources
are freed. To remove the file from the filesystem you could use standard
C <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">remove</span></code> or <span class="bold"><strong>Boost.Filesystem</strong></span>'s
<code class="computeroutput"><span class="identifier">remove</span><span class="special">()</span></code>
functions, but file removing might fail if any process still has the file
mapped in memory or the file is open by any process.
</p>
<p>
To obtain a more portable behaviour, use <code class="computeroutput"><span class="identifier">file_mapping</span><span class="special">::</span><span class="identifier">remove</span><span class="special">(</span><span class="keyword">const</span> <span class="keyword">char</span> <span class="special">*)</span></code>
operation, which will remove the file even if it's being mapped. However,
removal will fail in some OS systems if the file (eg. by C++ file streams)
and no delete share permission was granted to the file. But in most common
cases <code class="computeroutput"><span class="identifier">file_mapping</span><span class="special">::</span><span class="identifier">remove</span></code> is portable enough.
</p>
</div>
<p>
For more information about managed mapped file capabilities, see <code class="computeroutput"><a class="link" href="../boost/interprocess/basic_managed_mapped_file.html" title="Class template basic_managed_mapped_file">basic_managed_mapped_file</a></code>
class reference.
</p>
</div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="interprocess.managed_memory_segments.managed_memory_segment_features"></a><a class="link" href="managed_memory_segments.html#interprocess.managed_memory_segments.managed_memory_segment_features" title="Managed Memory Segment Features">
Managed Memory Segment Features</a>
</h3></div></div></div>
<div class="toc"><dl>
<dt><span class="section"><a href="managed_memory_segments.html#interprocess.managed_memory_segments.managed_memory_segment_features.allocate_deallocate">
Allocating fragments of a managed memory segment</a></span></dt>
<dt><span class="section"><a href="managed_memory_segments.html#interprocess.managed_memory_segments.managed_memory_segment_features.segment_offset">
Obtaining handles to identify data</a></span></dt>
<dt><span class="section"><a href="managed_memory_segments.html#interprocess.managed_memory_segments.managed_memory_segment_features.allocation_types">
Object construction function family</a></span></dt>
<dt><span class="section"><a href="managed_memory_segments.html#interprocess.managed_memory_segments.managed_memory_segment_features.anonymous">
Anonymous instance construction</a></span></dt>
<dt><span class="section"><a href="managed_memory_segments.html#interprocess.managed_memory_segments.managed_memory_segment_features.unique">
Unique instance construction</a></span></dt>
<dt><span class="section"><a href="managed_memory_segments.html#interprocess.managed_memory_segments.managed_memory_segment_features.synchronization">
Synchronization guarantees</a></span></dt>
<dt><span class="section"><a href="managed_memory_segments.html#interprocess.managed_memory_segments.managed_memory_segment_features.index_types">
Index types for name/object mappings</a></span></dt>
<dt><span class="section"><a href="managed_memory_segments.html#interprocess.managed_memory_segments.managed_memory_segment_features.managed_memory_segment_segment_manager">
Segment Manager</a></span></dt>
<dt><span class="section"><a href="managed_memory_segments.html#interprocess.managed_memory_segments.managed_memory_segment_features.managed_memory_segment_information">
Obtaining information about a constructed object</a></span></dt>
<dt><span class="section"><a href="managed_memory_segments.html#interprocess.managed_memory_segments.managed_memory_segment_features.managed_memory_segment_atomic_func">
Executing an object function atomically</a></span></dt>
</dl></div>
<p>
The following features are common to all managed memory segment classes,
but we will use managed shared memory in our examples. We can do the same
with memory mapped files or other managed memory segment classes.
</p>
<div class="section">
<div class="titlepage"><div><div><h4 class="title">
<a name="interprocess.managed_memory_segments.managed_memory_segment_features.allocate_deallocate"></a><a class="link" href="managed_memory_segments.html#interprocess.managed_memory_segments.managed_memory_segment_features.allocate_deallocate" title="Allocating fragments of a managed memory segment">
Allocating fragments of a managed memory segment</a>
</h4></div></div></div>
<p>
If a basic raw-byte allocation is needed from a managed memory segment,
(for example, a managed shared memory), to implement top-level interprocess
communications, this class offers <span class="bold"><strong>allocate</strong></span>
and <span class="bold"><strong>deallocate</strong></span> functions. The allocation
function comes with throwing and no throwing versions. Throwing version
throws boost::interprocess::bad_alloc (which derives from <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">bad_alloc</span></code>) if there is no more memory
and the non-throwing version returns 0 pointer.
</p>
<p>
</p>
<p>
</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">managed_shared_memory</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span>
<span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</span>
<span class="special">{</span>
<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="comment">//Remove shared memory on construction and destruction
</span> <span class="keyword">struct</span> <span class="identifier">shm_remove</span>
<span class="special">{</span>
<span class="identifier">shm_remove</span><span class="special">()</span> <span class="special">{</span> <span class="identifier">shared_memory_object</span><span class="special">::</span><span class="identifier">remove</span><span class="special">(</span><span class="string">"MySharedMemory"</span><span class="special">);</span> <span class="special">}</span>
<span class="special">~</span><span class="identifier">shm_remove</span><span class="special">(){</span> <span class="identifier">shared_memory_object</span><span class="special">::</span><span class="identifier">remove</span><span class="special">(</span><span class="string">"MySharedMemory"</span><span class="special">);</span> <span class="special">}</span>
<span class="special">}</span> <span class="identifier">remover</span><span class="special">;</span>
<span class="comment">//Managed memory segment that allocates portions of a shared memory
</span> <span class="comment">//segment with the default management algorithm
</span> <span class="identifier">managed_shared_memory</span> <span class="identifier">managed_shm</span><span class="special">(</span><span class="identifier">create_only</span><span class="special">,</span><span class="string">"MySharedMemory"</span><span class="special">,</span> <span class="number">65536</span><span class="special">);</span>
<span class="comment">//Allocate 100 bytes of memory from segment, throwing version
</span> <span class="keyword">void</span> <span class="special">*</span><span class="identifier">ptr</span> <span class="special">=</span> <span class="identifier">managed_shm</span><span class="special">.</span><span class="identifier">allocate</span><span class="special">(</span><span class="number">100</span><span class="special">);</span>
<span class="comment">//Deallocate it
</span> <span class="identifier">managed_shm</span><span class="special">.</span><span class="identifier">deallocate</span><span class="special">(</span><span class="identifier">ptr</span><span class="special">);</span>
<span class="comment">//Non throwing version
</span> <span class="identifier">ptr</span> <span class="special">=</span> <span class="identifier">managed_shm</span><span class="special">.</span><span class="identifier">allocate</span><span class="special">(</span><span class="number">100</span><span class="special">,</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">nothrow</span><span class="special">);</span>
<span class="comment">//Deallocate it
</span> <span class="identifier">managed_shm</span><span class="special">.</span><span class="identifier">deallocate</span><span class="special">(</span><span class="identifier">ptr</span><span class="special">);</span>
<span class="keyword">return</span> <span class="number">0</span><span class="special">;</span>
<span class="special">}</span>
</pre>
<p>
</p>
<p>
</p>
</div>
<div class="section">
<div class="titlepage"><div><div><h4 class="title">
<a name="interprocess.managed_memory_segments.managed_memory_segment_features.segment_offset"></a><a class="link" href="managed_memory_segments.html#interprocess.managed_memory_segments.managed_memory_segment_features.segment_offset" title="Obtaining handles to identify data">
Obtaining handles to identify data</a>
</h4></div></div></div>
<p>
The class also offers conversions between absolute addresses that belong
to a managed memory segment and a handle that can be passed using any interprocess
mechanism. That handle can be transformed again to an absolute address
using a managed memory segment that also contains that object. Handles
can be used as keys between processes to identify allocated portions of
a managed memory segment or objects constructed in the managed segment.
</p>
<p>
</p>
<pre class="programlisting"><span class="comment">//Process A obtains the offset of the address
</span><span class="identifier">managed_shared_memory</span><span class="special">::</span><span class="identifier">handle</span> <span class="identifier">handle</span> <span class="special">=</span>
<span class="identifier">segment</span><span class="special">.</span><span class="identifier">get_handle_from_address</span><span class="special">(</span><span class="identifier">processA_address</span><span class="special">);</span>
<span class="comment">//Process A sends this address using any mechanism to process B
</span>
<span class="comment">//Process B obtains the handle and transforms it again to an address
</span><span class="identifier">managed_shared_memory</span><span class="special">::</span><span class="identifier">handle</span> <span class="identifier">handle</span> <span class="special">=</span> <span class="special">...</span>
<span class="keyword">void</span> <span class="special">*</span> <span class="identifier">processB_address</span> <span class="special">=</span> <span class="identifier">segment</span><span class="special">.</span><span class="identifier">get_address_from_handle</span><span class="special">(</span><span class="identifier">handle</span><span class="special">);</span>
</pre>
</div>
<div class="section">
<div class="titlepage"><div><div><h4 class="title">
<a name="interprocess.managed_memory_segments.managed_memory_segment_features.allocation_types"></a><a class="link" href="managed_memory_segments.html#interprocess.managed_memory_segments.managed_memory_segment_features.allocation_types" title="Object construction function family">
Object construction function family</a>
</h4></div></div></div>
<p>
When constructing objects in a managed memory segment (managed shared memory,
managed mapped files...) associated with a name, the user has a varied
object construction family to "construct" or to "construct
if not found". <span class="bold"><strong>Boost.Interprocess</strong></span>
can construct a single object or an array of objects. The array can be
constructed with the same parameters for all objects or we can define each
parameter from a list of iterators:
</p>
<p>
</p>
<pre class="programlisting"><span class="comment">//!Allocates and constructs an object of type MyType (throwing version)
</span><span class="identifier">MyType</span> <span class="special">*</span><span class="identifier">ptr</span> <span class="special">=</span> <span class="identifier">managed_memory_segment</span><span class="special">.</span><span class="identifier">construct</span><span class="special"><</span><span class="identifier">MyType</span><span class="special">>(</span><span class="string">"Name"</span><span class="special">)</span> <span class="special">(</span><span class="identifier">par1</span><span class="special">,</span> <span class="identifier">par2</span><span class="special">...);</span>
<span class="comment">//!Allocates and constructs an array of objects of type MyType (throwing version)
</span><span class="comment">//!Each object receives the same parameters (par1, par2, ...)
</span><span class="identifier">MyType</span> <span class="special">*</span><span class="identifier">ptr</span> <span class="special">=</span> <span class="identifier">managed_memory_segment</span><span class="special">.</span><span class="identifier">construct</span><span class="special"><</span><span class="identifier">MyType</span><span class="special">>(</span><span class="string">"Name"</span><span class="special">)[</span><span class="identifier">count</span><span class="special">](</span><span class="identifier">par1</span><span class="special">,</span> <span class="identifier">par2</span><span class="special">...);</span>
<span class="comment">//!Tries to find a previously created object. If not present, allocates
</span><span class="comment">//!and constructs an object of type MyType (throwing version)
</span><span class="identifier">MyType</span> <span class="special">*</span><span class="identifier">ptr</span> <span class="special">=</span> <span class="identifier">managed_memory_segment</span><span class="special">.</span><span class="identifier">find_or_construct</span><span class="special"><</span><span class="identifier">MyType</span><span class="special">>(</span><span class="string">"Name"</span><span class="special">)</span> <span class="special">(</span><span class="identifier">par1</span><span class="special">,</span> <span class="identifier">par2</span><span class="special">...);</span>
<span class="comment">//!Tries to find a previously created object. If not present, allocates and
</span><span class="comment">//!constructs an array of objects of type MyType (throwing version). Each object
</span><span class="comment">//!receives the same parameters (par1, par2, ...)
</span><span class="identifier">MyType</span> <span class="special">*</span><span class="identifier">ptr</span> <span class="special">=</span> <span class="identifier">managed_memory_segment</span><span class="special">.</span><span class="identifier">find_or_construct</span><span class="special"><</span><span class="identifier">MyType</span><span class="special">>(</span><span class="string">"Name"</span><span class="special">)[</span><span class="identifier">count</span><span class="special">](</span><span class="identifier">par1</span><span class="special">,</span> <span class="identifier">par2</span><span class="special">...);</span>
<span class="comment">//!Allocates and constructs an array of objects of type MyType (throwing version)
</span><span class="comment">//!Each object receives parameters returned with the expression (*it1++, *it2++,... )
</span><span class="identifier">MyType</span> <span class="special">*</span><span class="identifier">ptr</span> <span class="special">=</span> <span class="identifier">managed_memory_segment</span><span class="special">.</span><span class="identifier">construct_it</span><span class="special"><</span><span class="identifier">MyType</span><span class="special">>(</span><span class="string">"Name"</span><span class="special">)[</span><span class="identifier">count</span><span class="special">](</span><span class="identifier">it1</span><span class="special">,</span> <span class="identifier">it2</span><span class="special">...);</span>
<span class="comment">//!Tries to find a previously created object. If not present, allocates and constructs
</span><span class="comment">//!an array of objects of type MyType (throwing version). Each object receives
</span><span class="comment">//!parameters returned with the expression (*it1++, *it2++,... )
</span><span class="identifier">MyType</span> <span class="special">*</span><span class="identifier">ptr</span> <span class="special">=</span> <span class="identifier">managed_memory_segment</span><span class="special">.</span><span class="identifier">find_or_construct_it</span><span class="special"><</span><span class="identifier">MyType</span><span class="special">>(</span><span class="string">"Name"</span><span class="special">)[</span><span class="identifier">count</span><span class="special">](</span><span class="identifier">it1</span><span class="special">,</span> <span class="identifier">it2</span><span class="special">...);</span>
<span class="comment">//!Tries to find a previously created object. Returns a pointer to the object and the
</span><span class="comment">//!count (if it is not an array, returns 1). If not present, the returned pointer is 0
</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">pair</span><span class="special"><</span><span class="identifier">MyType</span> <span class="special">*,</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span><span class="special">></span> <span class="identifier">ret</span> <span class="special">=</span> <span class="identifier">managed_memory_segment</span><span class="special">.</span><span class="identifier">find</span><span class="special"><</span><span class="identifier">MyType</span><span class="special">>(</span><span class="string">"Name"</span><span class="special">);</span>
<span class="comment">//!Destroys the created object, returns false if not present
</span><span class="keyword">bool</span> <span class="identifier">destroyed</span> <span class="special">=</span> <span class="identifier">managed_memory_segment</span><span class="special">.</span><span class="identifier">destroy</span><span class="special"><</span><span class="identifier">MyType</span><span class="special">>(</span><span class="string">"Name"</span><span class="special">);</span>
<span class="comment">//!Destroys the created object via pointer
</span><span class="identifier">managed_memory_segment</span><span class="special">.</span><span class="identifier">destroy_ptr</span><span class="special">(</span><span class="identifier">ptr</span><span class="special">);</span>
</pre>
<p>
All these functions have a non-throwing version, that is invoked with an
additional parameter std::nothrow. For example, for simple object construction:
</p>
<p>
</p>
<pre class="programlisting"><span class="comment">//!Allocates and constructs an object of type MyType (no throwing version)
</span><span class="identifier">MyType</span> <span class="special">*</span><span class="identifier">ptr</span> <span class="special">=</span> <span class="identifier">managed_memory_segment</span><span class="special">.</span><span class="identifier">construct</span><span class="special"><</span><span class="identifier">MyType</span><span class="special">>(</span><span class="string">"Name"</span><span class="special">,</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">nothrow</span><span class="special">)</span> <span class="special">(</span><span class="identifier">par1</span><span class="special">,</span> <span class="identifier">par2</span><span class="special">...);</span>
</pre>
</div>
<div class="section">
<div class="titlepage"><div><div><h4 class="title">
<a name="interprocess.managed_memory_segments.managed_memory_segment_features.anonymous"></a><a class="link" href="managed_memory_segments.html#interprocess.managed_memory_segments.managed_memory_segment_features.anonymous" title="Anonymous instance construction">
Anonymous instance construction</a>
</h4></div></div></div>
<p>
Sometimes, the user doesn't want to create class objects associated with
a name. For this purpose, <span class="bold"><strong>Boost.Interprocess</strong></span>
can create anonymous objects in a managed memory segment. All named object
construction functions are available to construct anonymous objects. To
allocate an anonymous objects, the user must use "boost::interprocess::anonymous_instance"
name instead of a normal name:
</p>
<p>
</p>
<pre class="programlisting"><span class="identifier">MyType</span> <span class="special">*</span><span class="identifier">ptr</span> <span class="special">=</span> <span class="identifier">managed_memory_segment</span><span class="special">.</span><span class="identifier">construct</span><span class="special"><</span><span class="identifier">MyType</span><span class="special">>(</span><span class="identifier">anonymous_instance</span><span class="special">)</span> <span class="special">(</span><span class="identifier">par1</span><span class="special">,</span> <span class="identifier">par2</span><span class="special">...);</span>
<span class="comment">//Other construct variants can also be used (including non-throwing ones)
</span><span class="special">...</span>
<span class="comment">//We can only destroy the anonymous object via pointer
</span><span class="identifier">managed_memory_segment</span><span class="special">.</span><span class="identifier">destroy_ptr</span><span class="special">(</span><span class="identifier">ptr</span><span class="special">);</span>
</pre>
<p>
Find functions have no sense here, since anonymous objects have no name.
We can only destroy the anonymous object via pointer.
</p>
</div>
<div class="section">
<div class="titlepage"><div><div><h4 class="title">
<a name="interprocess.managed_memory_segments.managed_memory_segment_features.unique"></a><a class="link" href="managed_memory_segments.html#interprocess.managed_memory_segments.managed_memory_segment_features.unique" title="Unique instance construction">
Unique instance construction</a>
</h4></div></div></div>
<p>
Sometimes, the user wants to emulate a singleton in a managed memory segment.
Obviously, as the managed memory segment is constructed at run-time, the
user must construct and destroy this object explicitly. But how can the
user be sure that the object is the only object of its type in the managed
memory segment? This can be emulated using a named object and checking
if it is present before trying to create one, but all processes must agree
in the object's name, that can also conflict with other existing names.
</p>
<p>
To solve this, <span class="bold"><strong>Boost.Interprocess</strong></span> offers
a "unique object" creation in a managed memory segment. Only
one instance of a class can be created in a managed memory segment using
this "unique object" service (you can create more named objects
of this class, though) so it makes easier the emulation of singleton-like
objects across processes, for example, to design pooled, shared memory
allocators. The object can be searched using the type of the class as a
key.
</p>
<p>
</p>
<pre class="programlisting"><span class="comment">// Construct
</span><span class="identifier">MyType</span> <span class="special">*</span><span class="identifier">ptr</span> <span class="special">=</span> <span class="identifier">managed_memory_segment</span><span class="special">.</span><span class="identifier">construct</span><span class="special"><</span><span class="identifier">MyType</span><span class="special">>(</span><span class="identifier">unique_instance</span><span class="special">)</span> <span class="special">(</span><span class="identifier">par1</span><span class="special">,</span> <span class="identifier">par2</span><span class="special">...);</span>
<span class="comment">// Find it
</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">pair</span><span class="special"><</span><span class="identifier">MyType</span> <span class="special">*,</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span><span class="special">></span> <span class="identifier">ret</span> <span class="special">=</span> <span class="identifier">managed_memory_segment</span><span class="special">.</span><span class="identifier">find</span><span class="special"><</span><span class="identifier">MyType</span><span class="special">>(</span><span class="identifier">unique_instance</span><span class="special">);</span>
<span class="comment">// Destroy it
</span><span class="identifier">managed_memory_segment</span><span class="special">.</span><span class="identifier">destroy</span><span class="special"><</span><span class="identifier">MyType</span><span class="special">>(</span><span class="identifier">unique_instance</span><span class="special">);</span>
<span class="comment">// Other construct and find variants can also be used (including non-throwing ones)
</span><span class="comment">//...
</span></pre>
<p>
</p>
<pre class="programlisting"><span class="comment">// We can also destroy the unique object via pointer
</span><span class="identifier">MyType</span> <span class="special">*</span><span class="identifier">ptr</span> <span class="special">=</span> <span class="identifier">managed_memory_segment</span><span class="special">.</span><span class="identifier">construct</span><span class="special"><</span><span class="identifier">MyType</span><span class="special">>(</span><span class="identifier">unique_instance</span><span class="special">)</span> <span class="special">(</span><span class="identifier">par1</span><span class="special">,</span> <span class="identifier">par2</span><span class="special">...);</span>
<span class="identifier">managed_shared_memory</span><span class="special">.</span><span class="identifier">destroy_ptr</span><span class="special">(</span><span class="identifier">ptr</span><span class="special">);</span>
</pre>
<p>
The find function obtains a pointer to the only object of type T that can
be created using this "unique instance" mechanism.
</p>
</div>
<div class="section">
<div class="titlepage"><div><div><h4 class="title">
<a name="interprocess.managed_memory_segments.managed_memory_segment_features.synchronization"></a><a class="link" href="managed_memory_segments.html#interprocess.managed_memory_segments.managed_memory_segment_features.synchronization" title="Synchronization guarantees">
Synchronization guarantees</a>
</h4></div></div></div>
<p>
One of the features of named/unique allocations/searches/destructions is
that they are <span class="bold"><strong>atomic</strong></span>. Named allocations
use the recursive synchronization scheme defined by the internal <code class="computeroutput"><span class="identifier">mutex_family</span></code> typedef defined of the memory
allocation algorithm template parameter (<code class="computeroutput"><span class="identifier">MemoryAlgorithm</span></code>).
That is, the mutex type used to synchronize named/unique allocations is
defined by the <code class="computeroutput"><span class="identifier">MemoryAlgorithm</span><span class="special">::</span><span class="identifier">mutex_family</span><span class="special">::</span><span class="identifier">recursive_mutex_type</span></code>
type. For shared memory, and memory mapped file based managed segments
this recursive mutex is defined as <code class="computeroutput"><a class="link" href="../boost/interprocess/interprocess_recursive__id829710.html" title="Class interprocess_recursive_mutex">interprocess_recursive_mutex</a></code>.
</p>
<p>
If two processes can call:
</p>
<p>
</p>
<pre class="programlisting"><span class="identifier">MyType</span> <span class="special">*</span><span class="identifier">ptr</span> <span class="special">=</span> <span class="identifier">managed_shared_memory</span><span class="special">.</span><span class="identifier">find_or_construct</span><span class="special"><</span><span class="identifier">MyType</span><span class="special">>(</span><span class="string">"Name"</span><span class="special">)[</span><span class="identifier">count</span><span class="special">](</span><span class="identifier">par1</span><span class="special">,</span> <span class="identifier">par2</span><span class="special">...);</span>
</pre>
<p>
at the same time, but only one process will create the object and the other
will obtain a pointer to the created object.
</p>
<p>
Raw allocation using <code class="computeroutput"><span class="identifier">allocate</span><span class="special">()</span></code> can be called also safely while executing
named/anonymous/unique allocations, just like when programming a multithreaded
application inserting an object in a mutex-protected map does not block
other threads from calling new[] while the map thread is searching the
place where it has to insert the new object. The synchronization does happen
once the map finds the correct place and it has to allocate raw memory
to construct the new value.
</p>
<p>
This means that if we are creating or searching for a lot of named objects,
we only block creation/searches from other processes but we don't block
another process if that process is inserting elements in a shared memory
vector.
</p>
</div>
<div class="section">
<div class="titlepage"><div><div><h4 class="title">
<a name="interprocess.managed_memory_segments.managed_memory_segment_features.index_types"></a><a class="link" href="managed_memory_segments.html#interprocess.managed_memory_segments.managed_memory_segment_features.index_types" title="Index types for name/object mappings">
Index types for name/object mappings</a>
</h4></div></div></div>
<p>
As seen, managed memory segments, when creating named objects, store the
name/object association in an index. The index is a map with the name of
the object as a key and a pointer to the object as the mapped type. The
default specializations, <span class="bold"><strong>managed_shared_memory</strong></span>
and <span class="bold"><strong>wmanaged_shared_memory</strong></span>, use <span class="bold"><strong>flat_map_index</strong></span> as the index type.
</p>
<p>
Each index has its own characteristics, like search-time, insertion time,
deletion time, memory use, and memory allocation patterns. <span class="bold"><strong>Boost.Interprocess</strong></span>
offers 3 index types right now:
</p>
<div class="itemizedlist"><ul class="itemizedlist" type="disc">
<li class="listitem">
<span class="bold"><strong>boost::interprocess::flat_map_index flat_map_index</strong></span>:
Based on boost::interprocess::flat_map, an ordered vector similar to
Loki library's AssocVector class, offers great search time and minimum
memory use. But the vector must be reallocated when is full, so all
data must be copied to the new buffer. Ideal when insertions are mainly
in initialization time and in run-time we just need searches.
</li>
<li class="listitem">
<span class="bold"><strong>boost::interprocess::map_index map_index</strong></span>:
Based on boost::interprocess::map, a managed memory ready version of
std::map. Since it's a node based container, it has no reallocations,
the tree must be just rebalanced sometimes. Offers equilibrated insertion/deletion/search
times with more overhead per node comparing to <span class="bold"><strong>boost::interprocess::flat_map_index</strong></span>.
Ideal when searches/insertions/deletions are in random order.
</li>
<li class="listitem">
<span class="bold"><strong>boost::interprocess::null_index null_index</strong></span>:
This index is for people using a managed memory segment just for raw
memory buffer allocations and they don't make use of named/unique allocations.
This class is just empty and saves some space and compilation time.
If you try to use named object creation with a managed memory segment
using this index, you will get a compilation error.
</li>
</ul></div>
<p>
As an example, if we want to define new managed shared memory class using
<span class="bold"><strong>boost::interprocess::map</strong></span> as the index
type we just must specify [boost::interprocess::map_index map_index] as
a template parameter:
</p>
<p>
</p>
<pre class="programlisting"><span class="comment">//This managed memory segment can allocate objects with:
</span><span class="comment">// -> a wchar_t string as key
</span><span class="comment">// -> boost::interprocess::rbtree_best_fit with process-shared mutexes
</span><span class="comment">// as memory allocation algorithm.
</span><span class="comment">// -> boost::interprocess::map<...> as the index to store name/object mappings
</span><span class="comment">//
</span><span class="keyword">typedef</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">interprocess</span><span class="special">::</span><span class="identifier">basic_managed_shared_memory</span>
<span class="special"><</span> <span class="keyword">wchar_t</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">rbtree_best_fit</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">mutex_family</span><span class="special">,</span> <span class="identifier">offset_ptr</span><span class="special"><</span><span class="keyword">void</span><span class="special">></span> <span class="special">></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">map_index</span>
<span class="special">></span> <span class="identifier">my_managed_shared_memory</span><span class="special">;</span>
</pre>
<p>
<span class="bold"><strong>Boost.Interprocess</strong></span> plans to offer an
<span class="bold"><strong>unordered_map</strong></span> based index as soon as this
container is included in Boost. If these indexes are not enough for you,
you can define your own index type. To know how to do this, go to <a class="link" href="customizing_interprocess.html#interprocess.customizing_interprocess.custom_indexes" title="Building custom indexes">Building
custom indexes</a> section.
</p>
</div>
<div class="section">
<div class="titlepage"><div><div><h4 class="title">
<a name="interprocess.managed_memory_segments.managed_memory_segment_features.managed_memory_segment_segment_manager"></a><a class="link" href="managed_memory_segments.html#interprocess.managed_memory_segments.managed_memory_segment_features.managed_memory_segment_segment_manager" title="Segment Manager">
Segment Manager</a>
</h4></div></div></div>
<p>
All <span class="bold"><strong>Boost.Interprocess</strong></span> managed memory
segment classes construct in their respective memory segments (shared memory,
memory mapped files, heap memory...) some structures to implement the memory
management algorithm, named allocations, synchronization objects... All
these objects are encapsulated in a single object called <span class="bold"><strong>segment
manager</strong></span>. A managed memory mapped file and a managed shared memory
use the same <span class="bold"><strong>segment manager</strong></span> to implement
all managed memory segment features, due to the fact that a <span class="bold"><strong>segment
manager</strong></span> is a class that manages a fixed size memory buffer.
Since both shared memory or memory mapped files are accessed though a mapped
region, and a mapped region is a fixed size memory buffer, a single <span class="bold"><strong>segment manager</strong></span> class can manage several managed
memory segment types.
</p>
<p>
Some <span class="bold"><strong>Boost.Interprocess</strong></span> classes require
a pointer to the segment manager in their constructors, and the segment
manager can be obtained from any managed memory segment using <code class="computeroutput"><span class="identifier">get_segment_manager</span></code> member:
</p>
<p>
</p>
<pre class="programlisting"><span class="identifier">managed_shared_memory</span><span class="special">::</span><span class="identifier">segment_manager</span> <span class="special">*</span><span class="identifier">seg_manager</span> <span class="special">=</span>
<span class="identifier">managed_shm</span><span class="special">.</span><span class="identifier">get_segment_manager</span><span class="special">();</span>
</pre>
</div>
<div class="section">
<div class="titlepage"><div><div><h4 class="title">
<a name="interprocess.managed_memory_segments.managed_memory_segment_features.managed_memory_segment_information"></a><a class="link" href="managed_memory_segments.html#interprocess.managed_memory_segments.managed_memory_segment_features.managed_memory_segment_information" title="Obtaining information about a constructed object">
Obtaining information about a constructed object</a>
</h4></div></div></div>
<p>
Once an object is constructed using <code class="computeroutput"><span class="identifier">construct</span><span class="special"><></span></code> function family, the programmer
can obtain information about the object using a pointer to the object.
The programmer can obtain the following information:
</p>
<div class="itemizedlist"><ul class="itemizedlist" type="disc">
<li class="listitem">
Name of the object: If it's a named instance, the name used in the
construction function is returned, otherwise 0 is returned.
</li>
<li class="listitem">
Length of the object: Returns the number of elements of the object
(1 if it's a single value, >=1 if it's an array).
</li>
<li class="listitem">
The type of construction: Whether the object was constructed using
a named, unique or anonymous construction.
</li>
</ul></div>
<p>
Here is an example showing this functionality:
</p>
<p>
</p>
<p>
</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">managed_shared_memory</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span>
<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">cassert</span><span class="special">></span>
<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">cstring</span><span class="special">></span>
<span class="keyword">class</span> <span class="identifier">my_class</span>
<span class="special">{</span>
<span class="comment">//...
</span><span class="special">};</span>
<span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</span>
<span class="special">{</span>
<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="comment">//Remove shared memory on construction and destruction
</span> <span class="keyword">struct</span> <span class="identifier">shm_remove</span>
<span class="special">{</span>
<span class="identifier">shm_remove</span><span class="special">()</span> <span class="special">{</span> <span class="identifier">shared_memory_object</span><span class="special">::</span><span class="identifier">remove</span><span class="special">(</span><span class="string">"MySharedMemory"</span><span class="special">);</span> <span class="special">}</span>
<span class="special">~</span><span class="identifier">shm_remove</span><span class="special">(){</span> <span class="identifier">shared_memory_object</span><span class="special">::</span><span class="identifier">remove</span><span class="special">(</span><span class="string">"MySharedMemory"</span><span class="special">);</span> <span class="special">}</span>
<span class="special">}</span> <span class="identifier">remover</span><span class="special">;</span>
<span class="identifier">managed_shared_memory</span> <span class="identifier">managed_shm</span><span class="special">(</span><span class="identifier">create_only</span><span class="special">,</span> <span class="string">"MySharedMemory"</span><span class="special">,</span> <span class="number">10000</span><span class="special">*</span><span class="keyword">sizeof</span><span class="special">(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span><span class="special">));</span>
<span class="comment">//Construct objects
</span> <span class="identifier">my_class</span> <span class="special">*</span><span class="identifier">named_object</span> <span class="special">=</span> <span class="identifier">managed_shm</span><span class="special">.</span><span class="identifier">construct</span><span class="special"><</span><span class="identifier">my_class</span><span class="special">>(</span><span class="string">"Object name"</span><span class="special">)[</span><span class="number">1</span><span class="special">]();</span>
<span class="identifier">my_class</span> <span class="special">*</span><span class="identifier">unique_object</span> <span class="special">=</span> <span class="identifier">managed_shm</span><span class="special">.</span><span class="identifier">construct</span><span class="special"><</span><span class="identifier">my_class</span><span class="special">>(</span><span class="identifier">unique_instance</span><span class="special">)[</span><span class="number">2</span><span class="special">]();</span>
<span class="identifier">my_class</span> <span class="special">*</span><span class="identifier">anon_object</span> <span class="special">=</span> <span class="identifier">managed_shm</span><span class="special">.</span><span class="identifier">construct</span><span class="special"><</span><span class="identifier">my_class</span><span class="special">>(</span><span class="identifier">anonymous_instance</span><span class="special">)[</span><span class="number">3</span><span class="special">]();</span>
<span class="comment">//Now test "get_instance_name" function.
</span> <span class="identifier">assert</span><span class="special">(</span><span class="number">0</span> <span class="special">==</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">strcmp</span><span class="special">(</span><span class="identifier">managed_shared_memory</span><span class="special">::</span><span class="identifier">get_instance_name</span><span class="special">(</span><span class="identifier">named_object</span><span class="special">),</span> <span class="string">"Object name"</span><span class="special">));</span>
<span class="identifier">assert</span><span class="special">(</span><span class="number">0</span> <span class="special">==</span> <span class="identifier">managed_shared_memory</span><span class="special">::</span><span class="identifier">get_instance_name</span><span class="special">(</span><span class="identifier">unique_object</span><span class="special">));</span>
<span class="identifier">assert</span><span class="special">(</span><span class="number">0</span> <span class="special">==</span> <span class="identifier">managed_shared_memory</span><span class="special">::</span><span class="identifier">get_instance_name</span><span class="special">(</span><span class="identifier">anon_object</span><span class="special">));</span>
<span class="comment">//Now test "get_instance_type" function.
</span> <span class="identifier">assert</span><span class="special">(</span><span class="identifier">named_type</span> <span class="special">==</span> <span class="identifier">managed_shared_memory</span><span class="special">::</span><span class="identifier">get_instance_type</span><span class="special">(</span><span class="identifier">named_object</span><span class="special">));</span>
<span class="identifier">assert</span><span class="special">(</span><span class="identifier">unique_type</span> <span class="special">==</span> <span class="identifier">managed_shared_memory</span><span class="special">::</span><span class="identifier">get_instance_type</span><span class="special">(</span><span class="identifier">unique_object</span><span class="special">));</span>
<span class="identifier">assert</span><span class="special">(</span><span class="identifier">anonymous_type</span> <span class="special">==</span> <span class="identifier">managed_shared_memory</span><span class="special">::</span><span class="identifier">get_instance_type</span><span class="special">(</span><span class="identifier">anon_object</span><span class="special">));</span>
<span class="comment">//Now test "get_instance_length" function.
</span> <span class="identifier">assert</span><span class="special">(</span><span class="number">1</span> <span class="special">==</span> <span class="identifier">managed_shared_memory</span><span class="special">::</span><span class="identifier">get_instance_length</span><span class="special">(</span><span class="identifier">named_object</span><span class="special">));</span>
<span class="identifier">assert</span><span class="special">(</span><span class="number">2</span> <span class="special">==</span> <span class="identifier">managed_shared_memory</span><span class="special">::</span><span class="identifier">get_instance_length</span><span class="special">(</span><span class="identifier">unique_object</span><span class="special">));</span>
<span class="identifier">assert</span><span class="special">(</span><span class="number">3</span> <span class="special">==</span> <span class="identifier">managed_shared_memory</span><span class="special">::</span><span class="identifier">get_instance_length</span><span class="special">(</span><span class="identifier">anon_object</span><span class="special">));</span>
<span class="identifier">managed_shm</span><span class="special">.</span><span class="identifier">destroy_ptr</span><span class="special">(</span><span class="identifier">named_object</span><span class="special">);</span>
<span class="identifier">managed_shm</span><span class="special">.</span><span class="identifier">destroy_ptr</span><span class="special">(</span><span class="identifier">unique_object</span><span class="special">);</span>
<span class="identifier">managed_shm</span><span class="special">.</span><span class="identifier">destroy_ptr</span><span class="special">(</span><span class="identifier">anon_object</span><span class="special">);</span>
<span class="keyword">return</span> <span class="number">0</span><span class="special">;</span>
<span class="special">}</span>
</pre>
<p>
</p>
<p>
</p>
</div>
<div class="section">
<div class="titlepage"><div><div><h4 class="title">
<a name="interprocess.managed_memory_segments.managed_memory_segment_features.managed_memory_segment_atomic_func"></a><a class="link" href="managed_memory_segments.html#interprocess.managed_memory_segments.managed_memory_segment_features.managed_memory_segment_atomic_func" title="Executing an object function atomically">
Executing an object function atomically</a>
</h4></div></div></div>
<p>
Sometimes the programmer must execute some code, and needs to execute it
with the guarantee that no other process or thread will create or destroy
any named, unique or anonymous object while executing the functor. A user
might want to create several named objects and initialize them, but those
objects should be available for the rest of processes at once.
</p>
<p>
To achieve this, the programmer can use the <code class="computeroutput"><span class="identifier">atomic_func</span><span class="special">()</span></code> function offered by managed classes:
</p>
<p>
</p>
<pre class="programlisting"><span class="comment">//This object function will create several named objects
</span><span class="identifier">create_several_objects_func</span> <span class="identifier">func</span><span class="special">(/**/);</span>
<span class="comment">//While executing the function, no other process will be
</span><span class="comment">//able to create or destroy objects
</span><span class="identifier">managed_memory</span><span class="special">.</span><span class="identifier">atomic_func</span><span class="special">(</span><span class="identifier">func</span><span class="special">);</span>
</pre>
<p>
Note that <code class="computeroutput"><span class="identifier">atomic_func</span></code> does
not prevent other processes from allocating raw memory or executing member
functions for already constructed objects (e.g.: another process might
be pushing elements into a vector placed in the segment). The atomic function
only blocks named, unique and anonymous creation, search and destruction
(concurrent calls to <code class="computeroutput"><span class="identifier">construct</span><span class="special"><></span></code>, <code class="computeroutput"><span class="identifier">find</span><span class="special"><></span></code>, <code class="computeroutput"><span class="identifier">find_or_construct</span><span class="special"><></span></code>, <code class="computeroutput"><span class="identifier">destroy</span><span class="special"><></span></code>...) from other processes.
</p>
</div>
</div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="interprocess.managed_memory_segments.managed_memory_segment_advanced_features"></a><a class="link" href="managed_memory_segments.html#interprocess.managed_memory_segments.managed_memory_segment_advanced_features" title="Managed Memory Segment Advanced Features">
Managed Memory Segment Advanced Features</a>
</h3></div></div></div>
<div class="toc"><dl>
<dt><span class="section"><a href="managed_memory_segments.html#interprocess.managed_memory_segments.managed_memory_segment_advanced_features.managed_memory_segment_information">
Obtaining information about the managed segment</a></span></dt>
<dt><span class="section"><a href="managed_memory_segments.html#interprocess.managed_memory_segments.managed_memory_segment_advanced_features.growing_managed_memory">
Growing managed segments</a></span></dt>
<dt><span class="section"><a href="managed_memory_segments.html#interprocess.managed_memory_segments.managed_memory_segment_advanced_features.managed_memory_segment_advanced_index_functions">
Advanced index functions</a></span></dt>
<dt><span class="section"><a href="managed_memory_segments.html#interprocess.managed_memory_segments.managed_memory_segment_advanced_features.allocate_aligned">
Allocating aligned memory portions</a></span></dt>
<dt><span class="section"><a href="managed_memory_segments.html#interprocess.managed_memory_segments.managed_memory_segment_advanced_features.managed_memory_segment_multiple_allocations">
Multiple allocation functions</a></span></dt>
<dt><span class="section"><a href="managed_memory_segments.html#interprocess.managed_memory_segments.managed_memory_segment_advanced_features.managed_memory_segment_expand_in_place">
Expand in place memory allocation</a></span></dt>
<dt><span class="section"><a href="managed_memory_segments.html#interprocess.managed_memory_segments.managed_memory_segment_advanced_features.copy_on_write_read_only">
Opening managed shared memory and mapped files with Copy On Write or Read
Only modes</a></span></dt>
</dl></div>
<div class="section">
<div class="titlepage"><div><div><h4 class="title">
<a name="interprocess.managed_memory_segments.managed_memory_segment_advanced_features.managed_memory_segment_information"></a><a class="link" href="managed_memory_segments.html#interprocess.managed_memory_segments.managed_memory_segment_advanced_features.managed_memory_segment_information" title="Obtaining information about the managed segment">
Obtaining information about the managed segment</a>
</h4></div></div></div>
<p>
These functions are available to obtain information about the managed memory
segments:
</p>
<p>
Obtain the size of the memory segment:
</p>
<p>
</p>
<pre class="programlisting"><span class="identifier">managed_shm</span><span class="special">.</span><span class="identifier">get_size</span><span class="special">();</span>
</pre>
<p>
Obtain the number of free bytes of the segment:
</p>
<p>
</p>
<pre class="programlisting"><span class="identifier">managed_shm</span><span class="special">.</span><span class="identifier">get_free_memory</span><span class="special">();</span>
</pre>
<p>
Clear to zero the free memory:
</p>
<p>
</p>
<pre class="programlisting"><span class="identifier">managed_shm</span><span class="special">.</span><span class="identifier">zero_free_memory</span><span class="special">();</span>
</pre>
<p>
Know if all memory has been deallocated, false otherwise:
</p>
<p>
</p>
<pre class="programlisting"><span class="identifier">managed_shm</span><span class="special">.</span><span class="identifier">all_memory_deallocated</span><span class="special">();</span>
</pre>
<p>
Test internal structures of the managed segment. Returns true if no errors
are detected:
</p>
<p>
</p>
<pre class="programlisting"><span class="identifier">managed_shm</span><span class="special">.</span><span class="identifier">check_sanity</span><span class="special">();</span>
</pre>
<p>
Obtain the number of named and unique objects allocated in the segment:
</p>
<p>
</p>
<pre class="programlisting"><span class="identifier">managed_shm</span><span class="special">.</span><span class="identifier">get_num_named_objects</span><span class="special">();</span>
<span class="identifier">managed_shm</span><span class="special">.</span><span class="identifier">get_num_unique_objects</span><span class="special">();</span>
</pre>
</div>
<div class="section">
<div class="titlepage"><div><div><h4 class="title">
<a name="interprocess.managed_memory_segments.managed_memory_segment_advanced_features.growing_managed_memory"></a><a class="link" href="managed_memory_segments.html#interprocess.managed_memory_segments.managed_memory_segment_advanced_features.growing_managed_memory" title="Growing managed segments">
Growing managed segments</a>
</h4></div></div></div>
<p>
Once a managed segment is created the managed segment can't be grown. The
limitation is not easily solvable: every process attached to the managed
segment would need to be stopped, notified of the new size, they would
need to remap the managed segment and continue working. Nearly impossible
to achieve with a user-level library without the help of the operating
system kernel.
</p>
<p>
On the other hand, <span class="bold"><strong>Boost.Interprocess</strong></span>
offers off-line segment growing. What does this mean? That the segment
can be grown if no process has mapped the managed segment. If the application
can find a moment where no process is attached it can grow or shrink to
fit the managed segment.
</p>
<p>
Here we have an example showing how to grow and shrink to fit <code class="computeroutput"><a class="link" href="../boost_interprocess_reference.html#boost.interprocess.managed_shared_memory">managed_shared_memory</a></code>:
</p>
<p>
</p>
<p>
</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">managed_shared_memory</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span>
<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">managed_mapped_file</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span>
<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">cassert</span><span class="special">></span>
<span class="keyword">class</span> <span class="identifier">MyClass</span>
<span class="special">{</span>
<span class="comment">//...
</span><span class="special">};</span>
<span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</span>
<span class="special">{</span>
<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="comment">//Remove shared memory on construction and destruction
</span> <span class="keyword">struct</span> <span class="identifier">shm_remove</span>
<span class="special">{</span>
<span class="identifier">shm_remove</span><span class="special">()</span> <span class="special">{</span> <span class="identifier">shared_memory_object</span><span class="special">::</span><span class="identifier">remove</span><span class="special">(</span><span class="string">"MySharedMemory"</span><span class="special">);</span> <span class="special">}</span>
<span class="special">~</span><span class="identifier">shm_remove</span><span class="special">(){</span> <span class="identifier">shared_memory_object</span><span class="special">::</span><span class="identifier">remove</span><span class="special">(</span><span class="string">"MySharedMemory"</span><span class="special">);</span> <span class="special">}</span>
<span class="special">}</span> <span class="identifier">remover</span><span class="special">;</span>
<span class="special">{</span>
<span class="comment">//Create a managed shared memory
</span> <span class="identifier">managed_shared_memory</span> <span class="identifier">shm</span><span class="special">(</span><span class="identifier">create_only</span><span class="special">,</span> <span class="string">"MySharedMemory"</span><span class="special">,</span> <span class="number">1000</span><span class="special">);</span>
<span class="comment">//Check size
</span> <span class="identifier">assert</span><span class="special">(</span><span class="identifier">shm</span><span class="special">.</span><span class="identifier">get_size</span><span class="special">()</span> <span class="special">==</span> <span class="number">1000</span><span class="special">);</span>
<span class="comment">//Construct a named object
</span> <span class="identifier">MyClass</span> <span class="special">*</span><span class="identifier">myclass</span> <span class="special">=</span> <span class="identifier">shm</span><span class="special">.</span><span class="identifier">construct</span><span class="special"><</span><span class="identifier">MyClass</span><span class="special">>(</span><span class="string">"MyClass"</span><span class="special">)();</span>
<span class="comment">//The managed segment is unmapped here
</span> <span class="special">}</span>
<span class="special">{</span>
<span class="comment">//Now that the segment is not mapped grow it adding extra 500 bytes
</span> <span class="identifier">managed_shared_memory</span><span class="special">::</span><span class="identifier">grow</span><span class="special">(</span><span class="string">"MySharedMemory"</span><span class="special">,</span> <span class="number">500</span><span class="special">);</span>
<span class="comment">//Map it again
</span> <span class="identifier">managed_shared_memory</span> <span class="identifier">shm</span><span class="special">(</span><span class="identifier">open_only</span><span class="special">,</span> <span class="string">"MySharedMemory"</span><span class="special">);</span>
<span class="comment">//Check size
</span> <span class="identifier">assert</span><span class="special">(</span><span class="identifier">shm</span><span class="special">.</span><span class="identifier">get_size</span><span class="special">()</span> <span class="special">==</span> <span class="number">1500</span><span class="special">);</span>
<span class="comment">//Check "MyClass" is still there
</span> <span class="identifier">MyClass</span> <span class="special">*</span><span class="identifier">myclass</span> <span class="special">=</span> <span class="identifier">shm</span><span class="special">.</span><span class="identifier">find</span><span class="special"><</span><span class="identifier">MyClass</span><span class="special">>(</span><span class="string">"MyClass"</span><span class="special">).</span><span class="identifier">first</span><span class="special">;</span>
<span class="identifier">assert</span><span class="special">(</span><span class="identifier">myclass</span> <span class="special">!=</span> <span class="number">0</span><span class="special">);</span>
<span class="comment">//The managed segment is unmapped here
</span> <span class="special">}</span>
<span class="special">{</span>
<span class="comment">//Now minimize the size of the segment
</span> <span class="identifier">managed_shared_memory</span><span class="special">::</span><span class="identifier">shrink_to_fit</span><span class="special">(</span><span class="string">"MySharedMemory"</span><span class="special">);</span>
<span class="comment">//Map it again
</span> <span class="identifier">managed_shared_memory</span> <span class="identifier">shm</span><span class="special">(</span><span class="identifier">open_only</span><span class="special">,</span> <span class="string">"MySharedMemory"</span><span class="special">);</span>
<span class="comment">//Check size
</span> <span class="identifier">assert</span><span class="special">(</span><span class="identifier">shm</span><span class="special">.</span><span class="identifier">get_size</span><span class="special">()</span> <span class="special"><</span> <span class="number">1000</span><span class="special">);</span>
<span class="comment">//Check "MyClass" is still there
</span> <span class="identifier">MyClass</span> <span class="special">*</span><span class="identifier">myclass</span> <span class="special">=</span> <span class="identifier">shm</span><span class="special">.</span><span class="identifier">find</span><span class="special"><</span><span class="identifier">MyClass</span><span class="special">>(</span><span class="string">"MyClass"</span><span class="special">).</span><span class="identifier">first</span><span class="special">;</span>
<span class="identifier">assert</span><span class="special">(</span><span class="identifier">myclass</span> <span class="special">!=</span> <span class="number">0</span><span class="special">);</span>
<span class="comment">//The managed segment is unmapped here
</span> <span class="special">}</span>
<span class="keyword">return</span> <span class="number">0</span><span class="special">;</span>
<span class="special">}</span>
</pre>
<p>
</p>
<p>
</p>
<p>
<code class="computeroutput"><a class="link" href="../boost_interprocess_reference.html#boost.interprocess.managed_mapped_file">managed_mapped_file</a></code>
also offers a similar function to grow or shrink_to_fit the managed file.
Please, remember that <span class="bold"><strong>no process should be modifying
the file/shared memory while the growing/shrinking process is performed</strong></span>.
Otherwise, the managed segment will be corrupted.
</p>
</div>
<div class="section">
<div class="titlepage"><div><div><h4 class="title">
<a name="interprocess.managed_memory_segments.managed_memory_segment_advanced_features.managed_memory_segment_advanced_index_functions"></a><a class="link" href="managed_memory_segments.html#interprocess.managed_memory_segments.managed_memory_segment_advanced_features.managed_memory_segment_advanced_index_functions" title="Advanced index functions">
Advanced index functions</a>
</h4></div></div></div>
<p>
As mentioned, the managed segment stores the information about named and
unique objects in two indexes. Depending on the type of those indexes,
the index must reallocate some auxiliary structures when new named or unique
allocations are made. For some indexes, if the user knows how many named
or unique objects are going to be created it's possible to preallocate
some structures to obtain much better performance. (If the index is an
ordered vector it can preallocate memory to avoid reallocations. If the
index is a hash structure it can preallocate the bucket array).
</p>
<p>
The following functions reserve memory to make the subsequent allocation
of named or unique objects more efficient. These functions are only useful
for pseudo-intrusive or non-node indexes (like <code class="computeroutput"><span class="identifier">flat_map_index</span></code>,
<code class="computeroutput"><span class="identifier">iunordered_set_index</span></code>).
These functions have no effect with the default index (<code class="computeroutput"><span class="identifier">iset_index</span></code>)
or other indexes (<code class="computeroutput"><span class="identifier">map_index</span></code>):
</p>
<p>
</p>
<pre class="programlisting"><span class="identifier">managed_shm</span><span class="special">.</span><span class="identifier">reserve_named_objects</span><span class="special">(</span><span class="number">1000</span><span class="special">);</span>
<span class="identifier">managed_shm</span><span class="special">.</span><span class="identifier">reserve_unique_objects</span><span class="special">(</span><span class="number">1000</span><span class="special">);</span>
</pre>
<p>
</p>
<pre class="programlisting"><span class="identifier">managed_shm</span><span class="special">.</span><span class="identifier">reserve_named_objects</span><span class="special">(</span><span class="number">1000</span><span class="special">);</span>
<span class="identifier">managed_shm</span><span class="special">.</span><span class="identifier">reserve_unique_objects</span><span class="special">(</span><span class="number">1000</span><span class="special">);</span>
</pre>
<p>
Managed memory segments also offer the possibility to iterate through constructed
named and unique objects for debugging purposes. <span class="bold"><strong>Caution:
this iteration is not thread-safe</strong></span> so the user should make sure
that no other thread is manipulating named or unique indexes (creating,
erasing, reserving...) in the segment. Other operations not involving indexes
can be concurrently executed (raw memory allocation/deallocations, for
example).
</p>
<p>
The following functions return constant iterators to the range of named
and unique objects stored in the managed segment. Depending on the index
type, iterators might be invalidated after a named or unique creation/erasure/reserve
operation:
</p>
<p>
</p>
<pre class="programlisting"><span class="keyword">typedef</span> <span class="identifier">managed_shared_memory</span><span class="special">::</span><span class="identifier">const_named_iterator</span> <span class="identifier">const_named_it</span><span class="special">;</span>
<span class="identifier">const_named_it</span> <span class="identifier">named_beg</span> <span class="special">=</span> <span class="identifier">managed_shm</span><span class="special">.</span><span class="identifier">named_begin</span><span class="special">();</span>
<span class="identifier">const_named_it</span> <span class="identifier">named_end</span> <span class="special">=</span> <span class="identifier">managed_shm</span><span class="special">.</span><span class="identifier">named_end</span><span class="special">();</span>
<span class="keyword">typedef</span> <span class="identifier">managed_shared_memory</span><span class="special">::</span><span class="identifier">const_unique_iterator</span> <span class="identifier">const_unique_it</span><span class="special">;</span>
<span class="identifier">const_unique_it</span> <span class="identifier">unique_beg</span> <span class="special">=</span> <span class="identifier">managed_shm</span><span class="special">.</span><span class="identifier">unique_begin</span><span class="special">();</span>
<span class="identifier">const_unique_it</span> <span class="identifier">unique_end</span> <span class="special">=</span> <span class="identifier">managed_shm</span><span class="special">.</span><span class="identifier">unique_end</span><span class="special">();</span>
<span class="keyword">for</span><span class="special">(;</span> <span class="identifier">named_beg</span> <span class="special">!=</span> <span class="identifier">named_end</span><span class="special">;</span> <span class="special">++</span><span class="identifier">named_beg</span><span class="special">){</span>
<span class="comment">//A pointer to the name of the named object
</span> <span class="keyword">const</span> <span class="identifier">managed_shared_memory</span><span class="special">::</span><span class="identifier">char_type</span> <span class="special">*</span><span class="identifier">name</span> <span class="special">=</span> <span class="identifier">named_beg</span><span class="special">-></span><span class="identifier">name</span><span class="special">();</span>
<span class="comment">//The length of the name
</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span> <span class="identifier">name_len</span> <span class="special">=</span> <span class="identifier">named_beg</span><span class="special">-></span><span class="identifier">name_length</span><span class="special">();</span>
<span class="comment">//A constant void pointer to the named object
</span> <span class="keyword">const</span> <span class="keyword">void</span> <span class="special">*</span><span class="identifier">value</span> <span class="special">=</span> <span class="identifier">named_beg</span><span class="special">-></span><span class="identifier">value</span><span class="special">();</span>
<span class="special">}</span>
<span class="keyword">for</span><span class="special">(;</span> <span class="identifier">unique_beg</span> <span class="special">!=</span> <span class="identifier">unique_end</span><span class="special">;</span> <span class="special">++</span><span class="identifier">unique_beg</span><span class="special">){</span>
<span class="comment">//The typeid(T).name() of the unique object
</span> <span class="keyword">const</span> <span class="keyword">char</span> <span class="special">*</span><span class="identifier">typeid_name</span> <span class="special">=</span> <span class="identifier">unique_beg</span><span class="special">-></span><span class="identifier">name</span><span class="special">();</span>
<span class="comment">//The length of the name
</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span> <span class="identifier">name_len</span> <span class="special">=</span> <span class="identifier">unique_beg</span><span class="special">-></span><span class="identifier">name_length</span><span class="special">();</span>
<span class="comment">//A constant void pointer to the unique object
</span> <span class="keyword">const</span> <span class="keyword">void</span> <span class="special">*</span><span class="identifier">value</span> <span class="special">=</span> <span class="identifier">unique_beg</span><span class="special">-></span><span class="identifier">value</span><span class="special">();</span>
<span class="special">}</span>
</pre>
</div>
<div class="section">
<div class="titlepage"><div><div><h4 class="title">
<a name="interprocess.managed_memory_segments.managed_memory_segment_advanced_features.allocate_aligned"></a><a class="link" href="managed_memory_segments.html#interprocess.managed_memory_segments.managed_memory_segment_advanced_features.allocate_aligned" title="Allocating aligned memory portions">
Allocating aligned memory portions</a>
</h4></div></div></div>
<p>
Sometimes it's interesting to be able to allocate aligned fragments of
memory because of some hardware or software restrictions. Sometimes, having
aligned memory is a feature that can be used to improve several memory
algorithms.
</p>
<p>
This allocation is similar to the previously shown raw memory allocation
but it takes an additional parameter specifying the alignment. There is
a restriction for the alignment: <span class="bold"><strong>the alignment must
be power of two</strong></span>.
</p>
<p>
If a user wants to allocate many aligned blocks (for example aligned to
128 bytes), the size that minimizes the memory waste is a value that's
is nearly a multiple of that alignment (for example 2*128 - some bytes).
The reason for this is that every memory allocation usually needs some
additional metadata in the first bytes of the allocated buffer. If the
user can know the value of "some bytes" and if the first bytes
of a free block of memory are used to fulfill the aligned allocation, the
rest of the block can be left also aligned and ready for the next aligned
allocation. Note that requesting <span class="bold"><strong>a size multiple
of the alignment is not optimal</strong></span> because lefts the next block
of memory unaligned due to the needed metadata.
</p>
<p>
Once the programmer knows the size of the payload of every memory allocation,
he can request a size that will be optimal to allocate aligned chunks of
memory maximizing both the size of the request <span class="bold"><strong>and</strong></span>
the possibilities of future aligned allocations. This information is stored
in the PayloadPerAllocation constant of managed memory segments.
</p>
<p>
Here is a small example showing how aligned allocation is used:
</p>
<p>
</p>
<p>
</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">managed_shared_memory</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span>
<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">cassert</span><span class="special">></span>
<span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</span>
<span class="special">{</span>
<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="comment">//Remove shared memory on construction and destruction
</span> <span class="keyword">struct</span> <span class="identifier">shm_remove</span>
<span class="special">{</span>
<span class="identifier">shm_remove</span><span class="special">()</span> <span class="special">{</span> <span class="identifier">shared_memory_object</span><span class="special">::</span><span class="identifier">remove</span><span class="special">(</span><span class="string">"MySharedMemory"</span><span class="special">);</span> <span class="special">}</span>
<span class="special">~</span><span class="identifier">shm_remove</span><span class="special">(){</span> <span class="identifier">shared_memory_object</span><span class="special">::</span><span class="identifier">remove</span><span class="special">(</span><span class="string">"MySharedMemory"</span><span class="special">);</span> <span class="special">}</span>
<span class="special">}</span> <span class="identifier">remover</span><span class="special">;</span>
<span class="comment">//Managed memory segment that allocates portions of a shared memory
</span> <span class="comment">//segment with the default management algorithm
</span> <span class="identifier">managed_shared_memory</span> <span class="identifier">managed_shm</span><span class="special">(</span><span class="identifier">create_only</span><span class="special">,</span> <span class="string">"MySharedMemory"</span><span class="special">,</span> <span class="number">65536</span><span class="special">);</span>
<span class="keyword">const</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span> <span class="identifier">Alignment</span> <span class="special">=</span> <span class="number">128</span><span class="special">;</span>
<span class="comment">//Allocate 100 bytes aligned to Alignment from segment, throwing version
</span> <span class="keyword">void</span> <span class="special">*</span><span class="identifier">ptr</span> <span class="special">=</span> <span class="identifier">managed_shm</span><span class="special">.</span><span class="identifier">allocate_aligned</span><span class="special">(</span><span class="number">100</span><span class="special">,</span> <span class="identifier">Alignment</span><span class="special">);</span>
<span class="comment">//Check alignment
</span> <span class="identifier">assert</span><span class="special">((</span><span class="keyword">static_cast</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="keyword">static_cast</span><span class="special"><</span><span class="keyword">char</span><span class="special">*>(</span><span class="number">0</span><span class="special">))</span> <span class="special">%</span> <span class="identifier">Alignment</span> <span class="special">==</span> <span class="number">0</span><span class="special">);</span>
<span class="comment">//Deallocate it
</span> <span class="identifier">managed_shm</span><span class="special">.</span><span class="identifier">deallocate</span><span class="special">(</span><span class="identifier">ptr</span><span class="special">);</span>
<span class="comment">//Non throwing version
</span> <span class="identifier">ptr</span> <span class="special">=</span> <span class="identifier">managed_shm</span><span class="special">.</span><span class="identifier">allocate_aligned</span><span class="special">(</span><span class="number">100</span><span class="special">,</span> <span class="identifier">Alignment</span><span class="special">,</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">nothrow</span><span class="special">);</span>
<span class="comment">//Check alignment
</span> <span class="identifier">assert</span><span class="special">((</span><span class="keyword">static_cast</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="keyword">static_cast</span><span class="special"><</span><span class="keyword">char</span><span class="special">*>(</span><span class="number">0</span><span class="special">))</span> <span class="special">%</span> <span class="identifier">Alignment</span> <span class="special">==</span> <span class="number">0</span><span class="special">);</span>
<span class="comment">//Deallocate it
</span> <span class="identifier">managed_shm</span><span class="special">.</span><span class="identifier">deallocate</span><span class="special">(</span><span class="identifier">ptr</span><span class="special">);</span>
<span class="comment">//If we want to efficiently allocate aligned blocks of memory
</span> <span class="comment">//use managed_shared_memory::PayloadPerAllocation value
</span> <span class="identifier">assert</span><span class="special">(</span><span class="identifier">Alignment</span> <span class="special">></span> <span class="identifier">managed_shared_memory</span><span class="special">::</span><span class="identifier">PayloadPerAllocation</span><span class="special">);</span>
<span class="comment">//This allocation will maximize the size of the aligned memory
</span> <span class="comment">//and will increase the possibility of finding more aligned memory
</span> <span class="identifier">ptr</span> <span class="special">=</span> <span class="identifier">managed_shm</span><span class="special">.</span><span class="identifier">allocate_aligned</span>
<span class="special">(</span><span class="number">3</span><span class="special">*</span><span class="identifier">Alignment</span> <span class="special">-</span> <span class="identifier">managed_shared_memory</span><span class="special">::</span><span class="identifier">PayloadPerAllocation</span><span class="special">,</span> <span class="identifier">Alignment</span><span class="special">);</span>
<span class="comment">//Check alignment
</span> <span class="identifier">assert</span><span class="special">((</span><span class="keyword">static_cast</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="keyword">static_cast</span><span class="special"><</span><span class="keyword">char</span><span class="special">*>(</span><span class="number">0</span><span class="special">))</span> <span class="special">%</span> <span class="identifier">Alignment</span> <span class="special">==</span> <span class="number">0</span><span class="special">);</span>
<span class="comment">//Deallocate it
</span> <span class="identifier">managed_shm</span><span class="special">.</span><span class="identifier">deallocate</span><span class="special">(</span><span class="identifier">ptr</span><span class="special">);</span>
<span class="keyword">return</span> <span class="number">0</span><span class="special">;</span>
<span class="special">}</span>
</pre>
<p>
</p>
<p>
</p>
</div>
<div class="section">
<div class="titlepage"><div><div><h4 class="title">
<a name="interprocess.managed_memory_segments.managed_memory_segment_advanced_features.managed_memory_segment_multiple_allocations"></a><a class="link" href="managed_memory_segments.html#interprocess.managed_memory_segments.managed_memory_segment_advanced_features.managed_memory_segment_multiple_allocations" title="Multiple allocation functions">
Multiple allocation functions</a>
</h4></div></div></div>
<p>
If an application needs to allocate a lot of memory buffers but it needs
to deallocate them independently, the application is normally forced to
loop calling <code class="computeroutput"><span class="identifier">allocate</span><span class="special">()</span></code>.
Managed memory segments offer an alternative function to pack several allocations
in a single call obtaining memory buffers that:
</p>
<div class="itemizedlist"><ul class="itemizedlist" type="disc">
<li class="listitem">
are packed contiguously in memory (which improves locality)
</li>
<li class="listitem">
can be independently deallocated.
</li>
</ul></div>
<p>
This allocation method is much faster than calling <code class="computeroutput"><span class="identifier">allocate</span><span class="special">()</span></code> in a loop. The downside is that the segment
must provide a contiguous memory segment big enough to hold all the allocations.
Managed memory segments offer this functionality through <code class="computeroutput"><span class="identifier">allocate_many</span><span class="special">()</span></code>
functions. There are 2 types of <code class="computeroutput"><span class="identifier">allocate_many</span></code>
functions:
</p>
<div class="itemizedlist"><ul class="itemizedlist" type="disc">
<li class="listitem">
Allocation of N buffers of memory with the same size.
</li>
<li class="listitem">
Allocation ot N buffers of memory, each one of different size.
</li>
</ul></div>
<p>
</p>
<pre class="programlisting"><span class="comment">//!Allocates n_elements of elem_size bytes.
</span><span class="identifier">multiallocation_iterator</span> <span class="identifier">allocate_many</span><span class="special">(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span> <span class="identifier">elem_size</span><span class="special">,</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span> <span class="identifier">min_elements</span><span class="special">,</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span> <span class="identifier">preferred_elements</span><span class="special">,</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span> <span class="special">&</span><span class="identifier">received_elements</span><span class="special">);</span>
<span class="comment">//!Allocates n_elements, each one of elem_sizes[i] bytes.
</span><span class="identifier">multiallocation_iterator</span> <span class="identifier">allocate_many</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span> <span class="special">*</span><span class="identifier">elem_sizes</span><span class="special">,</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span> <span class="identifier">n_elements</span><span class="special">);</span>
<span class="comment">//!Allocates n_elements of elem_size bytes. No throwing version.
</span><span class="identifier">multiallocation_iterator</span> <span class="identifier">allocate_many</span><span class="special">(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span> <span class="identifier">elem_size</span><span class="special">,</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span> <span class="identifier">min_elements</span><span class="special">,</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span> <span class="identifier">preferred_elements</span><span class="special">,</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span> <span class="special">&</span><span class="identifier">received_elements</span><span class="special">,</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">nothrow_t</span> <span class="identifier">nothrow</span><span class="special">);</span>
<span class="comment">//!Allocates n_elements, each one of elem_sizes[i] bytes. No throwing version.
</span><span class="identifier">multiallocation_iterator</span> <span class="identifier">allocate_many</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span> <span class="special">*</span><span class="identifier">elem_sizes</span><span class="special">,</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span> <span class="identifier">n_elements</span><span class="special">,</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">nothrow_t</span> <span class="identifier">nothrow</span><span class="special">);</span>
</pre>
<p>
All functions return a <code class="computeroutput"><span class="identifier">multiallocation</span>
<span class="identifier">iterator</span></code> that can be used to
obtain pointers to memory the user can overwrite. A <code class="computeroutput"><span class="identifier">multiallocation_iterator</span></code>:
</p>
<div class="itemizedlist"><ul class="itemizedlist" type="disc">
<li class="listitem">
Becomes invalidated if the memory is pointing to is deallocated or
the next iterators (which previously were reachable with <code class="computeroutput"><span class="keyword">operator</span><span class="special">++</span></code>)
become invalid.
</li>
<li class="listitem">
Returned from <code class="computeroutput"><span class="identifier">allocate_many</span></code>
can be checked in a boolean expression to know if the allocation has
been successful.
</li>
<li class="listitem">
A default constructed <code class="computeroutput"><span class="identifier">multiallocation</span>
<span class="identifier">iterator</span></code> indicates both an
invalid iterator and the "end" iterator.
</li>
<li class="listitem">
Dereferencing an iterator (<code class="computeroutput"><span class="keyword">operator</span>
<span class="special">*()</span></code>) returns a <code class="computeroutput"><span class="keyword">char</span> <span class="special">&</span></code>
referencing the first byte user can overwrite in the memory buffer.
</li>
<li class="listitem">
The iterator category depends on the memory allocation algorithm, but
it's at least a forward iterator.
</li>
</ul></div>
<p>
Here is a small example showing all this functionality:
</p>
<p>
</p>
<p>
</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">managed_shared_memory</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span>
<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">detail</span><span class="special">/</span><span class="identifier">move</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> <span class="comment">//boost::interprocess::move
</span><span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">cassert</span><span class="special">>//</span><span class="identifier">assert</span>
<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">cstring</span><span class="special">>//</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">memset</span>
<span class="preprocessor">#include</span> <span class="special"><</span><span class="keyword">new</span><span class="special">></span> <span class="comment">//std::nothrow
</span><span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">vector</span><span class="special">></span> <span class="comment">//std::vector
</span>
<span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</span>
<span class="special">{</span>
<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="keyword">typedef</span> <span class="identifier">managed_shared_memory</span><span class="special">::</span><span class="identifier">multiallocation_chain</span> <span class="identifier">multiallocation_chain</span><span class="special">;</span>
<span class="comment">//Remove shared memory on construction and destruction
</span> <span class="keyword">struct</span> <span class="identifier">shm_remove</span>
<span class="special">{</span>
<span class="identifier">shm_remove</span><span class="special">()</span> <span class="special">{</span> <span class="identifier">shared_memory_object</span><span class="special">::</span><span class="identifier">remove</span><span class="special">(</span><span class="string">"MySharedMemory"</span><span class="special">);</span> <span class="special">}</span>
<span class="special">~</span><span class="identifier">shm_remove</span><span class="special">(){</span> <span class="identifier">shared_memory_object</span><span class="special">::</span><span class="identifier">remove</span><span class="special">(</span><span class="string">"MySharedMemory"</span><span class="special">);</span> <span class="special">}</span>
<span class="special">}</span> <span class="identifier">remover</span><span class="special">;</span>
<span class="identifier">managed_shared_memory</span> <span class="identifier">managed_shm</span><span class="special">(</span><span class="identifier">create_only</span><span class="special">,</span><span class="string">"MySharedMemory"</span><span class="special">,</span> <span class="number">65536</span><span class="special">);</span>
<span class="comment">//Allocate 16 elements of 100 bytes in a single call. Non-throwing version.
</span> <span class="identifier">multiallocation_chain</span> <span class="identifier">chain</span><span class="special">(</span><span class="identifier">managed_shm</span><span class="special">.</span><span class="identifier">allocate_many</span><span class="special">(</span><span class="number">100</span><span class="special">,</span> <span class="number">16</span><span class="special">,</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">nothrow</span><span class="special">));</span>
<span class="comment">//Check if the memory allocation was successful
</span> <span class="keyword">if</span><span class="special">(</span><span class="identifier">chain</span><span class="special">.</span><span class="identifier">empty</span><span class="special">())</span> <span class="keyword">return</span> <span class="number">1</span><span class="special">;</span>
<span class="comment">//Allocated buffers
</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span><span class="special"><</span><span class="keyword">void</span><span class="special">*></span> <span class="identifier">allocated_buffers</span><span class="special">;</span>
<span class="comment">//Initialize our data
</span> <span class="keyword">while</span><span class="special">(!</span><span class="identifier">chain</span><span class="special">.</span><span class="identifier">empty</span><span class="special">()){</span>
<span class="keyword">void</span> <span class="special">*</span><span class="identifier">buf</span> <span class="special">=</span> <span class="identifier">chain</span><span class="special">.</span><span class="identifier">front</span><span class="special">();</span>
<span class="identifier">chain</span><span class="special">.</span><span class="identifier">pop_front</span><span class="special">();</span>
<span class="identifier">allocated_buffers</span><span class="special">.</span><span class="identifier">push_back</span><span class="special">(</span><span class="identifier">buf</span><span class="special">);</span>
<span class="comment">//The iterator must be incremented before overwriting memory
</span> <span class="comment">//because otherwise, the iterator is invalidated.
</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">memset</span><span class="special">(</span><span class="identifier">buf</span><span class="special">,</span> <span class="number">0</span><span class="special">,</span> <span class="number">100</span><span class="special">);</span>
<span class="special">}</span>
<span class="comment">//Now deallocate
</span> <span class="keyword">while</span><span class="special">(!</span><span class="identifier">allocated_buffers</span><span class="special">.</span><span class="identifier">empty</span><span class="special">()){</span>
<span class="identifier">managed_shm</span><span class="special">.</span><span class="identifier">deallocate</span><span class="special">(</span><span class="identifier">allocated_buffers</span><span class="special">.</span><span class="identifier">back</span><span class="special">());</span>
<span class="identifier">allocated_buffers</span><span class="special">.</span><span class="identifier">pop_back</span><span class="special">();</span>
<span class="special">}</span>
<span class="comment">//Allocate 10 buffers of different sizes in a single call. Throwing version
</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span> <span class="identifier">sizes</span><span class="special">[</span><span class="number">10</span><span class="special">];</span>
<span class="keyword">for</span><span class="special">(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span> <span class="identifier">i</span> <span class="special">=</span> <span class="number">0</span><span class="special">;</span> <span class="identifier">i</span> <span class="special"><</span> <span class="number">10</span><span class="special">;</span> <span class="special">++</span><span class="identifier">i</span><span class="special">)</span>
<span class="identifier">sizes</span><span class="special">[</span><span class="identifier">i</span><span class="special">]</span> <span class="special">=</span> <span class="identifier">i</span><span class="special">*</span><span class="number">3</span><span class="special">;</span>
<span class="identifier">chain</span> <span class="special">=</span> <span class="identifier">managed_shm</span><span class="special">.</span><span class="identifier">allocate_many</span><span class="special">(</span><span class="identifier">sizes</span><span class="special">,</span> <span class="number">10</span><span class="special">);</span>
<span class="identifier">managed_shm</span><span class="special">.</span><span class="identifier">deallocate_many</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">move</span><span class="special">(</span><span class="identifier">chain</span><span class="special">));</span>
<span class="keyword">return</span> <span class="number">0</span><span class="special">;</span>
<span class="special">}</span>
</pre>
<p>
</p>
<p>
</p>
<p>
Allocating N buffers of the same size improves the performance of pools
and node containers (for example STL-like lists): when inserting a range
of forward iterators in a STL-like list, the insertion function can detect
the number of needed elements and allocate in a single call. The nodes
still can be deallocated.
</p>
<p>
Allocating N buffers of different sizes can be used to speed up allocation
in cases where several objects must always be allocated at the same time
but deallocated at different times. For example, a class might perform
several initial allocations (some header data for a network packet, for
example) in its constructor but also allocations of buffers that might
be reallocated in the future (the data to be sent through the network).
Instead of allocating all the data independently, the constructor might
use <code class="computeroutput"><span class="identifier">allocate_many</span><span class="special">()</span></code>
to speed up the initialization, but it still can deallocate and expand
the memory of the variable size element.
</p>
<p>
In general, <code class="computeroutput"><span class="identifier">allocate_many</span></code>
is useful with large values of N. Overuse of <code class="computeroutput"><span class="identifier">allocate_many</span></code>
can increase the effective memory usage, because it can't reuse existing
non-contiguous memory fragments that might be available for some of the
elements.
</p>
</div>
<div class="section">
<div class="titlepage"><div><div><h4 class="title">
<a name="interprocess.managed_memory_segments.managed_memory_segment_advanced_features.managed_memory_segment_expand_in_place"></a><a class="link" href="managed_memory_segments.html#interprocess.managed_memory_segments.managed_memory_segment_advanced_features.managed_memory_segment_expand_in_place" title="Expand in place memory allocation">
Expand in place memory allocation</a>
</h4></div></div></div>
<p>
When programming some data structures such as vectors, memory reallocation
becomes an important tool to improve performance. Managed memory segments
offer an advanced reallocation function that offers:
</p>
<div class="itemizedlist"><ul class="itemizedlist" type="disc">
<li class="listitem">
Forward expansion: An allocated buffer can be expanded so that the
end of the buffer is moved further. New data can be written between
the old end and the new end.
</li>
<li class="listitem">
Backwards expansion: An allocated buffer can be expanded so that the
beginning of the buffer is moved backwards. New data can be written
between the new beginning and the old beginning.
</li>
<li class="listitem">
Shrinking: An allocated buffer can be shrunk so that the end of the
buffer is moved backwards. The memory between the new end and the old
end can be reused for future allocations.
</li>
</ul></div>
<p>
The expansion can be combined with the allocation of a new buffer if the
expansion fails obtaining a function with "expand, if fails allocate
a new buffer" semantics.
</p>
<p>
Apart from this features, the function always returns the real size of
the allocated buffer, because many times, due to alignment issues the allocated
buffer a bit bigger than the requested size. Thus, the programmer can maximize
the memory use using <code class="computeroutput"><span class="identifier">allocation_command</span></code>.
</p>
<p>
Here is the declaration of the function:
</p>
<p>
</p>
<pre class="programlisting"><span class="keyword">enum</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">interprocess</span><span class="special">::</span><span class="identifier">allocation_type</span>
<span class="special">{</span>
<span class="comment">//Bitwise OR (|) combinable values
</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">interprocess</span><span class="special">::</span><span class="identifier">allocate_new</span> <span class="special">=</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">expand_fwd</span> <span class="special">=</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">expand_bwd</span> <span class="special">=</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">shrink_in_place</span> <span class="special">=</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">nothrow_allocation</span> <span class="special">=</span> <span class="special">...</span>
<span class="special">};</span>
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">pair</span><span class="special"><</span><span class="identifier">T</span> <span class="special">*,</span> <span class="keyword">bool</span><span class="special">></span>
<span class="identifier">allocation_command</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">allocation_type</span> <span class="identifier">command</span>
<span class="special">,</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span> <span class="identifier">limit_size</span>
<span class="special">,</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span> <span class="identifier">preferred_size</span>
<span class="special">,</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span> <span class="special">&</span><span class="identifier">received_size</span>
<span class="special">,</span> <span class="identifier">T</span> <span class="special">*</span><span class="identifier">reuse_ptr</span> <span class="special">=</span> <span class="number">0</span><span class="special">);</span>
</pre>
<p>
<span class="bold"><strong>Preconditions for the function</strong></span>:
</p>
<div class="itemizedlist"><ul class="itemizedlist" type="disc">
<li class="listitem">
If the parameter command contains the value <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">interprocess</span><span class="special">::</span><span class="identifier">shrink_in_place</span></code>
it can't contain any of these values: <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">interprocess</span><span class="special">::</span><span class="identifier">expand_fwd</span></code>,
<code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">interprocess</span><span class="special">::</span><span class="identifier">expand_bwd</span></code>.
</li>
<li class="listitem">
If the parameter command contains <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">interprocess</span><span class="special">::</span><span class="identifier">expand_fwd</span></code>
or <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">interprocess</span><span class="special">::</span><span class="identifier">expand_bwd</span></code>, the parameter <code class="computeroutput"><span class="identifier">reuse_ptr</span></code> must be non-null and returned
by a previous allocation function.
</li>
<li class="listitem">
If the parameter command contains the value <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">interprocess</span><span class="special">::</span><span class="identifier">shrink_in_place</span></code>,
the parameter <code class="computeroutput"><span class="identifier">limit_size</span></code>
must be equal or greater than the parameter <code class="computeroutput"><span class="identifier">preferred_size</span></code>.
</li>
<li class="listitem">
If the parameter <code class="computeroutput"><span class="identifier">command</span></code>
contains any of these values: <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">interprocess</span><span class="special">::</span><span class="identifier">expand_fwd</span></code>
or <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">interprocess</span><span class="special">::</span><span class="identifier">expand_bwd</span></code>, the parameter <code class="computeroutput"><span class="identifier">limit_size</span></code> must be equal or less
than the parameter <code class="computeroutput"><span class="identifier">preferred_size</span></code>.
</li>
</ul></div>
<p>
<span class="bold"><strong>Which are the effects of this function:</strong></span>
</p>
<div class="itemizedlist"><ul class="itemizedlist" type="disc">
<li class="listitem">
If the parameter command contains the value <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">interprocess</span><span class="special">::</span><span class="identifier">shrink_in_place</span></code>,
the function will try to reduce the size of the memory block referenced
by pointer <code class="computeroutput"><span class="identifier">reuse_ptr</span></code>
to the value <code class="computeroutput"><span class="identifier">preferred_size</span></code>
moving only the end of the block. If it's not possible, it will try
to reduce the size of the memory block as much as possible as long
as this results in <code class="computeroutput"><span class="identifier">size</span><span class="special">(</span><span class="identifier">p</span><span class="special">)</span> <span class="special"><=</span> <span class="identifier">limit_size</span></code>. Success is reported only
if this results in <code class="computeroutput"><span class="identifier">preferred_size</span>
<span class="special"><=</span> <span class="identifier">size</span><span class="special">(</span><span class="identifier">p</span><span class="special">)</span></code> and <code class="computeroutput"><span class="identifier">size</span><span class="special">(</span><span class="identifier">p</span><span class="special">)</span> <span class="special"><=</span> <span class="identifier">limit_size</span></code>.
</li>
<li class="listitem">
If the parameter <code class="computeroutput"><span class="identifier">command</span></code>
only contains the value <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">interprocess</span><span class="special">::</span><span class="identifier">expand_fwd</span></code>
(with optional additional <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">interprocess</span><span class="special">::</span><span class="identifier">nothrow_allocation</span></code>),
the allocator will try to increase the size of the memory block referenced
by pointer reuse moving only the end of the block to the value <code class="computeroutput"><span class="identifier">preferred_size</span></code>. If it's not possible,
it will try to increase the size of the memory block as much as possible
as long as this results in <code class="computeroutput"><span class="identifier">size</span><span class="special">(</span><span class="identifier">p</span><span class="special">)</span> <span class="special">>=</span> <span class="identifier">limit_size</span></code>. Success is reported only
if this results in <code class="computeroutput"><span class="identifier">limit_size</span>
<span class="special"><=</span> <span class="identifier">size</span><span class="special">(</span><span class="identifier">p</span><span class="special">)</span></code>.
</li>
<li class="listitem">
If the parameter <code class="computeroutput"><span class="identifier">command</span></code>
only contains the value <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">interprocess</span><span class="special">::</span><span class="identifier">expand_bwd</span></code>
(with optional additional <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">interprocess</span><span class="special">::</span><span class="identifier">nothrow_allocation</span></code>),
the allocator will try to increase the size of the memory block referenced
by pointer <code class="computeroutput"><span class="identifier">reuse_ptr</span></code>
only moving the start of the block to a returned new position <code class="computeroutput"><span class="identifier">new_ptr</span></code>. If it's not possible, it
will try to move the start of the block as much as possible as long
as this results in <code class="computeroutput"><span class="identifier">size</span><span class="special">(</span><span class="identifier">new_ptr</span><span class="special">)</span> <span class="special">>=</span> <span class="identifier">limit_size</span></code>. Success is reported only
if this results in <code class="computeroutput"><span class="identifier">limit_size</span>
<span class="special"><=</span> <span class="identifier">size</span><span class="special">(</span><span class="identifier">new_ptr</span><span class="special">)</span></code>.
</li>
<li class="listitem">
If the parameter <code class="computeroutput"><span class="identifier">command</span></code>
only contains the value <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">interprocess</span><span class="special">::</span><span class="identifier">allocate_new</span></code>
(with optional additional <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">interprocess</span><span class="special">::</span><span class="identifier">nothrow_allocation</span></code>),
the allocator will try to allocate memory for <code class="computeroutput"><span class="identifier">preferred_size</span></code>
objects. If it's not possible it will try to allocate memory for at
least <code class="computeroutput"><span class="identifier">limit_size</span></code> objects.
</li>
<li class="listitem">
If the parameter <code class="computeroutput"><span class="identifier">command</span></code>
only contains a combination of <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">interprocess</span><span class="special">::</span><span class="identifier">expand_fwd</span></code>
and <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">interprocess</span><span class="special">::</span><span class="identifier">allocate_new</span></code>, (with optional additional
<code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">interprocess</span><span class="special">::</span><span class="identifier">nothrow_allocation</span></code>) the allocator
will try first the forward expansion. If this fails, it would try a
new allocation.
</li>
<li class="listitem">
If the parameter <code class="computeroutput"><span class="identifier">command</span></code>
only contains a combination of <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">interprocess</span><span class="special">::</span><span class="identifier">expand_bwd</span></code>
and <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">interprocess</span><span class="special">::</span><span class="identifier">allocate_new</span></code> (with optional additional
<code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">interprocess</span><span class="special">::</span><span class="identifier">nothrow_allocation</span></code>), the allocator
will try first to obtain <code class="computeroutput"><span class="identifier">preferred_size</span></code>
objects using both methods if necessary. If this fails, it will try
to obtain <code class="computeroutput"><span class="identifier">limit_size</span></code>
objects using both methods if necessary.
</li>
<li class="listitem">
If the parameter <code class="computeroutput"><span class="identifier">command</span></code>
only contains a combination of <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">interprocess</span><span class="special">::</span><span class="identifier">expand_fwd</span></code>
and <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">interprocess</span><span class="special">::</span><span class="identifier">expand_bwd</span></code> (with optional additional
<code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">interprocess</span><span class="special">::</span><span class="identifier">nothrow_allocation</span></code>), the allocator
will try first forward expansion. If this fails it will try to obtain
preferred_size objects using backwards expansion or a combination of
forward and backwards expansion. If this fails, it will try to obtain
<code class="computeroutput"><span class="identifier">limit_size</span></code> objects
using both methods if necessary.
</li>
<li class="listitem">
If the parameter <code class="computeroutput"><span class="identifier">command</span></code>
only contains a combination of allocation_new, <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">interprocess</span><span class="special">::</span><span class="identifier">expand_fwd</span></code>
and <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">interprocess</span><span class="special">::</span><span class="identifier">expand_bwd</span></code>, (with optional additional
<code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">interprocess</span><span class="special">::</span><span class="identifier">nothrow_allocation</span></code>) the allocator
will try first forward expansion. If this fails it will try to obtain
preferred_size objects using new allocation, backwards expansion or
a combination of forward and backwards expansion. If this fails, it
will try to obtain <code class="computeroutput"><span class="identifier">limit_size</span></code>
objects using the same methods.
</li>
<li class="listitem">
The allocator always writes the size or the expanded/allocated/shrunk
memory block in <code class="computeroutput"><span class="identifier">received_size</span></code>.
On failure the allocator writes in <code class="computeroutput"><span class="identifier">received_size</span></code>
a possibly successful <code class="computeroutput"><span class="identifier">limit_size</span></code>
parameter for a new call.
</li>
</ul></div>
<p>
<span class="bold"><strong>Throws an exception if two conditions are met:</strong></span>
</p>
<div class="itemizedlist"><ul class="itemizedlist" type="disc">
<li class="listitem">
The allocator is unable to allocate/expand/shrink the memory or there
is an error in preconditions
</li>
<li class="listitem">
The parameter command does not contain <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">interprocess</span><span class="special">::</span><span class="identifier">nothrow_allocation</span></code>.
</li>
</ul></div>
<p>
<span class="bold"><strong>This function returns:</strong></span>
</p>
<div class="itemizedlist"><ul class="itemizedlist" type="disc">
<li class="listitem">
The address of the allocated memory or the new address of the expanded
memory as the first member of the pair. If the parameter command contains
<code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">interprocess</span><span class="special">::</span><span class="identifier">nothrow_allocation</span></code> the first member
will be 0 if the allocation/expansion fails or there is an error in
preconditions.
</li>
<li class="listitem">
The second member of the pair will be false if the memory has been
allocated, true if the memory has been expanded. If the first member
is 0, the second member has an undefined value.
</li>
</ul></div>
<p>
<span class="bold"><strong>Notes:</strong></span>
</p>
<div class="itemizedlist"><ul class="itemizedlist" type="disc">
<li class="listitem">
If the user chooses <code class="computeroutput"><span class="keyword">char</span></code>
as template argument the returned buffer will be suitably aligned to
hold any type.
</li>
<li class="listitem">
If the user chooses <code class="computeroutput"><span class="keyword">char</span></code>
as template argument and a backwards expansion is performed, although
properly aligned, the returned buffer might not be suitable because
the distance between the new beginning and the old beginning might
not multiple of the type the user wants to construct, since due to
internal restrictions the expansion can be slightly bigger than the
requested bytes. <span class="bold"><strong>When performing backwards expansion,
if you have already constructed objects in the old buffer, make sure
to specify correctly the type.</strong></span>
</li>
</ul></div>
<p>
Here is a small example that shows the use of <code class="computeroutput"><span class="identifier">allocation_command</span></code>:
</p>
<p>
</p>
<p>
</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">managed_shared_memory</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span>
<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">cassert</span><span class="special">></span>
<span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</span>
<span class="special">{</span>
<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="comment">//Remove shared memory on construction and destruction
</span> <span class="keyword">struct</span> <span class="identifier">shm_remove</span>
<span class="special">{</span>
<span class="identifier">shm_remove</span><span class="special">()</span> <span class="special">{</span> <span class="identifier">shared_memory_object</span><span class="special">::</span><span class="identifier">remove</span><span class="special">(</span><span class="string">"MySharedMemory"</span><span class="special">);</span> <span class="special">}</span>
<span class="special">~</span><span class="identifier">shm_remove</span><span class="special">(){</span> <span class="identifier">shared_memory_object</span><span class="special">::</span><span class="identifier">remove</span><span class="special">(</span><span class="string">"MySharedMemory"</span><span class="special">);</span> <span class="special">}</span>
<span class="special">}</span> <span class="identifier">remover</span><span class="special">;</span>
<span class="comment">//Managed memory segment that allocates portions of a shared memory
</span> <span class="comment">//segment with the default management algorithm
</span> <span class="identifier">managed_shared_memory</span> <span class="identifier">managed_shm</span><span class="special">(</span><span class="identifier">create_only</span><span class="special">,</span> <span class="string">"MySharedMemory"</span><span class="special">,</span> <span class="number">10000</span><span class="special">*</span><span class="keyword">sizeof</span><span class="special">(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span><span class="special">));</span>
<span class="comment">//Allocate at least 100 bytes, 1000 bytes if possible
</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span> <span class="identifier">received_size</span><span class="special">,</span> <span class="identifier">min_size</span> <span class="special">=</span> <span class="number">100</span><span class="special">,</span> <span class="identifier">preferred_size</span> <span class="special">=</span> <span class="number">1000</span><span class="special">;</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span> <span class="special">*</span><span class="identifier">ptr</span> <span class="special">=</span> <span class="identifier">managed_shm</span><span class="special">.</span><span class="identifier">allocation_command</span><span class="special"><</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span><span class="special">></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">allocate_new</span><span class="special">,</span> <span class="identifier">min_size</span><span class="special">,</span> <span class="identifier">preferred_size</span><span class="special">,</span> <span class="identifier">received_size</span><span class="special">).</span><span class="identifier">first</span><span class="special">;</span>
<span class="comment">//Received size must be bigger than min_size
</span> <span class="identifier">assert</span><span class="special">(</span><span class="identifier">received_size</span> <span class="special">>=</span> <span class="identifier">min_size</span><span class="special">);</span>
<span class="comment">//Get free memory
</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span> <span class="identifier">free_memory_after_allocation</span> <span class="special">=</span> <span class="identifier">managed_shm</span><span class="special">.</span><span class="identifier">get_free_memory</span><span class="special">();</span>
<span class="comment">//Now write the data
</span> <span class="keyword">for</span><span class="special">(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span> <span class="identifier">i</span> <span class="special">=</span> <span class="number">0</span><span class="special">;</span> <span class="identifier">i</span> <span class="special"><</span> <span class="identifier">received_size</span><span class="special">;</span> <span class="special">++</span><span class="identifier">i</span><span class="special">)</span> <span class="identifier">ptr</span><span class="special">[</span><span class="identifier">i</span><span class="special">]</span> <span class="special">=</span> <span class="identifier">i</span><span class="special">;</span>
<span class="comment">//Now try to triplicate the buffer. We won't admit an expansion
</span> <span class="comment">//lower to the double of the original buffer.
</span> <span class="comment">//This "should" be successful since no other class is allocating
</span> <span class="comment">//memory from the segment
</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span> <span class="identifier">expanded_size</span><span class="special">;</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">pair</span><span class="special"><</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span> <span class="special">*,</span> <span class="keyword">bool</span><span class="special">></span> <span class="identifier">ret</span> <span class="special">=</span> <span class="identifier">managed_shm</span><span class="special">.</span><span class="identifier">allocation_command</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">expand_fwd</span><span class="special">,</span> <span class="identifier">received_size</span><span class="special">*</span><span class="number">2</span><span class="special">,</span> <span class="identifier">received_size</span><span class="special">*</span><span class="number">3</span><span class="special">,</span> <span class="identifier">expanded_size</span><span class="special">,</span> <span class="identifier">ptr</span><span class="special">);</span>
<span class="comment">//Check invariants
</span> <span class="identifier">assert</span><span class="special">(</span><span class="identifier">ret</span><span class="special">.</span><span class="identifier">second</span> <span class="special">==</span> <span class="keyword">true</span><span class="special">);</span>
<span class="identifier">assert</span><span class="special">(</span><span class="identifier">ret</span><span class="special">.</span><span class="identifier">first</span> <span class="special">==</span> <span class="identifier">ptr</span><span class="special">);</span>
<span class="identifier">assert</span><span class="special">(</span><span class="identifier">expanded_size</span> <span class="special">>=</span> <span class="identifier">received_size</span><span class="special">*</span><span class="number">2</span><span class="special">);</span>
<span class="comment">//Get free memory and compare
</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span> <span class="identifier">free_memory_after_expansion</span> <span class="special">=</span> <span class="identifier">managed_shm</span><span class="special">.</span><span class="identifier">get_free_memory</span><span class="special">();</span>
<span class="identifier">assert</span><span class="special">(</span><span class="identifier">free_memory_after_expansion</span> <span class="special"><</span> <span class="identifier">free_memory_after_allocation</span><span class="special">);</span>
<span class="comment">//Write new values
</span> <span class="keyword">for</span><span class="special">(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span> <span class="identifier">i</span> <span class="special">=</span> <span class="identifier">received_size</span><span class="special">;</span> <span class="identifier">i</span> <span class="special"><</span> <span class="identifier">expanded_size</span><span class="special">;</span> <span class="special">++</span><span class="identifier">i</span><span class="special">)</span> <span class="identifier">ptr</span><span class="special">[</span><span class="identifier">i</span><span class="special">]</span> <span class="special">=</span> <span class="identifier">i</span><span class="special">;</span>
<span class="comment">//Try to shrink approximately to min_size, but the new size
</span> <span class="comment">//should be smaller than min_size*2.
</span> <span class="comment">//This "should" be successful since no other class is allocating
</span> <span class="comment">//memory from the segment
</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span> <span class="identifier">shrunk_size</span><span class="special">;</span>
<span class="identifier">ret</span> <span class="special">=</span> <span class="identifier">managed_shm</span><span class="special">.</span><span class="identifier">allocation_command</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">shrink_in_place</span><span class="special">,</span> <span class="identifier">min_size</span><span class="special">*</span><span class="number">2</span><span class="special">,</span> <span class="identifier">min_size</span><span class="special">,</span> <span class="identifier">shrunk_size</span><span class="special">,</span> <span class="identifier">ptr</span><span class="special">);</span>
<span class="comment">//Check invariants
</span> <span class="identifier">assert</span><span class="special">(</span><span class="identifier">ret</span><span class="special">.</span><span class="identifier">second</span> <span class="special">==</span> <span class="keyword">true</span><span class="special">);</span>
<span class="identifier">assert</span><span class="special">(</span><span class="identifier">ret</span><span class="special">.</span><span class="identifier">first</span> <span class="special">==</span> <span class="identifier">ptr</span><span class="special">);</span>
<span class="identifier">assert</span><span class="special">(</span><span class="identifier">shrunk_size</span> <span class="special"><=</span> <span class="identifier">min_size</span><span class="special">*</span><span class="number">2</span><span class="special">);</span>
<span class="identifier">assert</span><span class="special">(</span><span class="identifier">shrunk_size</span> <span class="special">>=</span> <span class="identifier">min_size</span><span class="special">);</span>
<span class="comment">//Get free memory and compare
</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span> <span class="identifier">free_memory_after_shrinking</span> <span class="special">=</span> <span class="identifier">managed_shm</span><span class="special">.</span><span class="identifier">get_free_memory</span><span class="special">();</span>
<span class="identifier">assert</span><span class="special">(</span><span class="identifier">free_memory_after_shrinking</span> <span class="special">></span> <span class="identifier">free_memory_after_expansion</span><span class="special">);</span>
<span class="comment">//Deallocate the buffer
</span> <span class="identifier">managed_shm</span><span class="special">.</span><span class="identifier">deallocate</span><span class="special">(</span><span class="identifier">ptr</span><span class="special">);</span>
<span class="keyword">return</span> <span class="number">0</span><span class="special">;</span>
<span class="special">}</span>
</pre>
<p>
</p>
<p>
</p>
<p>
<code class="computeroutput"><span class="identifier">allocation_command</span></code> is a
very powerful function that can lead to important performance gains. It's
specially useful when programming vector-like data structures where the
programmer can minimize both the number of allocation requests and the
memory waste.
</p>
</div>
<div class="section">
<div class="titlepage"><div><div><h4 class="title">
<a name="interprocess.managed_memory_segments.managed_memory_segment_advanced_features.copy_on_write_read_only"></a><a class="link" href="managed_memory_segments.html#interprocess.managed_memory_segments.managed_memory_segment_advanced_features.copy_on_write_read_only" title="Opening managed shared memory and mapped files with Copy On Write or Read Only modes">
Opening managed shared memory and mapped files with Copy On Write or Read
Only modes</a>
</h4></div></div></div>
<p>
When mapping a memory segment based on shared memory or files, there is
an option to open them using <span class="bold"><strong>open_copy_on_write</strong></span>
option. This option is similar to <code class="computeroutput"><span class="identifier">open_only</span></code>
but every change the programmer does with this managed segment is kept
private to this process and is not translated to the underlying device
(shared memory or file).
</p>
<p>
The underlying shared memory or file is opened as read-only so several
processes can share an initial managed segment and make private changes
to it. If many processes open a managed segment in copy on write mode and
not modified pages from the managed segment will be shared between all
those processes, with considerable memory savings.
</p>
<p>
Opening managed shared memory and mapped files with <span class="bold"><strong>open_read_only</strong></span>
maps the underlying device in memory with <span class="bold"><strong>read-only</strong></span>
attributes. This means that any attempt to write that memory, either creating
objects or locking any mutex might result in an page-fault error (and thus,
program termination) from the OS. Read-only mode opens the underlying device
(shared memory, file...) in read-only mode and can result in considerable
memory savings if several processes just want to process a managed memory
segment without modifying it. Read-only mode operations are limited:
</p>
<div class="itemizedlist"><ul class="itemizedlist" type="disc">
<li class="listitem">
Read-only mode must be used only from managed classes. If the programmer
obtains the segment manager and tries to use it directly it might result
in an access violation. The reason for this is that the segment manager
is placed in the underlying device and does not nothing about the mode
it's been mapped in memory.
</li>
<li class="listitem">
Only const member functions from managed segments should be used.
</li>
<li class="listitem">
Additionally, the <code class="computeroutput"><span class="identifier">find</span><span class="special"><></span></code> member function avoids using
internal locks and can be used to look for named and unique objects.
</li>
</ul></div>
<p>
Here is an example that shows the use of these two open modes:
</p>
<p>
</p>
<p>
</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">managed_mapped_file</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span>
<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">fstream</span><span class="special">></span> <span class="comment">//std::fstream
</span><span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">iterator</span><span class="special">>//</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">distance</span>
<span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</span>
<span class="special">{</span>
<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="comment">//Try to erase any previous managed segment with the same name
</span> <span class="identifier">file_mapping</span><span class="special">::</span><span class="identifier">remove</span><span class="special">(</span><span class="string">"MyManagedFile"</span><span class="special">);</span>
<span class="identifier">file_mapping</span><span class="special">::</span><span class="identifier">remove</span><span class="special">(</span><span class="string">"MyManagedFile2"</span><span class="special">);</span>
<span class="identifier">remove_file_on_destroy</span> <span class="identifier">destroyer1</span><span class="special">(</span><span class="string">"MyManagedFile"</span><span class="special">);</span>
<span class="identifier">remove_file_on_destroy</span> <span class="identifier">destroyer2</span><span class="special">(</span><span class="string">"MyManagedFile2"</span><span class="special">);</span>
<span class="special">{</span>
<span class="comment">//Create an named integer in a managed mapped file
</span> <span class="identifier">managed_mapped_file</span> <span class="identifier">managed_file</span><span class="special">(</span><span class="identifier">create_only</span><span class="special">,</span> <span class="string">"MyManagedFile"</span><span class="special">,</span> <span class="number">65536</span><span class="special">);</span>
<span class="identifier">managed_file</span><span class="special">.</span><span class="identifier">construct</span><span class="special"><</span><span class="keyword">int</span><span class="special">>(</span><span class="string">"MyInt"</span><span class="special">)(</span><span class="number">0u</span><span class="special">);</span>
<span class="comment">//Now create a copy on write version
</span> <span class="identifier">managed_mapped_file</span> <span class="identifier">managed_file_cow</span><span class="special">(</span><span class="identifier">open_copy_on_write</span><span class="special">,</span> <span class="string">"MyManagedFile"</span><span class="special">);</span>
<span class="comment">//Erase the int and create a new one
</span> <span class="keyword">if</span><span class="special">(!</span><span class="identifier">managed_file_cow</span><span class="special">.</span><span class="identifier">destroy</span><span class="special"><</span><span class="keyword">int</span><span class="special">>(</span><span class="string">"MyInt"</span><span class="special">))</span>
<span class="keyword">throw</span> <span class="keyword">int</span><span class="special">(</span><span class="number">0</span><span class="special">);</span>
<span class="identifier">managed_file_cow</span><span class="special">.</span><span class="identifier">construct</span><span class="special"><</span><span class="keyword">int</span><span class="special">>(</span><span class="string">"MyInt2"</span><span class="special">);</span>
<span class="comment">//Check changes
</span> <span class="keyword">if</span><span class="special">(</span><span class="identifier">managed_file_cow</span><span class="special">.</span><span class="identifier">find</span><span class="special"><</span><span class="keyword">int</span><span class="special">>(</span><span class="string">"MyInt"</span><span class="special">).</span><span class="identifier">first</span> <span class="special">&&</span> <span class="special">!</span><span class="identifier">managed_file_cow</span><span class="special">.</span><span class="identifier">find</span><span class="special"><</span><span class="keyword">int</span><span class="special">>(</span><span class="string">"MyInt2"</span><span class="special">).</span><span class="identifier">first</span><span class="special">)</span>
<span class="keyword">throw</span> <span class="keyword">int</span><span class="special">(</span><span class="number">0</span><span class="special">);</span>
<span class="comment">//Check the original is intact
</span> <span class="keyword">if</span><span class="special">(!</span><span class="identifier">managed_file</span><span class="special">.</span><span class="identifier">find</span><span class="special"><</span><span class="keyword">int</span><span class="special">>(</span><span class="string">"MyInt"</span><span class="special">).</span><span class="identifier">first</span> <span class="special">&&</span> <span class="identifier">managed_file</span><span class="special">.</span><span class="identifier">find</span><span class="special"><</span><span class="keyword">int</span><span class="special">>(</span><span class="string">"MyInt2"</span><span class="special">).</span><span class="identifier">first</span><span class="special">)</span>
<span class="keyword">throw</span> <span class="keyword">int</span><span class="special">(</span><span class="number">0</span><span class="special">);</span>
<span class="special">{</span> <span class="comment">//Dump the modified copy on write segment to a file
</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">fstream</span> <span class="identifier">file</span><span class="special">(</span><span class="string">"MyManagedFile2"</span><span class="special">,</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">ios_base</span><span class="special">::</span><span class="identifier">out</span> <span class="special">|</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">ios_base</span><span class="special">::</span><span class="identifier">binary</span><span class="special">);</span>
<span class="keyword">if</span><span class="special">(!</span><span class="identifier">file</span><span class="special">)</span>
<span class="keyword">throw</span> <span class="keyword">int</span><span class="special">(</span><span class="number">0</span><span class="special">);</span>
<span class="identifier">file</span><span class="special">.</span><span class="identifier">write</span><span class="special">(</span><span class="keyword">static_cast</span><span class="special"><</span><span class="keyword">const</span> <span class="keyword">char</span> <span class="special">*>(</span><span class="identifier">managed_file_cow</span><span class="special">.</span><span class="identifier">get_address</span><span class="special">()),</span> <span class="identifier">managed_file_cow</span><span class="special">.</span><span class="identifier">get_size</span><span class="special">());</span>
<span class="special">}</span>
<span class="comment">//Now open the modified file and test changes
</span> <span class="identifier">managed_mapped_file</span> <span class="identifier">managed_file_cow2</span><span class="special">(</span><span class="identifier">open_only</span><span class="special">,</span> <span class="string">"MyManagedFile2"</span><span class="special">);</span>
<span class="keyword">if</span><span class="special">(</span><span class="identifier">managed_file_cow2</span><span class="special">.</span><span class="identifier">find</span><span class="special"><</span><span class="keyword">int</span><span class="special">>(</span><span class="string">"MyInt"</span><span class="special">).</span><span class="identifier">first</span> <span class="special">&&</span> <span class="special">!</span><span class="identifier">managed_file_cow2</span><span class="special">.</span><span class="identifier">find</span><span class="special"><</span><span class="keyword">int</span><span class="special">>(</span><span class="string">"MyInt2"</span><span class="special">).</span><span class="identifier">first</span><span class="special">)</span>
<span class="keyword">throw</span> <span class="keyword">int</span><span class="special">(</span><span class="number">0</span><span class="special">);</span>
<span class="special">}</span>
<span class="special">{</span>
<span class="comment">//Now create a read-only version
</span> <span class="identifier">managed_mapped_file</span> <span class="identifier">managed_file_ro</span><span class="special">(</span><span class="identifier">open_read_only</span><span class="special">,</span> <span class="string">"MyManagedFile"</span><span class="special">);</span>
<span class="comment">//Check the original is intact
</span> <span class="keyword">if</span><span class="special">(!</span><span class="identifier">managed_file_ro</span><span class="special">.</span><span class="identifier">find</span><span class="special"><</span><span class="keyword">int</span><span class="special">>(</span><span class="string">"MyInt"</span><span class="special">).</span><span class="identifier">first</span> <span class="special">&&</span> <span class="identifier">managed_file_ro</span><span class="special">.</span><span class="identifier">find</span><span class="special"><</span><span class="keyword">int</span><span class="special">>(</span><span class="string">"MyInt2"</span><span class="special">).</span><span class="identifier">first</span><span class="special">)</span>
<span class="keyword">throw</span> <span class="keyword">int</span><span class="special">(</span><span class="number">0</span><span class="special">);</span>
<span class="comment">//Check the number of named objects using the iterators
</span> <span class="keyword">if</span><span class="special">(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">distance</span><span class="special">(</span><span class="identifier">managed_file_ro</span><span class="special">.</span><span class="identifier">named_begin</span><span class="special">(),</span> <span class="identifier">managed_file_ro</span><span class="special">.</span><span class="identifier">named_end</span><span class="special">())</span> <span class="special">!=</span> <span class="number">1</span> <span class="special">&&</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">distance</span><span class="special">(</span><span class="identifier">managed_file_ro</span><span class="special">.</span><span class="identifier">unique_begin</span><span class="special">(),</span> <span class="identifier">managed_file_ro</span><span class="special">.</span><span class="identifier">unique_end</span><span class="special">())</span> <span class="special">!=</span> <span class="number">0</span> <span class="special">)</span>
<span class="keyword">throw</span> <span class="keyword">int</span><span class="special">(</span><span class="number">0</span><span class="special">);</span>
<span class="special">}</span>
<span class="keyword">return</span> <span class="number">0</span><span class="special">;</span>
<span class="special">}</span>
</pre>
<p>
</p>
<p>
</p>
</div>
</div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="interprocess.managed_memory_segments.managed_heap_memory_external_buffer"></a><a class="link" href="managed_memory_segments.html#interprocess.managed_memory_segments.managed_heap_memory_external_buffer" title="Managed Heap Memory And Managed External Buffer">
Managed Heap Memory And Managed External Buffer</a>
</h3></div></div></div>
<div class="toc"><dl>
<dt><span class="section"><a href="managed_memory_segments.html#interprocess.managed_memory_segments.managed_heap_memory_external_buffer.managed_external_buffer">
Managed External Buffer: Constructing all Boost.Interprocess objects in a
user provided buffer</a></span></dt>
<dt><span class="section"><a href="managed_memory_segments.html#interprocess.managed_memory_segments.managed_heap_memory_external_buffer.managed_heap_memory">
Managed Heap Memory: Boost.Interprocess machinery in heap memory</a></span></dt>
<dt><span class="section"><a href="managed_memory_segments.html#interprocess.managed_memory_segments.managed_heap_memory_external_buffer.managed_heap_memory_external_buffer_diff">
Differences between managed memory segments</a></span></dt>
<dt><span class="section"><a href="managed_memory_segments.html#interprocess.managed_memory_segments.managed_heap_memory_external_buffer.shared_message_queue_ex">
Example: Serializing a database through the message queue</a></span></dt>
</dl></div>
<p>
<span class="bold"><strong>Boost.Interprocess</strong></span> offers managed shared
memory between processes using <code class="computeroutput"><span class="identifier">managed_shared_memory</span></code>
or <code class="computeroutput"><span class="identifier">managed_mapped_file</span></code>. Two
processes just map the same the memory mappable resource and read from and
write to that object.
</p>
<p>
Many times, we don't want to use that shared memory approach and we prefer
to send serialized data through network, local socket or message queues.
Serialization can be done through <span class="bold"><strong>Boost.Serialization</strong></span>
or similar library. However, if two processes share the same ABI (application
binary interface), we could use the same object and container construction
capabilities of <code class="computeroutput"><span class="identifier">managed_shared_memory</span></code>
or <code class="computeroutput"><span class="identifier">managed_heap_memory</span></code> to
build all the information in a single buffer that will be sent, for example,
though message queues. The receiver would just copy the data to a local buffer,
and it could read or modify it directly without deserializing the data .
This approach can be much more efficient that a complex serialization mechanism.
</p>
<p>
Applications for <span class="bold"><strong>Boost.Interprocess</strong></span> services
using non-shared memory buffers:
</p>
<div class="itemizedlist"><ul class="itemizedlist" type="disc">
<li class="listitem">
Create and use STL compatible containers and allocators, in systems where
dynamic memory is not recommendable.
</li>
<li class="listitem">
Build complex, easily serializable databases in a single buffer:
<div class="itemizedlist"><ul class="itemizedlist" type="circle">
<li class="listitem">
To share data between threads
</li>
<li class="listitem">
To save and load information from/to files.
</li>
</ul></div>
</li>
<li class="listitem">
Duplicate information (containers, allocators, etc...) just copying the
contents of one buffer to another one.
</li>
<li class="listitem">
Send complex information and objects/databases using serial/inter-process/network
communications.
</li>
</ul></div>
<p>
To help with this management, <span class="bold"><strong>Boost.Interprocess</strong></span>
provides two useful classes, <code class="computeroutput"><span class="identifier">basic_managed_heap_memory</span></code>
and <code class="computeroutput"><span class="identifier">basic_managed_external_buffer</span></code>:
</p>
<div class="section">
<div class="titlepage"><div><div><h4 class="title">
<a name="interprocess.managed_memory_segments.managed_heap_memory_external_buffer.managed_external_buffer"></a><a class="link" href="managed_memory_segments.html#interprocess.managed_memory_segments.managed_heap_memory_external_buffer.managed_external_buffer" title="Managed External Buffer: Constructing all Boost.Interprocess objects in a user provided buffer">
Managed External Buffer: Constructing all Boost.Interprocess objects in a
user provided buffer</a>
</h4></div></div></div>
<p>
Sometimes, the user wants to create simple objects, STL compatible containers,
STL compatible strings and more, all in a single buffer. This buffer could
be a big static buffer, a memory-mapped auxiliary device or any other user
buffer.
</p>
<p>
This would allow an easy serialization and we-ll just need to copy the
buffer to duplicate all the objects created in the original buffer, including
complex objects like maps, lists.... <span class="bold"><strong>Boost.Interprocess</strong></span>
offers managed memory segment classes to handle user provided buffers that
allow the same functionality as shared memory classes:
</p>
<p>
</p>
<pre class="programlisting"><span class="comment">//Named object creation managed memory segment
</span><span class="comment">//All objects are constructed in a user provided buffer
</span><span class="keyword">template</span> <span class="special"><</span>
<span class="keyword">class</span> <span class="identifier">CharType</span><span class="special">,</span>
<span class="keyword">class</span> <span class="identifier">MemoryAlgorithm</span><span class="special">,</span>
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">IndexConfig</span><span class="special">></span> <span class="keyword">class</span> <span class="identifier">IndexType</span>
<span class="special">></span>
<span class="keyword">class</span> <span class="identifier">basic_managed_external_buffer</span><span class="special">;</span>
<span class="comment">//Named object creation managed memory segment
</span><span class="comment">//All objects are constructed in a user provided buffer
</span><span class="comment">// Names are c-strings,
</span><span class="comment">// Default memory management algorithm
</span><span class="comment">// (rbtree_best_fit with no mutexes and relative pointers)
</span><span class="comment">// Name-object mappings are stored in the default index type (flat_map)
</span><span class="keyword">typedef</span> <span class="identifier">basic_managed_external_buffer</span> <span class="special"><</span>
<span class="keyword">char</span><span class="special">,</span>
<span class="identifier">rbtree_best_fit</span><span class="special"><</span><span class="identifier">null_mutex_family</span><span class="special">,</span> <span class="identifier">offset_ptr</span><span class="special"><</span><span class="keyword">void</span><span class="special">></span> <span class="special">>,</span>
<span class="identifier">flat_map_index</span>
<span class="special">></span> <span class="identifier">managed_external_buffer</span><span class="special">;</span>
<span class="comment">//Named object creation managed memory segment
</span><span class="comment">//All objects are constructed in a user provided buffer
</span><span class="comment">// Names are wide-strings,
</span><span class="comment">// Default memory management algorithm
</span><span class="comment">// (rbtree_best_fit with no mutexes and relative pointers)
</span><span class="comment">// Name-object mappings are stored in the default index type (flat_map)
</span><span class="keyword">typedef</span> <span class="identifier">basic_managed_external_buffer</span><span class="special"><</span>
<span class="keyword">wchar_t</span><span class="special">,</span>
<span class="identifier">rbtree_best_fit</span><span class="special"><</span><span class="identifier">null_mutex_family</span><span class="special">,</span> <span class="identifier">offset_ptr</span><span class="special"><</span><span class="keyword">void</span><span class="special">></span> <span class="special">>,</span>
<span class="identifier">flat_map_index</span>
<span class="special">></span> <span class="identifier">wmanaged_external_buffer</span><span class="special">;</span>
</pre>
<p>
To use a managed external buffer, you must include the following header:
</p>
<p>
</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">managed_external_buffer</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span>
</pre>
<p>
Let's see an example of the use of managed_external_buffer:
</p>
<p>
</p>
<p>
</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">managed_external_buffer</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span>
<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">allocators</span><span class="special">/</span><span class="identifier">allocator</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span>
<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">containers</span><span class="special">/</span><span class="identifier">list</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span>
<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">cstring</span><span class="special">></span>
<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">aligned_storage</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span>
<span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</span>
<span class="special">{</span>
<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="comment">//Create the static memory who will store all objects
</span> <span class="keyword">const</span> <span class="keyword">int</span> <span class="identifier">memsize</span> <span class="special">=</span> <span class="number">65536</span><span class="special">;</span>
<span class="keyword">static</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">aligned_storage</span><span class="special"><</span><span class="identifier">memsize</span><span class="special">>::</span><span class="identifier">type</span> <span class="identifier">static_buffer</span><span class="special">;</span>
<span class="comment">//This managed memory will construct objects associated with
</span> <span class="comment">//a wide string in the static buffer
</span> <span class="identifier">wmanaged_external_buffer</span> <span class="identifier">objects_in_static_memory</span>
<span class="special">(</span><span class="identifier">create_only</span><span class="special">,</span> <span class="special">&</span><span class="identifier">static_buffer</span><span class="special">,</span> <span class="identifier">memsize</span><span class="special">);</span>
<span class="comment">//We optimize resources to create 100 named objects in the static buffer
</span> <span class="identifier">objects_in_static_memory</span><span class="special">.</span><span class="identifier">reserve_named_objects</span><span class="special">(</span><span class="number">100</span><span class="special">);</span>
<span class="comment">//Alias an integer node allocator type
</span> <span class="comment">//This allocator will allocate memory inside the static buffer
</span> <span class="keyword">typedef</span> <span class="identifier">allocator</span><span class="special"><</span><span class="keyword">int</span><span class="special">,</span> <span class="identifier">wmanaged_external_buffer</span><span class="special">::</span><span class="identifier">segment_manager</span><span class="special">></span>
<span class="identifier">allocator_t</span><span class="special">;</span>
<span class="comment">//Alias a STL compatible list to be constructed in the static buffer
</span> <span class="keyword">typedef</span> <span class="identifier">list</span><span class="special"><</span><span class="keyword">int</span><span class="special">,</span> <span class="identifier">allocator_t</span><span class="special">></span> <span class="identifier">MyBufferList</span><span class="special">;</span>
<span class="comment">//The list must be initialized with the allocator
</span> <span class="comment">//All objects created with objects_in_static_memory will
</span> <span class="comment">//be stored in the static_buffer!
</span> <span class="identifier">MyBufferList</span> <span class="special">*</span><span class="identifier">list</span> <span class="special">=</span> <span class="identifier">objects_in_static_memory</span><span class="special">.</span><span class="identifier">construct</span><span class="special"><</span><span class="identifier">MyBufferList</span><span class="special">>(</span><span class="identifier">L</span><span class="string">"MyList"</span><span class="special">)</span>
<span class="special">(</span><span class="identifier">objects_in_static_memory</span><span class="special">.</span><span class="identifier">get_segment_manager</span><span class="special">());</span>
<span class="comment">//Since the allocation algorithm from wmanaged_external_buffer uses relative
</span> <span class="comment">//pointers and all the pointers constructed int the static memory point
</span> <span class="comment">//to objects in the same segment, we can create another static buffer
</span> <span class="comment">//from the first one and duplicate all the data.
</span> <span class="keyword">static</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">aligned_storage</span><span class="special"><</span><span class="identifier">memsize</span><span class="special">>::</span><span class="identifier">type</span> <span class="identifier">static_buffer2</span><span class="special">;</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">memcpy</span><span class="special">(&</span><span class="identifier">static_buffer2</span><span class="special">,</span> <span class="special">&</span><span class="identifier">static_buffer</span><span class="special">,</span> <span class="identifier">memsize</span><span class="special">);</span>
<span class="comment">//Now open the duplicated managed memory passing the memory as argument
</span> <span class="identifier">wmanaged_external_buffer</span> <span class="identifier">objects_in_static_memory2</span>
<span class="special">(</span><span class="identifier">open_only</span><span class="special">,</span> <span class="special">&</span><span class="identifier">static_buffer2</span><span class="special">,</span> <span class="identifier">memsize</span><span class="special">);</span>
<span class="comment">//Check that "MyList" has been duplicated in the second buffer
</span> <span class="keyword">if</span><span class="special">(!</span><span class="identifier">objects_in_static_memory2</span><span class="special">.</span><span class="identifier">find</span><span class="special"><</span><span class="identifier">MyBufferList</span><span class="special">>(</span><span class="identifier">L</span><span class="string">"MyList"</span><span class="special">).</span><span class="identifier">first</span><span class="special">)</span>
<span class="keyword">return</span> <span class="number">1</span><span class="special">;</span>
<span class="comment">//Destroy the lists from the static buffers
</span> <span class="identifier">objects_in_static_memory</span><span class="special">.</span><span class="identifier">destroy</span><span class="special"><</span><span class="identifier">MyBufferList</span><span class="special">>(</span><span class="identifier">L</span><span class="string">"MyList"</span><span class="special">);</span>
<span class="identifier">objects_in_static_memory2</span><span class="special">.</span><span class="identifier">destroy</span><span class="special"><</span><span class="identifier">MyBufferList</span><span class="special">>(</span><span class="identifier">L</span><span class="string">"MyList"</span><span class="special">);</span>
<span class="keyword">return</span> <span class="number">0</span><span class="special">;</span>
<span class="special">}</span>
</pre>
<p>
</p>
<p>
</p>
<p>
<span class="bold"><strong>Boost.Interprocess</strong></span> STL compatible allocators
can also be used to place STL compatible containers in the user segment.
</p>
<p>
<code class="computeroutput"><a class="link" href="../boost/interprocess/basic_managed_external__id819571.html" title="Class template basic_managed_external_buffer">basic_managed_external_buffer</a></code>
can be also useful to build small databases for embedded systems limiting
the size of the used memory to a predefined memory chunk, instead of letting
the database fragment the heap memory.
</p>
</div>
<div class="section">
<div class="titlepage"><div><div><h4 class="title">
<a name="interprocess.managed_memory_segments.managed_heap_memory_external_buffer.managed_heap_memory"></a><a class="link" href="managed_memory_segments.html#interprocess.managed_memory_segments.managed_heap_memory_external_buffer.managed_heap_memory" title="Managed Heap Memory: Boost.Interprocess machinery in heap memory">
Managed Heap Memory: Boost.Interprocess machinery in heap memory</a>
</h4></div></div></div>
<p>
The use of heap memory (new/delete) to obtain a buffer where the user wants
to store all his data is very common, so <span class="bold"><strong>Boost.Interprocess</strong></span>
provides some specialized classes that work exclusively with heap memory.
</p>
<p>
These are the classes:
</p>
<p>
</p>
<pre class="programlisting"><span class="comment">//Named object creation managed memory segment
</span><span class="comment">//All objects are constructed in a single buffer allocated via new[]
</span><span class="keyword">template</span> <span class="special"><</span>
<span class="keyword">class</span> <span class="identifier">CharType</span><span class="special">,</span>
<span class="keyword">class</span> <span class="identifier">MemoryAlgorithm</span><span class="special">,</span>
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">IndexConfig</span><span class="special">></span> <span class="keyword">class</span> <span class="identifier">IndexType</span>
<span class="special">></span>
<span class="keyword">class</span> <span class="identifier">basic_managed_heap_memory</span><span class="special">;</span>
<span class="comment">//Named object creation managed memory segment
</span><span class="comment">//All objects are constructed in a single buffer allocated via new[]
</span><span class="comment">// Names are c-strings,
</span><span class="comment">// Default memory management algorithm
</span><span class="comment">// (rbtree_best_fit with no mutexes and relative pointers)
</span><span class="comment">// Name-object mappings are stored in the default index type (flat_map)
</span><span class="keyword">typedef</span> <span class="identifier">basic_managed_heap_memory</span> <span class="special"><</span>
<span class="keyword">char</span><span class="special">,</span>
<span class="identifier">rbtree_best_fit</span><span class="special"><</span><span class="identifier">null_mutex_family</span><span class="special">>,</span>
<span class="identifier">flat_map_index</span>
<span class="special">></span> <span class="identifier">managed_heap_memory</span><span class="special">;</span>
<span class="comment">//Named object creation managed memory segment
</span><span class="comment">//All objects are constructed in a single buffer allocated via new[]
</span><span class="comment">// Names are wide-strings,
</span><span class="comment">// Default memory management algorithm
</span><span class="comment">// (rbtree_best_fit with no mutexes and relative pointers)
</span><span class="comment">// Name-object mappings are stored in the default index type (flat_map)
</span><span class="keyword">typedef</span> <span class="identifier">basic_managed_heap_memory</span><span class="special"><</span>
<span class="keyword">wchar_t</span><span class="special">,</span>
<span class="identifier">rbtree_best_fit</span><span class="special"><</span><span class="identifier">null_mutex_family</span><span class="special">>,</span>
<span class="identifier">flat_map_index</span>
<span class="special">></span> <span class="identifier">wmanaged_heap_memory</span><span class="special">;</span>
</pre>
<p>
To use a managed heap memory, you must include the following header:
</p>
<p>
</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">managed_heap_memory</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span>
</pre>
<p>
The use is exactly the same as <code class="computeroutput"><a class="link" href="../boost/interprocess/basic_managed_external__id819571.html" title="Class template basic_managed_external_buffer">basic_managed_external_buffer</a></code>,
except that memory is created by the managed memory segment itself using
dynamic (new/delete) memory.
</p>
<p>
<span class="bold"><strong>basic_managed_heap_memory</strong></span> also offers
a <code class="computeroutput"><span class="identifier">grow</span><span class="special">(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span> <span class="identifier">extra_bytes</span><span class="special">)</span></code> function that tries to resize internal
heap memory so that we have room for more objects. But <span class="bold"><strong>be
careful</strong></span>, if memory is reallocated, the old buffer will be copied
into the new one so all the objects will be binary-copied to the new buffer.
To be able to use this function, all pointers constructed in the heap buffer
that point to objects in the heap buffer must be relative pointers (for
example <code class="computeroutput"><span class="identifier">offset_ptr</span></code>). Otherwise,
the result is undefined. Here is an example:
</p>
<p>
</p>
<p>
</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">containers</span><span class="special">/</span><span class="identifier">list</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span>
<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">managed_heap_memory</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span>
<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">allocators</span><span class="special">/</span><span class="identifier">allocator</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span>
<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">cstddef</span><span class="special">></span>
<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="keyword">typedef</span> <span class="identifier">list</span><span class="special"><</span><span class="keyword">int</span><span class="special">,</span> <span class="identifier">allocator</span><span class="special"><</span><span class="keyword">int</span><span class="special">,</span> <span class="identifier">managed_heap_memory</span><span class="special">::</span><span class="identifier">segment_manager</span><span class="special">></span> <span class="special">></span>
<span class="identifier">MyList</span><span class="special">;</span>
<span class="keyword">int</span> <span class="identifier">main</span> <span class="special">()</span>
<span class="special">{</span>
<span class="comment">//We will create a buffer of 1000 bytes to store a list
</span> <span class="identifier">managed_heap_memory</span> <span class="identifier">heap_memory</span><span class="special">(</span><span class="number">1000</span><span class="special">);</span>
<span class="identifier">MyList</span> <span class="special">*</span> <span class="identifier">mylist</span> <span class="special">=</span> <span class="identifier">heap_memory</span><span class="special">.</span><span class="identifier">construct</span><span class="special"><</span><span class="identifier">MyList</span><span class="special">>(</span><span class="string">"MyList"</span><span class="special">)</span>
<span class="special">(</span><span class="identifier">heap_memory</span><span class="special">.</span><span class="identifier">get_segment_manager</span><span class="special">());</span>
<span class="comment">//Obtain handle, that identifies the list in the buffer
</span> <span class="identifier">managed_heap_memory</span><span class="special">::</span><span class="identifier">handle_t</span> <span class="identifier">list_handle</span> <span class="special">=</span> <span class="identifier">heap_memory</span><span class="special">.</span><span class="identifier">get_handle_from_address</span><span class="special">(</span><span class="identifier">mylist</span><span class="special">);</span>
<span class="comment">//Fill list until there is no more memory in the buffer
</span> <span class="keyword">try</span><span class="special">{</span>
<span class="keyword">while</span><span class="special">(</span><span class="number">1</span><span class="special">)</span> <span class="special">{</span>
<span class="identifier">mylist</span><span class="special">-></span><span class="identifier">insert</span><span class="special">(</span><span class="identifier">mylist</span><span class="special">-></span><span class="identifier">begin</span><span class="special">(),</span> <span class="number">0</span><span class="special">);</span>
<span class="special">}</span>
<span class="special">}</span>
<span class="keyword">catch</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">bad_alloc</span> <span class="special">&){</span>
<span class="comment">//memory is full
</span> <span class="special">}</span>
<span class="comment">//Let's obtain the size of the list
</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span> <span class="identifier">old_size</span> <span class="special">=</span> <span class="identifier">mylist</span><span class="special">-></span><span class="identifier">size</span><span class="special">();</span>
<span class="comment">//To make the list bigger, let's increase the heap buffer
</span> <span class="comment">//in 1000 bytes more.
</span> <span class="identifier">heap_memory</span><span class="special">.</span><span class="identifier">grow</span><span class="special">(</span><span class="number">1000</span><span class="special">);</span>
<span class="comment">//If memory has been reallocated, the old pointer is invalid, so
</span> <span class="comment">//use previously obtained handle to find the new pointer.
</span> <span class="identifier">mylist</span> <span class="special">=</span> <span class="keyword">static_cast</span><span class="special"><</span><span class="identifier">MyList</span> <span class="special">*></span>
<span class="special">(</span><span class="identifier">heap_memory</span><span class="special">.</span><span class="identifier">get_address_from_handle</span><span class="special">(</span><span class="identifier">list_handle</span><span class="special">));</span>
<span class="comment">//Fill list until there is no more memory in the buffer
</span> <span class="keyword">try</span><span class="special">{</span>
<span class="keyword">while</span><span class="special">(</span><span class="number">1</span><span class="special">)</span> <span class="special">{</span>
<span class="identifier">mylist</span><span class="special">-></span><span class="identifier">insert</span><span class="special">(</span><span class="identifier">mylist</span><span class="special">-></span><span class="identifier">begin</span><span class="special">(),</span> <span class="number">0</span><span class="special">);</span>
<span class="special">}</span>
<span class="special">}</span>
<span class="keyword">catch</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">bad_alloc</span> <span class="special">&){</span>
<span class="comment">//memory is full
</span> <span class="special">}</span>
<span class="comment">//Let's obtain the new size of the list
</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span> <span class="identifier">new_size</span> <span class="special">=</span> <span class="identifier">mylist</span><span class="special">-></span><span class="identifier">size</span><span class="special">();</span>
<span class="identifier">assert</span><span class="special">(</span><span class="identifier">new_size</span> <span class="special">></span> <span class="identifier">old_size</span><span class="special">);</span>
<span class="comment">//Destroy list
</span> <span class="identifier">heap_memory</span><span class="special">.</span><span class="identifier">destroy_ptr</span><span class="special">(</span><span class="identifier">mylist</span><span class="special">);</span>
<span class="keyword">return</span> <span class="number">0</span><span class="special">;</span>
<span class="special">}</span>
</pre>
<p>
</p>
<p>
</p>
</div>
<div class="section">
<div class="titlepage"><div><div><h4 class="title">
<a name="interprocess.managed_memory_segments.managed_heap_memory_external_buffer.managed_heap_memory_external_buffer_diff"></a><a class="link" href="managed_memory_segments.html#interprocess.managed_memory_segments.managed_heap_memory_external_buffer.managed_heap_memory_external_buffer_diff" title="Differences between managed memory segments">
Differences between managed memory segments</a>
</h4></div></div></div>
<p>
All managed memory segments have similar capabilities (memory allocation
inside the memory segment, named object construction...), but there are
some remarkable differences between <span class="bold"><strong>managed_shared_memory</strong></span>,
<span class="bold"><strong>managed_mapped_file</strong></span> and <span class="bold"><strong>managed_heap_memory</strong></span>,
<span class="bold"><strong>managed_external_file</strong></span>.
</p>
<div class="itemizedlist"><ul class="itemizedlist" type="disc">
<li class="listitem">
Default specializations of managed shared memory and mapped file use
process-shared mutexes. Heap memory and external buffer have no internal
synchronization by default. The cause is that the first two are thought
to be shared between processes (although memory mapped files could
be used just to obtain a persistent object data-base for a process)
whereas the last two are thought to be used inside one process to construct
a serialized named object data-base that can be sent though serial
interprocess communications (like message queues, localhost network...).
</li>
<li class="listitem">
The first two create a system-global object (a shared memory object
or a file) shared by several processes, whereas the last two are objects
that don't create system-wide resources.
</li>
</ul></div>
</div>
<div class="section">
<div class="titlepage"><div><div><h4 class="title">
<a name="interprocess.managed_memory_segments.managed_heap_memory_external_buffer.shared_message_queue_ex"></a><a class="link" href="managed_memory_segments.html#interprocess.managed_memory_segments.managed_heap_memory_external_buffer.shared_message_queue_ex" title="Example: Serializing a database through the message queue">
Example: Serializing a database through the message queue</a>
</h4></div></div></div>
<p>
To see the utility of managed heap memory and managed external buffer classes,
the following example shows how a message queue can be used to serialize
a whole database constructed in a memory buffer using <span class="bold"><strong>Boost.Interprocess</strong></span>,
send the database through a message queue and duplicated in another buffer:
</p>
<p>
</p>
<p>
</p>
<pre class="programlisting"><span class="comment">//This test creates a in memory data-base using Interprocess machinery and
</span><span class="comment">//serializes it through a message queue. Then rebuilds the data-base in
</span><span class="comment">//another buffer and checks it against the original data-base
</span><span class="keyword">bool</span> <span class="identifier">test_serialize_db</span><span class="special">()</span>
<span class="special">{</span>
<span class="comment">//Typedef data to create a Interprocess map
</span> <span class="keyword">typedef</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">pair</span><span class="special"><</span><span class="keyword">const</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span><span class="special">,</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span><span class="special">></span> <span class="identifier">MyPair</span><span class="special">;</span>
<span class="keyword">typedef</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">less</span><span class="special"><</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span><span class="special">></span> <span class="identifier">MyLess</span><span class="special">;</span>
<span class="keyword">typedef</span> <span class="identifier">node_allocator</span><span class="special"><</span><span class="identifier">MyPair</span><span class="special">,</span> <span class="identifier">managed_external_buffer</span><span class="special">::</span><span class="identifier">segment_manager</span><span class="special">></span>
<span class="identifier">node_allocator_t</span><span class="special">;</span>
<span class="keyword">typedef</span> <span class="identifier">map</span><span class="special"><</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span><span class="special">,</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span><span class="special">,</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">less</span><span class="special"><</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span><span class="special">>,</span>
<span class="identifier">node_allocator_t</span><span class="special">></span>
<span class="identifier">MyMap</span><span class="special">;</span>
<span class="comment">//Some constants
</span> <span class="keyword">const</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span> <span class="identifier">BufferSize</span> <span class="special">=</span> <span class="number">65536</span><span class="special">;</span>
<span class="keyword">const</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span> <span class="identifier">MaxMsgSize</span> <span class="special">=</span> <span class="number">100</span><span class="special">;</span>
<span class="comment">//Allocate a memory buffer to hold the destiny database using vector<char>
</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span><span class="special"><</span><span class="keyword">char</span><span class="special">></span> <span class="identifier">buffer_destiny</span><span class="special">(</span><span class="identifier">BufferSize</span><span class="special">,</span> <span class="number">0</span><span class="special">);</span>
<span class="identifier">message_queue</span><span class="special">::</span><span class="identifier">remove</span><span class="special">(</span><span class="identifier">test</span><span class="special">::</span><span class="identifier">get_process_id_name</span><span class="special">());</span>
<span class="special">{</span>
<span class="comment">//Create the message-queues
</span> <span class="identifier">message_queue</span> <span class="identifier">mq1</span><span class="special">(</span><span class="identifier">create_only</span><span class="special">,</span> <span class="identifier">test</span><span class="special">::</span><span class="identifier">get_process_id_name</span><span class="special">(),</span> <span class="number">1</span><span class="special">,</span> <span class="identifier">MaxMsgSize</span><span class="special">);</span>
<span class="comment">//Open previously created message-queue simulating other process
</span> <span class="identifier">message_queue</span> <span class="identifier">mq2</span><span class="special">(</span><span class="identifier">open_only</span><span class="special">,</span> <span class="identifier">test</span><span class="special">::</span><span class="identifier">get_process_id_name</span><span class="special">());</span>
<span class="comment">//A managed heap memory to create the origin database
</span> <span class="identifier">managed_heap_memory</span> <span class="identifier">db_origin</span><span class="special">(</span><span class="identifier">buffer_destiny</span><span class="special">.</span><span class="identifier">size</span><span class="special">());</span>
<span class="comment">//Construct the map in the first buffer
</span> <span class="identifier">MyMap</span> <span class="special">*</span><span class="identifier">map1</span> <span class="special">=</span> <span class="identifier">db_origin</span><span class="special">.</span><span class="identifier">construct</span><span class="special"><</span><span class="identifier">MyMap</span><span class="special">>(</span><span class="string">"MyMap"</span><span class="special">)</span>
<span class="special">(</span><span class="identifier">MyLess</span><span class="special">(),</span>
<span class="identifier">db_origin</span><span class="special">.</span><span class="identifier">get_segment_manager</span><span class="special">());</span>
<span class="keyword">if</span><span class="special">(!</span><span class="identifier">map1</span><span class="special">)</span>
<span class="keyword">return</span> <span class="keyword">false</span><span class="special">;</span>
<span class="comment">//Fill map1 until is full
</span> <span class="keyword">try</span><span class="special">{</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span> <span class="identifier">i</span> <span class="special">=</span> <span class="number">0</span><span class="special">;</span>
<span class="keyword">while</span><span class="special">(</span><span class="number">1</span><span class="special">){</span>
<span class="special">(*</span><span class="identifier">map1</span><span class="special">)[</span><span class="identifier">i</span><span class="special">]</span> <span class="special">=</span> <span class="identifier">i</span><span class="special">;</span>
<span class="special">++</span><span class="identifier">i</span><span class="special">;</span>
<span class="special">}</span>
<span class="special">}</span>
<span class="keyword">catch</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">bad_alloc</span> <span class="special">&){}</span>
<span class="comment">//Data control data sending through the message queue
</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span> <span class="identifier">sent</span> <span class="special">=</span> <span class="number">0</span><span class="special">;</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span> <span class="identifier">recvd</span> <span class="special">=</span> <span class="number">0</span><span class="special">;</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span> <span class="identifier">total_recvd</span> <span class="special">=</span> <span class="number">0</span><span class="special">;</span>
<span class="keyword">unsigned</span> <span class="keyword">int</span> <span class="identifier">priority</span><span class="special">;</span>
<span class="comment">//Send whole first buffer through the mq1, read it
</span> <span class="comment">//through mq2 to the second buffer
</span> <span class="keyword">while</span><span class="special">(</span><span class="number">1</span><span class="special">){</span>
<span class="comment">//Send a fragment of buffer1 through mq1
</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span> <span class="identifier">bytes_to_send</span> <span class="special">=</span> <span class="identifier">MaxMsgSize</span> <span class="special"><</span> <span class="special">(</span><span class="identifier">db_origin</span><span class="special">.</span><span class="identifier">get_size</span><span class="special">()</span> <span class="special">-</span> <span class="identifier">sent</span><span class="special">)</span> <span class="special">?</span>
<span class="identifier">MaxMsgSize</span> <span class="special">:</span> <span class="special">(</span><span class="identifier">db_origin</span><span class="special">.</span><span class="identifier">get_size</span><span class="special">()</span> <span class="special">-</span> <span class="identifier">sent</span><span class="special">);</span>
<span class="identifier">mq1</span><span class="special">.</span><span class="identifier">send</span><span class="special">(</span> <span class="special">&</span><span class="keyword">static_cast</span><span class="special"><</span><span class="keyword">char</span><span class="special">*>(</span><span class="identifier">db_origin</span><span class="special">.</span><span class="identifier">get_address</span><span class="special">())[</span><span class="identifier">sent</span><span class="special">]</span>
<span class="special">,</span> <span class="identifier">bytes_to_send</span>
<span class="special">,</span> <span class="number">0</span><span class="special">);</span>
<span class="identifier">sent</span> <span class="special">+=</span> <span class="identifier">bytes_to_send</span><span class="special">;</span>
<span class="comment">//Receive the fragment through mq2 to buffer_destiny
</span> <span class="identifier">mq2</span><span class="special">.</span><span class="identifier">receive</span><span class="special">(</span> <span class="special">&</span><span class="identifier">buffer_destiny</span><span class="special">[</span><span class="identifier">total_recvd</span><span class="special">]</span>
<span class="special">,</span> <span class="identifier">BufferSize</span> <span class="special">-</span> <span class="identifier">recvd</span>
<span class="special">,</span> <span class="identifier">recvd</span>
<span class="special">,</span> <span class="identifier">priority</span><span class="special">);</span>
<span class="identifier">total_recvd</span> <span class="special">+=</span> <span class="identifier">recvd</span><span class="special">;</span>
<span class="comment">//Check if we have received all the buffer
</span> <span class="keyword">if</span><span class="special">(</span><span class="identifier">total_recvd</span> <span class="special">==</span> <span class="identifier">BufferSize</span><span class="special">){</span>
<span class="keyword">break</span><span class="special">;</span>
<span class="special">}</span>
<span class="special">}</span>
<span class="comment">//The buffer will contain a copy of the original database
</span> <span class="comment">//so let's interpret the buffer with managed_external_buffer
</span> <span class="identifier">managed_external_buffer</span> <span class="identifier">db_destiny</span><span class="special">(</span><span class="identifier">open_only</span><span class="special">,</span> <span class="special">&</span><span class="identifier">buffer_destiny</span><span class="special">[</span><span class="number">0</span><span class="special">],</span> <span class="identifier">BufferSize</span><span class="special">);</span>
<span class="comment">//Let's find the map
</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">pair</span><span class="special"><</span><span class="identifier">MyMap</span> <span class="special">*,</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span><span class="special">></span> <span class="identifier">ret</span> <span class="special">=</span> <span class="identifier">db_destiny</span><span class="special">.</span><span class="identifier">find</span><span class="special"><</span><span class="identifier">MyMap</span><span class="special">>(</span><span class="string">"MyMap"</span><span class="special">);</span>
<span class="identifier">MyMap</span> <span class="special">*</span><span class="identifier">map2</span> <span class="special">=</span> <span class="identifier">ret</span><span class="special">.</span><span class="identifier">first</span><span class="special">;</span>
<span class="comment">//Check if we have found it
</span> <span class="keyword">if</span><span class="special">(!</span><span class="identifier">map2</span><span class="special">){</span>
<span class="keyword">return</span> <span class="keyword">false</span><span class="special">;</span>
<span class="special">}</span>
<span class="comment">//Check if it is a single variable (not an array)
</span> <span class="keyword">if</span><span class="special">(</span><span class="identifier">ret</span><span class="special">.</span><span class="identifier">second</span> <span class="special">!=</span> <span class="number">1</span><span class="special">){</span>
<span class="keyword">return</span> <span class="keyword">false</span><span class="special">;</span>
<span class="special">}</span>
<span class="comment">//Now let's compare size
</span> <span class="keyword">if</span><span class="special">(</span><span class="identifier">map1</span><span class="special">-></span><span class="identifier">size</span><span class="special">()</span> <span class="special">!=</span> <span class="identifier">map2</span><span class="special">-></span><span class="identifier">size</span><span class="special">()){</span>
<span class="keyword">return</span> <span class="keyword">false</span><span class="special">;</span>
<span class="special">}</span>
<span class="comment">//Now let's compare all db values
</span> <span class="keyword">for</span><span class="special">(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span> <span class="identifier">i</span> <span class="special">=</span> <span class="number">0</span><span class="special">,</span> <span class="identifier">num_elements</span> <span class="special">=</span> <span class="identifier">map1</span><span class="special">-></span><span class="identifier">size</span><span class="special">();</span> <span class="identifier">i</span> <span class="special"><</span> <span class="identifier">num_elements</span><span class="special">;</span> <span class="special">++</span><span class="identifier">i</span><span class="special">){</span>
<span class="keyword">if</span><span class="special">((*</span><span class="identifier">map1</span><span class="special">)[</span><span class="identifier">i</span><span class="special">]</span> <span class="special">!=</span> <span class="special">(*</span><span class="identifier">map2</span><span class="special">)[</span><span class="identifier">i</span><span class="special">]){</span>
<span class="keyword">return</span> <span class="keyword">false</span><span class="special">;</span>
<span class="special">}</span>
<span class="special">}</span>
<span class="comment">//Destroy maps from db-s
</span> <span class="identifier">db_origin</span><span class="special">.</span><span class="identifier">destroy_ptr</span><span class="special">(</span><span class="identifier">map1</span><span class="special">);</span>
<span class="identifier">db_destiny</span><span class="special">.</span><span class="identifier">destroy_ptr</span><span class="special">(</span><span class="identifier">map2</span><span class="special">);</span>
<span class="special">}</span>
<span class="identifier">message_queue</span><span class="special">::</span><span class="identifier">remove</span><span class="special">(</span><span class="identifier">test</span><span class="special">::</span><span class="identifier">get_process_id_name</span><span class="special">());</span>
<span class="keyword">return</span> <span class="keyword">true</span><span class="special">;</span>
<span class="special">}</span>
</pre>
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<td align="right"><div class="copyright-footer">Copyright © 2005 - 2008 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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