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<title>Some basic explanations</title>
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<div class="section">
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="interprocess.some_basic_explanations"></a><a class="link" href="some_basic_explanations.html" title="Some basic explanations">Some basic explanations</a>
</h2></div></div></div>
<div class="toc"><dl>
<dt><span class="section"><a href="some_basic_explanations.html#interprocess.some_basic_explanations.processes_and_threads">Processes
And Threads</a></span></dt>
<dt><span class="section"><a href="some_basic_explanations.html#interprocess.some_basic_explanations.sharing_information">Sharing
information between processes</a></span></dt>
<dt><span class="section"><a href="some_basic_explanations.html#interprocess.some_basic_explanations.persistence">Persistence
Of Interprocess Mechanisms</a></span></dt>
<dt><span class="section"><a href="some_basic_explanations.html#interprocess.some_basic_explanations.names">Names Of
Interprocess Mechanisms</a></span></dt>
<dt><span class="section"><a href="some_basic_explanations.html#interprocess.some_basic_explanations.constructors_destructors_and_resource_lifetime">Constructors,
destructors and lifetime of Interprocess named resources</a></span></dt>
<dt><span class="section"><a href="some_basic_explanations.html#interprocess.some_basic_explanations.permissions">Permissions</a></span></dt>
</dl></div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="interprocess.some_basic_explanations.processes_and_threads"></a><a class="link" href="some_basic_explanations.html#interprocess.some_basic_explanations.processes_and_threads" title="Processes And Threads">Processes
And Threads</a>
</h3></div></div></div>
<p>
<span class="bold"><strong>Boost.Interprocess</strong></span> does not work only with
processes but also with threads. <span class="bold"><strong>Boost.Interprocess</strong></span>
synchronization mechanisms can synchronize threads from different processes,
but also threads from the same process.
</p>
</div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="interprocess.some_basic_explanations.sharing_information"></a><a class="link" href="some_basic_explanations.html#interprocess.some_basic_explanations.sharing_information" title="Sharing information between processes">Sharing
information between processes</a>
</h3></div></div></div>
<p>
In the traditional programming model an operating system has multiple processes
running and each process has its own address space. To share information
between processes we have several alternatives:
</p>
<div class="itemizedlist"><ul class="itemizedlist" type="disc">
<li class="listitem">
Two processes share information using a <span class="bold"><strong>file</strong></span>.
To access to the data, each process uses the usual file read/write mechanisms.
When updating/reading a file shared between processes, we need some sort
of synchronization, to protect readers from writers.
</li>
<li class="listitem">
Two processes share information that resides in the <span class="bold"><strong>kernel</strong></span>
of the operating system. This is the case, for example, of traditional
message queues. The synchronization is guaranteed by the operating system
kernel.
</li>
<li class="listitem">
Two processes can share a <span class="bold"><strong>memory</strong></span> region.
This is the case of classical shared memory or memory mapped files. Once
the processes set up the memory region, the processes can read/write
the data like any other memory segment without calling the operating
system's kernel. This also requires some kind of manual synchronization
between processes.
</li>
</ul></div>
</div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="interprocess.some_basic_explanations.persistence"></a><a class="link" href="some_basic_explanations.html#interprocess.some_basic_explanations.persistence" title="Persistence Of Interprocess Mechanisms">Persistence
Of Interprocess Mechanisms</a>
</h3></div></div></div>
<p>
One of the biggest issues with interprocess communication mechanisms is the
lifetime of the interprocess communication mechanism. It's important to know
when an interprocess communication mechanism disappears from the system.
In <span class="bold"><strong>Boost.Interprocess</strong></span>, we can have 3 types
of persistence:
</p>
<div class="itemizedlist"><ul class="itemizedlist" type="disc">
<li class="listitem">
<span class="bold"><strong>Process-persistence</strong></span>: The mechanism lasts
until all the processes that have opened the mechanism close it, exit
or crash.
</li>
<li class="listitem">
<span class="bold"><strong>Kernel-persistence</strong></span>: The mechanism exists
until the kernel of the operating system reboots or the mechanism is
explicitly deleted.
</li>
<li class="listitem">
<span class="bold"><strong>Filesystem-persistence</strong></span>: The mechanism
exists until the mechanism is explicitly deleted.
</li>
</ul></div>
<p>
Some native POSIX and Windows IPC mechanisms have different persistence so
it's difficult to achieve portability between Windows and POSIX native mechanisms.
<span class="bold"><strong>Boost.Interprocess</strong></span> classes have the following
persistence:
</p>
<div class="table">
<a name="interprocess.some_basic_explanations.persistence.t0"></a><p class="title"><b>Table 12.1. Boost.Interprocess Persistence Table</b></p>
<div class="table-contents"><table class="table" summary="Boost.Interprocess Persistence Table">
<colgroup>
<col>
<col>
</colgroup>
<thead><tr>
<th>
<p>
Mechanism
</p>
</th>
<th>
<p>
Persistence
</p>
</th>
</tr></thead>
<tbody>
<tr>
<td>
<p>
Shared memory
</p>
</td>
<td>
<p>
Kernel or Filesystem
</p>
</td>
</tr>
<tr>
<td>
<p>
Memory mapped file
</p>
</td>
<td>
<p>
Filesystem
</p>
</td>
</tr>
<tr>
<td>
<p>
Process-shared mutex types
</p>
</td>
<td>
<p>
Process
</p>
</td>
</tr>
<tr>
<td>
<p>
Process-shared semaphore
</p>
</td>
<td>
<p>
Process
</p>
</td>
</tr>
<tr>
<td>
<p>
Process-shared condition
</p>
</td>
<td>
<p>
Process
</p>
</td>
</tr>
<tr>
<td>
<p>
File lock
</p>
</td>
<td>
<p>
Process
</p>
</td>
</tr>
<tr>
<td>
<p>
Message queue
</p>
</td>
<td>
<p>
Kernel or Filesystem
</p>
</td>
</tr>
<tr>
<td>
<p>
Named mutex
</p>
</td>
<td>
<p>
Kernel or Filesystem
</p>
</td>
</tr>
<tr>
<td>
<p>
Named semaphore
</p>
</td>
<td>
<p>
Kernel or Filesystem
</p>
</td>
</tr>
<tr>
<td>
<p>
Named condition
</p>
</td>
<td>
<p>
Kernel or Filesystem
</p>
</td>
</tr>
</tbody>
</table></div>
</div>
<br class="table-break"><p>
As you can see, <span class="bold"><strong>Boost.Interprocess</strong></span> defines
some mechanisms with "Kernel or Filesystem" persistence. This is
because POSIX allows this possibility to native interprocess communication
implementations. One could, for example, implement shared memory using memory
mapped files and obtain filesystem persistence (for example, there is no
proper known way to emulate kernel persistence with a user library for Windows
shared memory using native shared memory, or process persistence for POSIX
shared memory, so the only portable way is to define "Kernel or Filesystem"
persistence).
</p>
</div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="interprocess.some_basic_explanations.names"></a><a class="link" href="some_basic_explanations.html#interprocess.some_basic_explanations.names" title="Names Of Interprocess Mechanisms">Names Of
Interprocess Mechanisms</a>
</h3></div></div></div>
<p>
Some interprocess mechanisms are anonymous objects created in shared memory
or memory-mapped files but other interprocess mechanisms need a name or identifier
so that two unrelated processes can use the same interprocess mechanism object.
Examples of this are shared memory, named mutexes and named semaphores (for
example, native windows CreateMutex/CreateSemaphore API family).
</p>
<p>
The name used to identify an interprocess mechanism is not portable, even
between UNIX systems. For this reason, <span class="bold"><strong>Boost.Interprocess</strong></span>
limits this name to a C++ variable identifier or keyword:
</p>
<div class="itemizedlist"><ul class="itemizedlist" type="disc">
<li class="listitem">
Starts with a letter, lowercase or uppercase, such as a letter from a
to z or from A to Z. Examples: <span class="emphasis"><em>Sharedmemory, sharedmemory,
sHaReDmEmOrY...</em></span>
</li>
<li class="listitem">
Can include letters, underscore, or digits. Examples: <span class="emphasis"><em>shm1,
shm2and3, ShM3plus4...</em></span>
</li>
</ul></div>
</div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="interprocess.some_basic_explanations.constructors_destructors_and_resource_lifetime"></a><a class="link" href="some_basic_explanations.html#interprocess.some_basic_explanations.constructors_destructors_and_resource_lifetime" title="Constructors, destructors and lifetime of Interprocess named resources">Constructors,
destructors and lifetime of Interprocess named resources</a>
</h3></div></div></div>
<p>
Named <span class="bold"><strong>Boost.Interprocess</strong></span> resources (shared
memory, memory mapped files, named mutexes/conditions/semaphores) have kernel
or filesystem persistency. This means that even if all processes that have
opened those resources end, the resource will still be accessible to be opened
again and the resource can only be destructed via an explicit to their static
member <code class="computeroutput"><span class="identifier">remove</span></code> function. This
behavior can be easily understood, since it's the same mechanism used by
functions controlling file opening/creation/erasure:
</p>
<div class="table">
<a name="interprocess.some_basic_explanations.constructors_destructors_and_resource_lifetime.t0"></a><p class="title"><b>Table 12.2. Boost.Interprocess-Filesystem Analogy</b></p>
<div class="table-contents"><table class="table" summary="Boost.Interprocess-Filesystem Analogy">
<colgroup>
<col>
<col>
<col>
</colgroup>
<thead><tr>
<th>
<p>
Named Interprocess resource
</p>
</th>
<th>
<p>
Corresponding std file
</p>
</th>
<th>
<p>
Corresponding POSIX operation
</p>
</th>
</tr></thead>
<tbody>
<tr>
<td>
<p>
Constructor
</p>
</td>
<td>
<p>
std::fstream constructor
</p>
</td>
<td>
<p>
open
</p>
</td>
</tr>
<tr>
<td>
<p>
Destructor
</p>
</td>
<td>
<p>
std::fstream destructor
</p>
</td>
<td>
<p>
close
</p>
</td>
</tr>
<tr>
<td>
<p>
Member <code class="computeroutput"><span class="identifier">remove</span></code>
</p>
</td>
<td>
<p>
None. <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">remove</span></code>
</p>
</td>
<td>
<p>
unlink
</p>
</td>
</tr>
</tbody>
</table></div>
</div>
<br class="table-break"><p>
Now the correspondence between POSIX and Boost.Interprocess regarding shared
memory and named semaphores:
</p>
<div class="table">
<a name="interprocess.some_basic_explanations.constructors_destructors_and_resource_lifetime.t1"></a><p class="title"><b>Table 12.3. Boost.Interprocess-POSIX shared memory</b></p>
<div class="table-contents"><table class="table" summary="Boost.Interprocess-POSIX shared memory">
<colgroup>
<col>
<col>
</colgroup>
<thead><tr>
<th>
<p>
<code class="computeroutput"><span class="identifier">shared_memory_object</span></code>
operation
</p>
</th>
<th>
<p>
POSIX operation
</p>
</th>
</tr></thead>
<tbody>
<tr>
<td>
<p>
Constructor
</p>
</td>
<td>
<p>
shm_open
</p>
</td>
</tr>
<tr>
<td>
<p>
Destructor
</p>
</td>
<td>
<p>
close
</p>
</td>
</tr>
<tr>
<td>
<p>
Member <code class="computeroutput"><span class="identifier">remove</span></code>
</p>
</td>
<td>
<p>
shm_unlink
</p>
</td>
</tr>
</tbody>
</table></div>
</div>
<br class="table-break"><div class="table">
<a name="interprocess.some_basic_explanations.constructors_destructors_and_resource_lifetime.t2"></a><p class="title"><b>Table 12.4. Boost.Interprocess-POSIX named semaphore</b></p>
<div class="table-contents"><table class="table" summary="Boost.Interprocess-POSIX named semaphore">
<colgroup>
<col>
<col>
</colgroup>
<thead><tr>
<th>
<p>
<code class="computeroutput"><span class="identifier">named_semaphore</span></code>
operation
</p>
</th>
<th>
<p>
POSIX operation
</p>
</th>
</tr></thead>
<tbody>
<tr>
<td>
<p>
Constructor
</p>
</td>
<td>
<p>
sem_open
</p>
</td>
</tr>
<tr>
<td>
<p>
Destructor
</p>
</td>
<td>
<p>
close
</p>
</td>
</tr>
<tr>
<td>
<p>
Member <code class="computeroutput"><span class="identifier">remove</span></code>
</p>
</td>
<td>
<p>
sem_unlink
</p>
</td>
</tr>
</tbody>
</table></div>
</div>
<br class="table-break"><p>
The most important property is that <span class="bold"><strong>destructors of
named resources don't remove the resource from the system</strong></span>, they
only liberate resources allocated by the system for use by the process for
the named resource. <span class="bold"><strong>To remove the resource from the
system the programmer must use <code class="computeroutput"><span class="identifier">remove</span></code></strong></span>.
</p>
</div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="interprocess.some_basic_explanations.permissions"></a><a class="link" href="some_basic_explanations.html#interprocess.some_basic_explanations.permissions" title="Permissions">Permissions</a>
</h3></div></div></div>
<p>
Named resources offered by <span class="bold"><strong>Boost.Interprocess</strong></span>
must cope with platform-dependant permission issues also present when creating
files. If a programmer wants to shared shared memory, memory mapped files
or named synchronization mechanisms (mutexes, semaphores, etc...) between
users, it's necessary to specify those permissions. Sadly, traditional UNIX
and Windows permissions are very different and <span class="bold"><strong>Boost.Interprocess</strong></span>
does not try to standardize permissions, but does not ignore them.
</p>
<p>
All named resource creation functions take an optional <code class="computeroutput"><a class="link" href="../boost/interprocess/permissions.html" title="Class permissions">permissions
object</a></code> that can be configured with platform-dependant permissions.
</p>
<p>
Since each mechanism can be emulated through diferent mechanisms (a semaphore
might be implement using mapped files or native semaphores) permissions types
could vary when the implementation of a named resource changes (eg.: in Windows
mutexes require <code class="computeroutput"><span class="identifier">synchronize</span> <span class="identifier">permissions</span></code>, but that's not the case of
files). To avoid this, <span class="bold"><strong>Boost.Interprocess</strong></span>
relies on file-like permissions, requiring file read-write-delete permissions
to open named synchronization mechanisms (mutex, semaphores, etc.) and appropiate
read or read-write-delete permissions for shared memory. This approach has
two advantages: it's similar to the UNIX philosophy and the programmer does
not need to know how the named resource is implemented.
</p>
</div>
</div>
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
<td align="left"></td>
<td align="right"><div class="copyright-footer">Copyright © 2005-2011 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>)
</p>
</div></td>
</tr></table>
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