<html> <head> <meta http-equiv="Content-Type" content="text/html; charset=US-ASCII"> <title>Some basic explanations</title> <link rel="stylesheet" href="../../../doc/src/boostbook.css" type="text/css"> <meta name="generator" content="DocBook XSL Stylesheets V1.75.2"> <link rel="home" href="../index.html" title="The Boost C++ Libraries BoostBook Documentation Subset"> <link rel="up" href="../interprocess.html" title="Chapter 9. Boost.Interprocess"> <link rel="prev" href="quick_guide.html" title="Quick Guide for the Impatient"> <link rel="next" href="sharedmemorybetweenprocesses.html" title="Sharing memory between processes"> </head> <body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"> <table cellpadding="2" width="100%"><tr> <td valign="top"><img alt="Boost C++ Libraries" width="277" height="86" src="../../../boost.png"></td> <td align="center"><a href="../../../index.html">Home</a></td> <td align="center"><a href="../../../libs/libraries.htm">Libraries</a></td> <td align="center"><a href="http://www.boost.org/users/people.html">People</a></td> <td align="center"><a href="http://www.boost.org/users/faq.html">FAQ</a></td> <td align="center"><a href="../../../more/index.htm">More</a></td> </tr></table> <hr> <div class="spirit-nav"> <a accesskey="p" href="quick_guide.html"><img src="../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../interprocess.html"><img src="../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../index.html"><img src="../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="sharedmemorybetweenprocesses.html"><img src="../../../doc/src/images/next.png" alt="Next"></a> </div> <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> </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="id1311363"></a><p class="title"><b>Table 9.1. 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="id1311710"></a><p class="title"><b>Table 9.2. 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="id1311862"></a><p class="title"><b>Table 9.3. 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="id1311971"></a><p class="title"><b>Table 9.4. 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> <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 - 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>) </p> </div></td> </tr></table> <hr> <div class="spirit-nav"> <a accesskey="p" href="quick_guide.html"><img src="../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../interprocess.html"><img src="../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../index.html"><img src="../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="sharedmemorybetweenprocesses.html"><img src="../../../doc/src/images/next.png" alt="Next"></a> </div> </body> </html>