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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="id1311317"></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="id1311665"></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="id1311817"></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="id1311926"></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 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 - 2010 Ion Gaztanaga<p> |
| Distributed under the Boost Software License, Version 1.0. (See accompanying |
| file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>) |
| </p> |
| </div></td> |
| </tr></table> |
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