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<a class="dropdown-item" tabindex="-1" href="POSIXvSysV.html#"><b>Chapter 1</b></a>
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<a class="dropdown-item" href="IntroConcSysOverview.html"> 1.1. Introduction to Concurrent Systems</a>
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<a class="dropdown-item" href="SequenceModels.html"> 1.6. Sequence Models in UML</a>
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<a class="dropdown-item disabled"><b>Chapter 2</b></a>
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<a class="dropdown-item" href="ProcessesOverview.html"> 2.1. Processes and OS Basics</a>
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<a class="dropdown-item" href="Multiprogramming.html"> 2.2. Processes and Multiprogramming</a>
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<a class="dropdown-item" href="IPCOverview.html"> 3.1. Concurrency with IPC</a>
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<a class="dropdown-item" href="MQueues.html"> 3.6. Message Passing With Message Queues</a>
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<a class="dropdown-item" href="ShMem.html"> 3.7. Shared Memory</a>
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<a class="dropdown-item" href="IPCSems.html"> 3.8. Semaphores</a>
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<a class="dropdown-item" href="Extended3Bash.html"> 3.9. Extended Example: Bash-lite: A Simple Command-line Shell</a>
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<a class="dropdown-item" href="SocketsOverview.html"> 4.1. Networked Concurrency</a>
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<a class="dropdown-item" href="ThreadsOverview.html"> 6.1. Concurrency with Multithreading</a>
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<a class="dropdown-item" href="SynchOverview.html"> 7.1. Synchronization Primitives</a>
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<a class="dropdown-item" href="SynchProblemsOverview.html"> 8.1. Synchronization Patterns and Problems</a>
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<a class="dropdown-item" href="ProdCons.html"> 8.3. Producer-Consumer Problem</a>
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<a class="dropdown-item" href="DiningPhil.html"> 8.5. Dining Philosophers Problem and Deadlock</a>
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<a class="dropdown-item" href="CigSmokers.html"> 8.6. Cigarette Smokers Problem and the Limits of Semaphores and Locks</a>
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<a class="dropdown-item" href="ParallelDistributedOverview.html"> 9.1. Parallel and Distributed Systems</a>
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<a class="dropdown-item" href="ParVConc.html"> 9.2. Parallelism vs. Concurrency</a>
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<a class="dropdown-item" href="Scaling.html"> 9.4. Limits of Parallelism and Scaling</a>
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<a class="dropdown-item" href="DistDataStorage.html"> 9.6. Reliable Data Storage and Location</a>
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<a class="dropdown-item" href="DistConsensus.html"> 9.7. Consensus in Distributed Systems</a>
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<a class="dropdown-item" href="Extended9Blockchain.html"> 9.8. Extended Example: Blockchain Proof-of-Work</a>
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<a class="dropdown-item" href="CLangOverview.html"> A.1. C Language Reintroduction</a>
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<a class="dropdown-item" href="Debugging.html"> A.2. Documentation and Debugging</a>
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<script>ODSA.SETTINGS.DISP_MOD_COMP = true;ODSA.SETTINGS.MODULE_NAME = "POSIXvSysV";ODSA.SETTINGS.MODULE_LONG_NAME = "POSIX vs. System V IPC";ODSA.SETTINGS.MODULE_CHAPTER = "Concurrency with IPC"; ODSA.SETTINGS.BUILD_DATE = "2021-06-01 15:31:50"; ODSA.SETTINGS.BUILD_CMAP = false;JSAV_OPTIONS['lang']='en';JSAV_EXERCISE_OPTIONS['code']='java';</script><div class="section" id="posix-vs-system-v-ipc">
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<h1>3.5. POSIX vs. System V IPC<a class="headerlink" href="POSIXvSysV.html#posix-vs-system-v-ipc" title="Permalink to this headline">¶</a></h1>
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<p>Throughout this book, we try to focus on platform-independent concepts and code examples as much as
|
||
possible. In particular, we tend to focus on the <a class="reference internal" href="Glossary.html#term-portable-operating-system-interface"><span class="xref std std-term">POSIX interface specification</span></a>, given its
|
||
broad base of support across the major modern OS. IPC is the exception that proves the rule. Some OS
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||
(e.g., macOS in particular) provide little or no support for POSIX IPC; instead, these OS uses the
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||
<a class="reference internal" href="Glossary.html#term-system-v"><span class="xref std std-term">System V interface</span></a> for IPC. In this section, we will continue to focus on POSIX and describe
|
||
the relevant interfaces for <a class="reference internal" href="Glossary.html#term-message-queue"><span class="xref std std-term">message queues</span></a>, <a class="reference internal" href="Glossary.html#term-semaphore"><span class="xref std std-term">semaphores</span></a>,
|
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and <a class="reference internal" href="Glossary.html#term-shared-memory"><span class="xref std std-term">shared memory</span></a>. Appendix B describes the System V IPC interfaces for reference and comparison.</p>
|
||
<div class="topic border border-dark rounded-lg alert-danger px-2 mb-3">
|
||
<div class="figure align-left">
|
||
<a class="reference internal image-reference" href="_images/CSF-Images-BugWarning.png"><img alt="Decorative bug warning" src="_images/CSF-Images-BugWarning.png" style="width: 90%;" /></a>
|
||
</div>
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||
<p class="topic-title first pt-2 mb-1">Bug Warning</p><hr class="mt-1" />
|
||
<p>POSIX IPC code will not work successfully on macOS. macOS does not provide the required header files
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for POSIX message queues, so the code will not successfully compile. Even worse, macOS provides the
|
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header files for POSIX unnamed semaphores (allowing the code to compile), but the implementation
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||
consists of empty stub functions. That is, the semaphore functions will simply return <strong>without
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||
actually acting like semaphores</strong>. To avoid these issues, code intended to run on macOS should use
|
||
the System V IPC interface instead. Pre-processor directives can be used to switch between the code
|
||
required at compile time.</p>
|
||
</div>
|
||
<div class="section" id="posix-ipc-fundamentals">
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<h2>3.5.1. POSIX IPC Fundamentals<a class="headerlink" href="POSIXvSysV.html#posix-ipc-fundamentals" title="Permalink to this headline">¶</a></h2>
|
||
<p>All POSIX IPC functions identify and refer to IPC objects with a string that adheres to a given
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||
format. The string must start with an initial slash, then have one or more non-slash characters. For
|
||
instance, <code class="docutils literal notranslate"><span class="pre">/OpenCSF_mqueue</span></code> is a valid name, presumably for a message queue. Each object’s name
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||
must be unique, so application designers need to ensure that any name they choose is unlikely to
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||
conflict with other applications.</p>
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||
<p>IPC objects are often represented as a file within a particular directory in the file system. In
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||
Linux, information related to the <code class="docutils literal notranslate"><span class="pre">/OpenCSF_mqueue</span></code> message queue would be stored in
|
||
<code class="docutils literal notranslate"><span class="pre">/dev/mqueue/OpenCSF_mqueue</span></code>, for example. These files can be accessed using the standard file
|
||
commands, such as ls, cat, or rm. However, manipulating these files with these standard utilities
|
||
may cause unexpected behavior, as running processes may be relying on these files.</p>
|
||
<p>In addition to a name, all of the functions for opening a POSIX IPC connection accept at least two
|
||
other bit mask parameters: <code class="docutils literal notranslate"><span class="pre">oflag</span></code> and <code class="docutils literal notranslate"><span class="pre">mode</span></code>. The <code class="docutils literal notranslate"><span class="pre">oflag</span></code> parameter specifies the requested
|
||
access needed (<code class="docutils literal notranslate"><span class="pre">O_RDONLY</span></code>, <code class="docutils literal notranslate"><span class="pre">O_WRONLY</span></code>, <code class="docutils literal notranslate"><span class="pre">O_RDWR</span></code>); note that these are identical to the
|
||
standard options for opening a file with <code class="docutils literal notranslate"><span class="pre">open()</span></code>. Similarly, the <code class="docutils literal notranslate"><span class="pre">mode</span></code> parameter uses the
|
||
standard file permissions (e.g., <code class="docutils literal notranslate"><span class="pre">S_IRUSR</span></code>, <code class="docutils literal notranslate"><span class="pre">S_IWGRP</span></code>) to set the permissions on a newly created
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||
object.</p>
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<p>If a new object is being created, the <code class="docutils literal notranslate"><span class="pre">O_CREAT</span></code> flag must be included in the oflag bit mask. If
|
||
<code class="docutils literal notranslate"><span class="pre">O_CREAT</span></code> is omitted and the object does not exist, an error will be returned. In addition, the
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||
<code class="docutils literal notranslate"><span class="pre">O_EXCL</span></code> flag controls the behavior when attempting to create an object, but one with the
|
||
specified name already exists. If <code class="docutils literal notranslate"><span class="pre">O_EXCL</span></code> is included and an object exists, the open fails and an
|
||
error is returned. If <code class="docutils literal notranslate"><span class="pre">O_EXCL</span></code> is omitted, no object is created, but the open returns a connection
|
||
to the existing object. Note that passing <code class="docutils literal notranslate"><span class="pre">O_EXCL</span></code> without <code class="docutils literal notranslate"><span class="pre">O_CREAT</span></code> is undefined in the POSIX
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||
specification and should not be used. The following examples illustrate how to use these options:</p>
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||
<blockquote>
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<div><ul>
|
||
<li><div class="first line-block">
|
||
<div class="line"><code class="docutils literal notranslate"><span class="pre">shm_open</span> <span class="pre">("/shared_mem",</span> <span class="pre">O_CREAT</span> <span class="pre">|</span> <span class="pre">O_RDWR,</span> <span class="pre">S_IWUSR</span> <span class="pre">|</span> <span class="pre">S_IRGRP);</span></code></div>
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||
<div class="line">Create and/or open shared memory; if it doesn’t exist, set permissions to 0420 (user has write, group has read).</div>
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||
</div>
|
||
</li>
|
||
<li><div class="first line-block">
|
||
<div class="line"><code class="docutils literal notranslate"><span class="pre">shm_open</span> <span class="pre">("/shared_mem",</span> <span class="pre">O_CREAT</span> <span class="pre">|</span> <span class="pre">O_EXCL,</span> <span class="pre">S_IWUSR</span> <span class="pre">|</span> <span class="pre">S_IRUSR);</span></code></div>
|
||
<div class="line">Create but do not open shared memory with 0600 permissions. Fail if it already exists.</div>
|
||
</div>
|
||
</li>
|
||
<li><div class="first line-block">
|
||
<div class="line"><code class="docutils literal notranslate"><span class="pre">shm_open</span> <span class="pre">("/shared_mem",</span> <span class="pre">O_RDONLY);</span></code></div>
|
||
<div class="line">Open a read-only connection to shared memory. If it does not exist, fail.</div>
|
||
</div>
|
||
</li>
|
||
</ul>
|
||
</div></blockquote>
|
||
<div class="topic border border-dark rounded-lg alert-danger px-2 mb-3">
|
||
<div class="figure align-left">
|
||
<a class="reference internal image-reference" href="_images/CSF-Images-BugWarning.png"><img alt="Decorative bug warning" src="_images/CSF-Images-BugWarning.png" style="width: 90%;" /></a>
|
||
</div>
|
||
<p class="topic-title first pt-2 mb-1">Bug Warning</p><hr class="mt-1" />
|
||
<p>The POSIX IPC implementations are typically provided within the <em>POSIX Real-time Library</em>
|
||
(<code class="docutils literal notranslate"><span class="pre">librt</span></code>). As such, the <code class="docutils literal notranslate"><span class="pre">-lrt</span></code> flag must be passed to gcc when compiling and linking programs
|
||
that use these mechanisms.</p>
|
||
</div>
|
||
<div
|
||
id="IPCPOSIXSumm"
|
||
class="embedContainer"
|
||
data-exer-name="IPCPOSIXSumm"
|
||
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|
||
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|
||
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|
||
data-frame-width="950"
|
||
data-frame-height="550"
|
||
data-external="false"
|
||
data-points="0"
|
||
data-required="False"
|
||
data-showhide="show"
|
||
data-threshold="3"
|
||
data-type="ka"
|
||
data-exer-id="">
|
||
|
||
<div class="center">
|
||
<div id="IPCPOSIXSumm_iframe"></div>
|
||
</div>
|
||
</div>
|
||
</div>
|
||
</div>
|
||
|
||
|
||
</div>
|
||
|
||
|
||
|
||
<div class="container">
|
||
|
||
<div class="mt-4 container center">
|
||
«  <a id="prevmod1" href="MMap.html">3.4. Shared Memory With Memory-mapped Files</a>
|
||
  ::  
|
||
<a class="uplink" href="index.html">Contents</a>
|
||
  ::  
|
||
<a id="nextmod1" href="MQueues.html">3.6. Message Passing With Message Queues</a>  »
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|
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|
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|
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|
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