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<a class="nav-link dropdown-toggle jmu-gold rounded" href="ShMem.html#" id="navbarDropdownChapters" role="button" data-toggle="dropdown" aria-haspopup="true" aria-expanded="false">Contents</a>
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<a class="dropdown-item" tabindex="-1" href="ShMem.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="SysAndModels.html"> 1.2. Systems and Models</a>
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<a class="dropdown-item" href="Themes.html"> 1.3. Themes and Guiding Principles</a>
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<a class="dropdown-item" href="Architectures.html"> 1.4. System Architectures</a>
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<a class="dropdown-item" href="StateModels.html"> 1.5. State Models in UML</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" href="StateModelImplementation.html"> 1.7. Extended Example: State Model Implementation</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="KernelMechanics.html"> 2.3. Kernel Mechanics</a>
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<a class="dropdown-item" href="Syscall.html"> 2.4. System Call Interface</a>
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<a class="dropdown-item" href="ProcessCycle.html"> 2.5. Process Life Cycle</a>
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<a class="dropdown-item" href="UnixFile.html"> 2.6. The UNIX File Abstraction</a>
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<a class="dropdown-item" href="EventsSignals.html"> 2.7. Events and Signals</a>
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<a class="dropdown-item" href="Extended2Processes.html"> 2.8. Extended Example: Listing Files with Processes</a>
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<a class="dropdown-item disabled"><b>Chapter 3</b></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="IPCModels.html"> 3.2. IPC Models</a>
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<a class="dropdown-item" href="Pipes.html"> 3.3. Pipes and FIFOs</a>
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<a class="dropdown-item" href="MMap.html"> 3.4. Shared Memory With Memory-mapped Files</a>
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<a class="dropdown-item" href="POSIXvSysV.html"> 3.5. POSIX vs. System V 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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<div class="dropdown-divider"></div>
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<a class="dropdown-item disabled"><b>Chapter 4</b></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="FiveLayer.html"> 4.2. The TCP/IP Internet Model</a>
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<a class="dropdown-item" href="NetApps.html"> 4.3. Network Applications and Protocols</a>
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<a class="dropdown-item" href="Sockets.html"> 4.4. The Socket Interface</a>
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<a class="dropdown-item" href="TCPSockets.html"> 4.5. TCP Socket Programming: HTTP</a>
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<a class="dropdown-item" href="UDPSockets.html"> 4.6. UDP Socket Programming: DNS</a>
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<a class="dropdown-item" href="AppBroadcast.html"> 4.7. Application-Layer Broadcasting: DHCP</a>
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<a class="dropdown-item" href="Extended4CGI.html"> 4.8. Extended Example: CGI Web Server</a>
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<a class="dropdown-item disabled"><b>Chapter 5</b></a>
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<a class="dropdown-item" href="InternetOverview.html"> 5.1. The Internet and Connectivity</a>
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<a class="dropdown-item" href="AppLayer.html"> 5.2. Application Layer: Overlay Networks</a>
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<a class="dropdown-item" href="TransLayer.html"> 5.3. Transport Layer</a>
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<a class="dropdown-item" href="NetSec.html"> 5.4. Network Security Fundamentals</a>
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<a class="dropdown-item" href="NetLayer.html"> 5.5. Network Layer: IP</a>
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<a class="dropdown-item" href="LinkLayer.html"> 5.6. Link Layer</a>
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<a class="dropdown-item" href="Wireless.html"> 5.7. Wireless Connectivity: Wi-Fi, Bluetooth, and Zigbee</a>
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<a class="dropdown-item" href="Extended5DNS.html"> 5.8. Extended Example: DNS client</a>
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<a class="dropdown-item disabled"><b>Chapter 6</b></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="ProcVThreads.html"> 6.2. Processes vs. Threads</a>
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<a class="dropdown-item" href="RaceConditions.html"> 6.3. Race Conditions and Critical Sections</a>
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<a class="dropdown-item" href="POSIXThreads.html"> 6.4. POSIX Thread Library</a>
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<a class="dropdown-item" href="ThreadArgs.html"> 6.5. Thread Arguments and Return Values</a>
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<a class="dropdown-item" href="ImplicitThreads.html"> 6.6. Implicit Threading and Language-based Threads</a>
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<a class="dropdown-item" href="Extended6Input.html"> 6.7. Extended Example: Keyboard Input Listener</a>
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<a class="dropdown-item" href="Extended6Primes.html"> 6.8. Extended Example: Concurrent Prime Number Search</a>
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<div class="dropdown-divider"></div>
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<a class="dropdown-item disabled"><b>Chapter 7</b></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="CritSect.html"> 7.2. Critical Sections and Peterson's Solution</a>
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<a class="dropdown-item" href="Locks.html"> 7.3. Locks</a>
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<a class="dropdown-item" href="Semaphores.html"> 7.4. Semaphores</a>
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<a class="dropdown-item" href="Barriers.html"> 7.5. Barriers</a>
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<a class="dropdown-item" href="Condvars.html"> 7.6. Condition Variables</a>
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<a class="dropdown-item" href="Deadlock.html"> 7.7. Deadlock</a>
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<a class="dropdown-item" href="Extended7Events.html"> 7.8. Extended Example: Event Log File</a>
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<div class="dropdown-divider"></div>
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<a class="dropdown-item disabled"><b>Chapter 8</b></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="SynchDesign.html"> 8.2. Basic Synchronization Design Patterns</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="ReadWrite.html"> 8.4. Readers-Writers 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="Extended8ModExp.html"> 8.7. Extended Example: Parallel Modular Exponentiation</a>
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<div class="dropdown-divider"></div>
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<a class="dropdown-item disabled"><b>Chapter 9</b></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="ParallelDesign.html"> 9.3. Parallel Design Patterns</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="DistTiming.html"> 9.5. Timing in Distributed Environments</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 disabled"><b>Appendix A</b></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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<a class="dropdown-item" href="BasicTypes.html"> A.3. Basic Types and Pointers</a>
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<a class="dropdown-item" href="Arrays.html"> A.4. Arrays, Structs, Enums, and Type Definitions</a>
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<a class="dropdown-item" href="Functions.html"> A.5. Functions and Scope</a>
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<a class="dropdown-item" href="Pointers.html"> A.6. Pointers and Dynamic Allocation</a>
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<a class="dropdown-item" href="Strings.html"> A.7. Strings</a>
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<a class="dropdown-item" href="FunctionPointers.html"> A.8. Function Pointers</a>
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<a class="dropdown-item" href="Files.html"> A.9. Files</a>
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</div>
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<li class="nav-item"><a class="nav-link jmu-gold" href="https://w3.cs.jmu.edu/kirkpams/OpenCSF/Books/csf/source/ShMem.rst"
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«  <a id="prevmod" href="MQueues.html">3.6. Message Passing With Message Queues</a>
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  ::  
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<a class="uplink" href="index.html">Contents</a>
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  ::  
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<a id="nextmod" href="IPCSems.html">3.8. Semaphores</a>  »
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<script>ODSA.SETTINGS.DISP_MOD_COMP = true;ODSA.SETTINGS.MODULE_NAME = "ShMem";ODSA.SETTINGS.MODULE_LONG_NAME = "Shared Memory";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="shared-memory">
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<h1>3.7. Shared Memory<a class="headerlink" href="ShMem.html#shared-memory" title="Permalink to this headline">¶</a></h1>
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<div class="figure mb-2 align-right" id="id2" style="width: 40%">
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<span id="ipcshmem"></span><a class="reference internal image-reference" href="_images/CSF-Images.3.6.png"><img class="p-3 mb-2 align-center border border-dark rounded-lg" alt="A shared memory region present in two processes" src="_images/CSF-Images.3.6.png" style="width: 90%;" /></a>
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<p class="caption align-center px-3"><span class="caption-text"> Figure 3.7.1: A shared memory region present in two processes</span></p>
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</div>
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<p>Setting up <a class="reference internal" href="Glossary.html#term-shared-memory"><span class="xref std std-term">shared memory</span></a> with the techniques in this section is very similar to using
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<a class="reference internal" href="Glossary.html#term-memory-mapped-file"><span class="xref std std-term">memory-mapped files</span></a>, with the exception that there is no pre-defined
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persistent file. That is, shared memory can be set up to allow for immediate data exchange between
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processes without having a persistent record of the communication. <a href="ShMem.html#ipcshmem">Figure 3.7.1</a>
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illustrates the logical structure of a shared memory region that is mapped into two different processes.</p>
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<p>For applications that exchange large amounts of data, shared memory is far superior to
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message-passing techniques like message queues, which require system calls for every data exchange.
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The major disadvantage of shared memory is that the processes must take extra precaution to
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synchronize access to the region. If process A writes into the shared region, that might cause
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unstable behavior in process B, or vice versa.</p>
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<div class="topic border border-dark rounded-lg alert-danger px-2 mb-3">
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<div class="figure align-left">
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<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>When a shared memory region is established in two or more processes, there is no guarantee that the
|
|||
|
|
regions will be placed at the same base address. For instance, one process might have the shared
|
|||
|
|
region starting at address <code class="docutils literal notranslate"><span class="pre">0x40000000</span></code> while the other process uses <code class="docutils literal notranslate"><span class="pre">0x50008000</span></code>. It is
|
|||
|
|
critical to understand that these two addresses refer to the exact same piece of data. So storing
|
|||
|
|
the number 1 in the first process’s address <code class="docutils literal notranslate"><span class="pre">0x40000000</span></code> means the second process has the value
|
|||
|
|
of 1 at <code class="docutils literal notranslate"><span class="pre">0x50008000</span></code>. The two (different) addresses refer to the exact same location.</p>
|
|||
|
|
<p>Given that the base addresses are different, all elements in the region must also have different
|
|||
|
|
addresses. The implication of this is that shared memory regions cannot use pointers to refer to
|
|||
|
|
other parts of the region. For instance, assume the shared memory contains a linked list with the
|
|||
|
|
first node at the beginning of the region. The next node appears 256 bytes later. Using the
|
|||
|
|
addresses above, in one process, <code class="docutils literal notranslate"><span class="pre">0x40000000</span></code> must contain a pointer to address <code class="docutils literal notranslate"><span class="pre">0x40000100</span></code>;
|
|||
|
|
in the other address, the pointer must point to <code class="docutils literal notranslate"><span class="pre">0x50008100</span></code>, given the different base address.
|
|||
|
|
This means that the beginning of the shared memory region must simultaneously store <cite>two different</cite>
|
|||
|
|
values, which is impossible.</p>
|
|||
|
|
<p>The solution is to avoid the use of pointers within the shared memory region, using pointer
|
|||
|
|
arithmetic instead. That is, if <code class="docutils literal notranslate"><span class="pre">char</span> <span class="pre">*baseaddr</span></code> is declared to point to the base address of the
|
|||
|
|
region, then the two processes would need to use <code class="docutils literal notranslate"><span class="pre">*(baseaddr</span> <span class="pre">+</span> <span class="pre">256)</span></code> to refer to the next node of
|
|||
|
|
the linked list. Or, as an alternative, avoid linked lists entirely and use a contiguous data
|
|||
|
|
structure instead.</p>
|
|||
|
|
</div>
|
|||
|
|
<div class="section" id="posix-shared-memory">
|
|||
|
|
<h2>3.7.1. POSIX Shared Memory<a class="headerlink" href="ShMem.html#posix-shared-memory" title="Permalink to this headline">¶</a></h2>
|
|||
|
|
<p>The POSIX interface for shared memory is very simple, consisting of two primary functions. The
|
|||
|
|
<code class="docutils literal notranslate"><span class="pre">shm_open()</span></code> takes the POSIX IPC object name, a bit-mask of flags (<code class="docutils literal notranslate"><span class="pre">oflag</span></code>) and a permission
|
|||
|
|
mode to apply new objects. <a class="footnote-reference" href="ShMem.html#f18" id="id1">[1]</a> Similarly, <code class="docutils literal notranslate"><span class="pre">shm_unlink()</span></code> deletes the shared memory object.</p>
|
|||
|
|
<div class="topic border border-dark rounded-lg bg-light px-2 mb-3">
|
|||
|
|
<div class="figure align-left">
|
|||
|
|
<a class="reference internal image-reference" href="_images/CSF-Images-Library.png"><img alt="Decorative C library image" src="_images/CSF-Images-Library.png" style="width: 100%;" /></a>
|
|||
|
|
</div>
|
|||
|
|
<p class="topic-title first pt-2 mb-1">C library functions – <sys/mman.h></p><hr class="mt-1" />
|
|||
|
|
<dl class="docutils">
|
|||
|
|
<dt><code class="docutils literal notranslate"><span class="pre">int</span> <span class="pre">shm_open</span> <span class="pre">(const</span> <span class="pre">char</span> <span class="pre">*name,</span> <span class="pre">int</span> <span class="pre">oflag,</span> <span class="pre">mode_t</span> <span class="pre">mode);</span></code></dt>
|
|||
|
|
<dd>Open a connection to a POSIX shared memory object.</dd>
|
|||
|
|
<dt><code class="docutils literal notranslate"><span class="pre">int</span> <span class="pre">shm_unlink</span> <span class="pre">(const</span> <span class="pre">char</span> <span class="pre">*name);</span></code></dt>
|
|||
|
|
<dd>Delete a POSIX shared memory object.</dd>
|
|||
|
|
</dl>
|
|||
|
|
</div>
|
|||
|
|
<p>The following code sample uses a <code class="docutils literal notranslate"><span class="pre">struct</span></code> declared as follows:</p>
|
|||
|
|
<div class="highlight-c border border-dark rounded-lg bg-light px-0 mb-3 notranslate"><table class="highlighttable"><tr><td class="linenos px-0 mx-0"><div class="linenodiv"><pre class="mb-0">1
|
|||
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2
|
|||
|
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3
|
|||
|
|
4
|
|||
|
|
5</pre></div></td><td class="code"><div class="highlight bg-light"><pre class="mb-0"><span></span><span class="k">struct</span> <span class="n">permission</span> <span class="p">{</span>
|
|||
|
|
<span class="kt">int</span> <span class="n">user</span><span class="p">;</span>
|
|||
|
|
<span class="kt">int</span> <span class="n">group</span><span class="p">;</span>
|
|||
|
|
<span class="kt">int</span> <span class="n">other</span><span class="p">;</span>
|
|||
|
|
<span class="p">};</span>
|
|||
|
|
</pre></div>
|
|||
|
|
</td></tr></table></div>
|
|||
|
|
<p><a class="reference external" href="ShMem.html#cl3-10">Code Listing 3.10</a> sets up a POSIX shared memory object, which is identified by a given name. Note
|
|||
|
|
that the call to <code class="docutils literal notranslate"><span class="pre">shm_open()</span></code> creates the object but does not specify a size; instead,
|
|||
|
|
<code class="docutils literal notranslate"><span class="pre">ftruncate()</span></code> resizes the object to be large enough to store one instance of the <code class="docutils literal notranslate"><span class="pre">struct</span></code>
|
|||
|
|
permission. Next, the process maps the shared memory object into memory with <code class="docutils literal notranslate"><span class="pre">mmap()</span></code> before using
|
|||
|
|
<code class="docutils literal notranslate"><span class="pre">fork()</span></code> to create a child process (which inherits both the shared memory identifier and the
|
|||
|
|
memory mapped region). The child writes to the region before exiting; the parent waits until the
|
|||
|
|
child exits and reads the data.</p>
|
|||
|
|
<div class="highlight-c border border-dark rounded-lg bg-light px-0 mb-3 notranslate" id="cl3-10"><table class="highlighttable"><tr><td class="linenos px-0 mx-0"><div class="linenodiv"><pre class="mb-0"> 1
|
|||
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|
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3
|
|||
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|
|||
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|
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|
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|
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|
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|
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|
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|
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|
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|
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|
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|
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|
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|
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|
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|
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|
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|
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|
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|
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|
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|
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|
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|
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|
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|
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|
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|
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38
|
|||
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39
|
|||
|
|
40
|
|||
|
|
41</pre></div></td><td class="code"><div class="highlight bg-light"><pre class="mb-0"><span></span><span class="cm">/* Code Listing 3.10:</span>
|
|||
|
|
<span class="cm"> Using POSIX shared memory to exchange data between processes</span>
|
|||
|
|
<span class="cm"> */</span>
|
|||
|
|
|
|||
|
|
<span class="cm">/* Create unsized shared memory object;</span>
|
|||
|
|
<span class="cm"> return value is a file descriptor */</span>
|
|||
|
|
<span class="kt">int</span> <span class="n">shmfd</span> <span class="o">=</span> <span class="n">shm_open</span> <span class="p">(</span><span class="s">"/OpenCSF_SHM"</span><span class="p">,</span> <span class="n">O_CREAT</span> <span class="o">|</span> <span class="n">O_EXCL</span> <span class="o">|</span> <span class="n">O_RDWR</span><span class="p">,</span> <span class="n">S_IRUSR</span> <span class="o">|</span> <span class="n">S_IWUSR</span><span class="p">);</span>
|
|||
|
|
<span class="n">assert</span> <span class="p">(</span><span class="n">shmfd</span> <span class="o">!=</span> <span class="o">-</span><span class="mi">1</span><span class="p">);</span>
|
|||
|
|
|
|||
|
|
<span class="cm">/* Resize the region to store 1 struct instance */</span>
|
|||
|
|
<span class="n">assert</span> <span class="p">(</span><span class="n">ftruncate</span> <span class="p">(</span><span class="n">shmfd</span><span class="p">,</span> <span class="k">sizeof</span> <span class="p">(</span><span class="k">struct</span> <span class="n">permission</span><span class="p">))</span> <span class="o">!=</span> <span class="o">-</span><span class="mi">1</span><span class="p">);</span>
|
|||
|
|
|
|||
|
|
<span class="cm">/* Map the object into memory so file operations aren't needed */</span>
|
|||
|
|
<span class="k">struct</span> <span class="n">permission</span> <span class="o">*</span><span class="n">perm</span> <span class="o">=</span> <span class="n">mmap</span> <span class="p">(</span><span class="nb">NULL</span><span class="p">,</span> <span class="k">sizeof</span> <span class="p">(</span><span class="k">struct</span> <span class="n">permission</span><span class="p">),</span>
|
|||
|
|
<span class="n">PROT_READ</span> <span class="o">|</span> <span class="n">PROT_WRITE</span><span class="p">,</span> <span class="n">MAP_SHARED</span><span class="p">,</span> <span class="n">shmfd</span><span class="p">,</span> <span class="mi">0</span><span class="p">);</span>
|
|||
|
|
<span class="n">assert</span> <span class="p">(</span><span class="n">perm</span> <span class="o">!=</span> <span class="n">MAP_FAILED</span><span class="p">);</span>
|
|||
|
|
|
|||
|
|
<span class="cm">/* Create a child process and write to the mapped/shared region */</span>
|
|||
|
|
<span class="kt">pid_t</span> <span class="n">child_pid</span> <span class="o">=</span> <span class="n">fork</span><span class="p">();</span>
|
|||
|
|
<span class="k">if</span> <span class="p">(</span><span class="n">child_pid</span> <span class="o">==</span> <span class="mi">0</span><span class="p">)</span>
|
|||
|
|
<span class="p">{</span>
|
|||
|
|
<span class="n">perm</span><span class="o">-></span><span class="n">user</span> <span class="o">=</span> <span class="mi">6</span><span class="p">;</span>
|
|||
|
|
<span class="n">perm</span><span class="o">-></span><span class="n">group</span> <span class="o">=</span> <span class="mi">4</span><span class="p">;</span>
|
|||
|
|
<span class="n">perm</span><span class="o">-></span><span class="n">other</span> <span class="o">=</span> <span class="mi">0</span><span class="p">;</span>
|
|||
|
|
|
|||
|
|
<span class="cm">/* Unmap and close the child's shared memory access */</span>
|
|||
|
|
<span class="n">munmap</span> <span class="p">(</span><span class="n">perm</span><span class="p">,</span> <span class="k">sizeof</span> <span class="p">(</span><span class="k">struct</span> <span class="n">permission</span><span class="p">));</span>
|
|||
|
|
<span class="n">close</span> <span class="p">(</span><span class="n">shmfd</span><span class="p">);</span>
|
|||
|
|
<span class="k">return</span> <span class="mi">0</span><span class="p">;</span>
|
|||
|
|
<span class="p">}</span>
|
|||
|
|
|
|||
|
|
<span class="cm">/* Make the parent wait until the child has exited */</span>
|
|||
|
|
<span class="n">wait</span> <span class="p">(</span><span class="nb">NULL</span><span class="p">);</span>
|
|||
|
|
|
|||
|
|
<span class="cm">/* Read from the mapped/shared memory region */</span>
|
|||
|
|
<span class="n">printf</span> <span class="p">(</span><span class="s">"Permission bit-mask: 0%d%d%d</span><span class="se">\n</span><span class="s">"</span><span class="p">,</span> <span class="n">perm</span><span class="o">-></span><span class="n">user</span><span class="p">,</span> <span class="n">perm</span><span class="o">-></span><span class="n">group</span><span class="p">,</span> <span class="n">perm</span><span class="o">-></span><span class="n">other</span><span class="p">);</span>
|
|||
|
|
|
|||
|
|
<span class="cm">/* Unmap, close, and delete the shared memory object */</span>
|
|||
|
|
<span class="n">munmap</span> <span class="p">(</span><span class="n">perm</span><span class="p">,</span> <span class="k">sizeof</span> <span class="p">(</span><span class="k">struct</span> <span class="n">permission</span><span class="p">));</span>
|
|||
|
|
<span class="n">close</span> <span class="p">(</span><span class="n">shmfd</span><span class="p">);</span>
|
|||
|
|
<span class="n">shm_unlink</span> <span class="p">(</span><span class="s">"/OpenCSF_SHM"</span><span class="p">);</span>
|
|||
|
|
</pre></div>
|
|||
|
|
</td></tr></table></div>
|
|||
|
|
<p>Combining <code class="docutils literal notranslate"><span class="pre">shm_open()</span></code> and <code class="docutils literal notranslate"><span class="pre">mmap()</span></code> in this way is a common technique that often confuses
|
|||
|
|
novices. Specifically, it seems redundant to use both IPC mechanisms, because the same task could be
|
|||
|
|
accomplished with either <code class="docutils literal notranslate"><span class="pre">shm_open()</span></code> or <code class="docutils literal notranslate"><span class="pre">mmap()</span></code> (not both). While it is true that both are not
|
|||
|
|
required, combining them leads to certain advantages:</p>
|
|||
|
|
<blockquote>
|
|||
|
|
<div><ul class="simple">
|
|||
|
|
<li>The <code class="docutils literal notranslate"><span class="pre">shm_open()</span></code> parameters adhere to the standard POSIX IPC conventions. This allows the
|
|||
|
|
processes to use POSIX names to identify the objects, rather than creating files in arbitrary locations.</li>
|
|||
|
|
<li>Using <code class="docutils literal notranslate"><span class="pre">shm_unlink()</span></code> provides a safety check that is not guaranteed with creating and mapping
|
|||
|
|
arbitrary files. Specifically, <code class="docutils literal notranslate"><span class="pre">shm_unlink()</span></code> will not delete the object immediately if any other
|
|||
|
|
process also has an open connection; this delay allows decreases the likelihood of other processes
|
|||
|
|
experiencing random and unexpected failures. In contrast, if an arbitrary non-IPC file was used for
|
|||
|
|
the <code class="docutils literal notranslate"><span class="pre">mmap()</span></code>, the timing of the deletion would be determined by the underlying file system; this
|
|||
|
|
introduces unnecessary maintenance and possibly restricts the portability of the program.</li>
|
|||
|
|
<li>Using <code class="docutils literal notranslate"><span class="pre">mmap()</span></code> allows the developer to cast the shared memory object to a more meaningful data
|
|||
|
|
structure type and to avoid using file operations. Specifically, observe that the return type from
|
|||
|
|
<code class="docutils literal notranslate"><span class="pre">shm_open()</span></code> is a file descriptor, which does not provide any information about the type of data
|
|||
|
|
stored in the object. By mapping it into memory, we can cast the pointer returned and access the
|
|||
|
|
<code class="docutils literal notranslate"><span class="pre">user</span></code>, <code class="docutils literal notranslate"><span class="pre">group</span></code>, and <code class="docutils literal notranslate"><span class="pre">other</span></code> fields contained in the <code class="docutils literal notranslate"><span class="pre">struct</span></code>.</li>
|
|||
|
|
</ul>
|
|||
|
|
</div></blockquote>
|
|||
|
|
<table class="docutils footnote" frame="void" id="f18" rules="none">
|
|||
|
|
<colgroup><col class="label" /><col /></colgroup>
|
|||
|
|
<tbody valign="top">
|
|||
|
|
<tr><td class="label"><a class="fn-backref" href="ShMem.html#id1">[1]</a></td><td>As a minor point of terminology, all forms of POSIX IPC, including shared memory, are
|
|||
|
|
referred to as <em>objects</em>. In contrast, System V shared memory is called a “segment” for historical purposes.</td></tr>
|
|||
|
|
</tbody>
|
|||
|
|
</table>
|
|||
|
|
<div
|
|||
|
|
id="IPCShmSumm"
|
|||
|
|
class="embedContainer"
|
|||
|
|
data-exer-name="IPCShmSumm"
|
|||
|
|
data-long-name="IPC shared memory questions"
|
|||
|
|
data-short-name="IPCShmSumm"
|
|||
|
|
data-frame-src="../../../Exercises/IPC/IPCShmSumm.html?selfLoggingEnabled=false&localMode=true&module=ShMem"
|
|||
|
|
data-frame-width="950"
|
|||
|
|
data-frame-height="550"
|
|||
|
|
data-external="false"
|
|||
|
|
data-points="0"
|
|||
|
|
data-required="False"
|
|||
|
|
data-showhide="show"
|
|||
|
|
data-threshold="2"
|
|||
|
|
data-type="ka"
|
|||
|
|
data-exer-id="">
|
|||
|
|
|
|||
|
|
<div class="center">
|
|||
|
|
<div id="IPCShmSumm_iframe"></div>
|
|||
|
|
</div>
|
|||
|
|
</div>
|
|||
|
|
</div>
|
|||
|
|
</div>
|
|||
|
|
|
|||
|
|
|
|||
|
|
</div>
|
|||
|
|
|
|||
|
|
|
|||
|
|
|
|||
|
|
<div class="container">
|
|||
|
|
|
|||
|
|
<div class="mt-4 container center">
|
|||
|
|
«  <a id="prevmod1" href="MQueues.html">3.6. Message Passing With Message Queues</a>
|
|||
|
|
  ::  
|
|||
|
|
<a class="uplink" href="index.html">Contents</a>
|
|||
|
|
  ::  
|
|||
|
|
<a id="nextmod1" href="IPCSems.html">3.8. Semaphores</a>  »
|
|||
|
|
</div>
|
|||
|
|
|
|||
|
|
|
|||
|
|
</div>
|
|||
|
|
|
|||
|
|
<br />
|
|||
|
|
|
|||
|
|
<div class="row jmu-dark-purple-bg">
|
|||
|
|
<div class="col-md-12">
|
|||
|
|
<center>
|
|||
|
|
<a id="contact_us" class="btn button-link-no-blue jmu-gold" rel="nofollow" href="mailto:webmaster@opencsf.org" role="button">Contact Us</a>
|
|||
|
|
<a id="license" class="btn button-link-no-blue jmu-gold" rel="nofollow" href="https://w3.cs.jmu.edu/kirkpams/OpenCSF/lib/license.html" target="_blank">License</a>
|
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</center>
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<script src="_static/js/popper.js-1.14.7-min.js" integrity="sha384-UO2eT0CpHqdSJQ6hJty5KVphtPhzWj9WO1clHTMGa3JDZwrnQq4sF86dIHNDz0W1" crossorigin="anonymous"></script>
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<script src="_static/js/bootstrap.min.js" integrity="sha384-JjSmVgyd0p3pXB1rRibZUAYoIIy6OrQ6VrjIEaFf/nJGzIxFDsf4x0xIM+B07jRM" crossorigin="anonymous"></script>
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</body>
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</html>
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