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| <section id="shared-memory-support"> |
| <h1>Shared Memory Support<a class="headerlink" href="#shared-memory-support" title="Permalink to this heading"></a></h1> |
| <section id="prerequisites"> |
| <h2>Prerequisites<a class="headerlink" href="#prerequisites" title="Permalink to this heading"></a></h2> |
| <p>These features must be enabled before shared memory support can be |
| provided:</p> |
| <ul class="simple"> |
| <li><p><code class="docutils literal notranslate"><span class="pre">CONFIG_ARCH_ADDRENV=y</span></code> - Support for per-task address environment using a |
| MMU.</p></li> |
| <li><p><code class="docutils literal notranslate"><span class="pre">CONFIG_BUILD_KERNEL=y</span></code> - Support for protected kernel-/user-space memory |
| regions must be provided by the MMU.</p></li> |
| <li><p><code class="docutils literal notranslate"><span class="pre">CONFIG_GRAN=y</span></code> - The granule allocation is the allocation underlying all |
| paged allocations.</p></li> |
| <li><p><code class="docutils literal notranslate"><span class="pre">CONFIG_MM_PGALLOC=y</span></code> - Enables the physical page allocator</p></li> |
| <li><p><code class="docutils literal notranslate"><span class="pre">CONFIG_MM_PGSIZE</span></code> - Determines the size of one page that can be mapped by |
| the MMU.</p></li> |
| </ul> |
| <p>And then finally:</p> |
| <ul class="simple"> |
| <li><p><code class="docutils literal notranslate"><span class="pre">CONFIG_MM_SHM=y</span></code> - Enables shared memory support</p></li> |
| <li><p><code class="docutils literal notranslate"><span class="pre">CONFIG_ARCH_SHM_VBASE</span></code> - The virtual address of the beginning of the |
| shared memory region.</p></li> |
| <li><p><code class="docutils literal notranslate"><span class="pre">CONFIG_ARCH_SHM_MAXREGIONS</span></code> - The maximum number of regions that can |
| allocated for the shared memory space. This hard-coded value permits |
| static allocation of the shared memory data structures and serves no |
| other purpose. Default is 1.</p></li> |
| <li><p><code class="docutils literal notranslate"><span class="pre">CONFIG_ARCH_SHM_NPAGES</span></code> - The maximum number of pages that can allocated |
| for the shared memory region. Default is 1.</p></li> |
| </ul> |
| <p>The size of the virtual shared memory address space is then determined by |
| the product of the maximum number of regions, the maximum number of pages |
| per region, and the configured size of each page.</p> |
| </section> |
| <section id="concepts"> |
| <h2>Concepts<a class="headerlink" href="#concepts" title="Permalink to this heading"></a></h2> |
| <p>Each process has a task group structure, struct task_group_s, that holds |
| information common to all threads in the group. If <code class="docutils literal notranslate"><span class="pre">CONFIG_MM_SHM=y</span></code>, then |
| this includes data structures for the per-process shared memory virtual |
| page allocator.</p> |
| <p>A memory region is accessed using:</p> |
| <div class="highlight-C notranslate"><div class="highlight"><pre><span></span><span class="kt">int</span><span class="w"> </span><span class="nf">shmget</span><span class="p">(</span><span class="kt">key_t</span><span class="w"> </span><span class="n">key</span><span class="p">,</span><span class="w"> </span><span class="kt">size_t</span><span class="w"> </span><span class="n">size</span><span class="p">,</span><span class="w"> </span><span class="kt">int</span><span class="w"> </span><span class="n">shmflg</span><span class="p">);</span> |
| </pre></div> |
| </div> |
| <p>by a lookup using internal shared memory data sets with key as the lookup |
| match value. On success, shmget returns the shared memory identifier for |
| the match – in this implementation that identifier is simply the table |
| index of the match.</p> |
| <p>If the memory region does not exist, it may also be created by shmget (if |
| the IPC_CREAT bit is set in the shmflag). When a shared memory region is |
| created, the following things happen:</p> |
| <ul class="simple"> |
| <li><p>A new entry is set aside in the internal data set. The key value is |
| assigned to the entry and the table index is the new shared memory |
| identifier.</p></li> |
| <li><p>The requested size is rounded up to rounded up to full pages, each of |
| size <code class="docutils literal notranslate"><span class="pre">CONFIG_MM_PGSIZE</span></code>.</p></li> |
| <li><p>A set of physical pages are allocated and the physical address of |
| these pages is retained in the internal data set.</p></li> |
| </ul> |
| <p>Now the key maps to and shared memory identifier (the table index) and |
| the table index provides access to the list of physical pages making up |
| the shared memory region.</p> |
| <p>NOTE: An improved implementation my perform a “lazy” back up of the |
| physical memory, i.e., do not allocate the physical memory until the |
| memory is required, for example, when a page fault occurs when a |
| application tries to allocate the memory.</p> |
| <p>A shared memory region is destroyed via:</p> |
| <div class="highlight-C notranslate"><div class="highlight"><pre><span></span><span class="kt">int</span><span class="w"> </span><span class="nf">shmctl</span><span class="p">(</span><span class="kt">int</span><span class="w"> </span><span class="n">shmid</span><span class="p">,</span><span class="w"> </span><span class="kt">int</span><span class="w"> </span><span class="n">cmd</span><span class="p">,</span><span class="w"> </span><span class="k">struct</span><span class="w"> </span><span class="nc">shmid_ds</span><span class="w"> </span><span class="o">*</span><span class="n">buf</span><span class="p">);</span> |
| </pre></div> |
| </div> |
| <p>In order for a process to make use of the memory region, it must be |
| “attached” the process using:</p> |
| <div class="highlight-C notranslate"><div class="highlight"><pre><span></span><span class="n">FAR</span><span class="w"> </span><span class="kt">void</span><span class="w"> </span><span class="o">*</span><span class="n">shmat</span><span class="p">(</span><span class="kt">int</span><span class="w"> </span><span class="n">shmid</span><span class="p">,</span><span class="w"> </span><span class="n">FAR</span><span class="w"> </span><span class="k">const</span><span class="w"> </span><span class="kt">void</span><span class="w"> </span><span class="o">*</span><span class="n">shmaddr</span><span class="p">,</span><span class="w"> </span><span class="kt">int</span><span class="w"> </span><span class="n">shmflg</span><span class="p">);</span> |
| </pre></div> |
| </div> |
| <p><code class="docutils literal notranslate"><span class="pre">shmat()</span></code> returns the virtual address where the shared memory can be found |
| in the user process. Attaching the shared memory region involves the |
| following steps:</p> |
| <ul class="simple"> |
| <li><p>Use the shmid as a table index to look up the mapping in the shared |
| memory internal data structures.</p></li> |
| <li><p>Allocate a virtual address spaces of the same size as the physical |
| address space using the per-process virtual shared memory virtual |
| page allocator that can be found in the calling process’s task group |
| structure.</p></li> |
| <li><p>Use platform specific interfaces to mapy the physical memory to the |
| selected virtual address space, and</p></li> |
| <li><p>Return the allocated virtual base address to the caller.</p></li> |
| </ul> |
| <p>The memory region can be detached from the user process using:</p> |
| <div class="highlight-C notranslate"><div class="highlight"><pre><span></span><span class="kt">int</span><span class="w"> </span><span class="nf">shmdt</span><span class="p">(</span><span class="n">FAR</span><span class="w"> </span><span class="k">const</span><span class="w"> </span><span class="kt">void</span><span class="w"> </span><span class="o">*</span><span class="n">shmaddr</span><span class="p">);</span> |
| </pre></div> |
| </div> |
| </section> |
| <section id="relevant-header-files"> |
| <h2>Relevant header files<a class="headerlink" href="#relevant-header-files" title="Permalink to this heading"></a></h2> |
| <ul class="simple"> |
| <li><p><code class="docutils literal notranslate"><span class="pre">include/sys/shm.h</span></code> - Shared memory interface declarations</p></li> |
| <li><p><code class="docutils literal notranslate"><span class="pre">include/sys/ipc.h</span></code> - Provides additional definitions used by the shared |
| memory interfaces</p></li> |
| <li><p><code class="docutils literal notranslate"><span class="pre">include/nuttx/addrenv.h</span></code> - Defines the virtual address space of the |
| process.</p></li> |
| <li><p><code class="docutils literal notranslate"><span class="pre">include/nuttx/pgalloc.h</span></code> - Page allocator interfaces</p></li> |
| <li><dl class="simple"> |
| <dt><code class="docutils literal notranslate"><span class="pre">mm/shm/shm.h</span></code> - Internal shared memory definitions. This includes the</dt><dd><p>definitions of the internal shared memory data structures.</p> |
| </dd> |
| </dl> |
| </li> |
| </ul> |
| </section> |
| </section> |
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