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| <section id="shared-memory-interfaces"> |
| <h1>Shared Memory Interfaces<a class="headerlink" href="#shared-memory-interfaces" title="Permalink to this heading"></a></h1> |
| <p>Shared memory interfaces are only available with the NuttX kernel build |
| (<code class="docutils literal notranslate"><span class="pre">CONFIG_BUILD_KERNEL=y</span></code>). These interfaces support user memory |
| regions that can be shared between multiple user processes. Shared |
| memory interfaces:</p> |
| <blockquote> |
| <div><ul class="simple"> |
| <li><p><a class="reference internal" href="#c.shmget" title="shmget"><code class="xref c c-func docutils literal notranslate"><span class="pre">shmget()</span></code></a></p></li> |
| <li><p><a class="reference internal" href="#c.shmat" title="shmat"><code class="xref c c-func docutils literal notranslate"><span class="pre">shmat()</span></code></a></p></li> |
| <li><p><a class="reference internal" href="#c.shmctl" title="shmctl"><code class="xref c c-func docutils literal notranslate"><span class="pre">shmctl()</span></code></a></p></li> |
| <li><p><a class="reference internal" href="#c.shmdt" title="shmdt"><code class="xref c c-func docutils literal notranslate"><span class="pre">shmdt()</span></code></a></p></li> |
| </ul> |
| </div></blockquote> |
| <section id="functions"> |
| <h2>Functions<a class="headerlink" href="#functions" title="Permalink to this heading"></a></h2> |
| <dl class="c function"> |
| <dt class="sig sig-object c" id="c.shmget"> |
| <span class="kt"><span class="pre">int</span></span><span class="w"> </span><span class="sig-name descname"><span class="n"><span class="pre">shmget</span></span></span><span class="sig-paren">(</span><span class="n"><span class="pre">key_t</span></span><span class="w"> </span><span class="n"><span class="pre">key</span></span>, <a class="reference internal" href="structures.html#c.size_t" title="size_t"><span class="n"><span class="pre">size_t</span></span></a><span class="w"> </span><span class="n"><span class="pre">size</span></span>, <span class="kt"><span class="pre">int</span></span><span class="w"> </span><span class="n"><span class="pre">shmflg</span></span><span class="sig-paren">)</span><a class="headerlink" href="#c.shmget" title="Permalink to this definition"></a><br /></dt> |
| <dd><p>Returns the shared memory identifier associated with <code class="docutils literal notranslate"><span class="pre">key</span></code>.</p> |
| <p>A shared memory identifier, associated data structure, and shared memory |
| segment of at least size bytes is created for <code class="docutils literal notranslate"><span class="pre">key</span></code> if one of the |
| following is true:</p> |
| <ul class="simple"> |
| <li><p>The argument <code class="docutils literal notranslate"><span class="pre">key</span></code> is equal to <code class="docutils literal notranslate"><span class="pre">IPC_PRIVATE</span></code>.</p></li> |
| <li><p>The argument <code class="docutils literal notranslate"><span class="pre">key</span></code> does not already have a shared memory identifier |
| associated with it and <code class="docutils literal notranslate"><span class="pre">(shmflg</span> <span class="pre">&</span> <span class="pre">IPC_CREAT)</span></code> is non-zero.</p></li> |
| </ul> |
| <p>Upon creation, the data structure associated with the new shared memory |
| identifier will be initialized as follows:</p> |
| <ul class="simple"> |
| <li><p>The low-order nine bits of <code class="docutils literal notranslate"><span class="pre">shm_perm.mode</span></code> are set equal to the |
| low-order nine bits of <code class="docutils literal notranslate"><span class="pre">shmflg</span></code>.</p></li> |
| <li><p>The value of <code class="docutils literal notranslate"><span class="pre">shm_segsz</span></code> is set equal to the value of size.</p></li> |
| <li><p>The values of <code class="docutils literal notranslate"><span class="pre">shm_lpid</span></code>, <code class="docutils literal notranslate"><span class="pre">shm_nattch</span></code>, <code class="docutils literal notranslate"><span class="pre">shm_atime</span></code>, and |
| <code class="docutils literal notranslate"><span class="pre">shm_dtime</span></code> are set equal to 0.</p></li> |
| <li><p>The value of <code class="docutils literal notranslate"><span class="pre">shm_ctime</span></code> is set equal to the current time.</p></li> |
| </ul> |
| <p>When the shared memory segment is created, it will be initialized with |
| all zero values.</p> |
| <dl class="field-list simple"> |
| <dt class="field-odd">Parameters<span class="colon">:</span></dt> |
| <dd class="field-odd"><ul class="simple"> |
| <li><p><strong>key</strong> – The key that is used to access the unique shared memory |
| identifier.</p></li> |
| <li><p><strong>size</strong> – The shared memory region that is created will be at least |
| this size in bytes.</p></li> |
| <li><p><strong>shmflg</strong> – See <code class="docutils literal notranslate"><span class="pre">IPC_*</span></code> definitions in <code class="docutils literal notranslate"><span class="pre">sys/ipc.h</span></code>. Only the |
| values <code class="docutils literal notranslate"><span class="pre">IPC_PRIVATE</span></code> or <code class="docutils literal notranslate"><span class="pre">IPC_CREAT</span></code> are supported.</p></li> |
| </ul> |
| </dd> |
| <dt class="field-even">Returns<span class="colon">:</span></dt> |
| <dd class="field-even"><p><p>Upon successful completion, <code class="docutils literal notranslate"><span class="pre">shmget()</span></code> will return |
| a non-negative integer, namely a shared memory identifier; otherwise, it |
| will return -1 and set <code class="docutils literal notranslate"><span class="pre">errno</span></code> to indicate the error.</p> |
| <ul class="simple"> |
| <li><p><code class="docutils literal notranslate"><span class="pre">EACCES</span></code>. A shared memory identifier exists for key but operation |
| permission as specified by the low-order nine bits of <code class="docutils literal notranslate"><span class="pre">shmflg</span></code> |
| would not be granted.</p></li> |
| <li><p><code class="docutils literal notranslate"><span class="pre">EEXIST</span></code>. A shared memory identifier exists for the argument key |
| but <code class="docutils literal notranslate"><span class="pre">(shmflg</span> <span class="pre">&</span> <span class="pre">IPC_CREAT)</span> <span class="pre">&&</span> <span class="pre">(shmflg</span> <span class="pre">&</span> <span class="pre">IPC_EXCL)</span></code> are non-zero.</p></li> |
| <li><p><code class="docutils literal notranslate"><span class="pre">EINVAL</span></code>. A shared memory segment is to be created and the value of |
| size is less than the system-imposed minimum or greater than the |
| system-imposed maximum.</p></li> |
| <li><p><code class="docutils literal notranslate"><span class="pre">EINVAL</span></code>. No shared memory segment is to be created and a shared |
| memory segment exists for key but the size of the segment associated |
| with it is less than size and size is not 0.</p></li> |
| <li><p><code class="docutils literal notranslate"><span class="pre">ENOENT</span></code>. A shared memory identifier does not exist for the |
| argument key and <code class="docutils literal notranslate"><span class="pre">(shmflg</span> <span class="pre">&</span> <span class="pre">IPC_CREAT)</span></code> is 0.</p></li> |
| <li><p><code class="docutils literal notranslate"><span class="pre">ENOMEM</span></code>. A shared memory identifier and associated shared memory |
| segment will be created, but the amount of available physical memory |
| is not sufficient to fill the request.</p></li> |
| <li><p><code class="docutils literal notranslate"><span class="pre">ENOSPC</span></code>. A shared memory identifier is to be created, but the |
| system-imposed limit on the maximum number of allowed shared memory |
| identifiers system-wide would be exceeded.</p></li> |
| </ul> |
| </p> |
| </dd> |
| </dl> |
| <p><strong>POSIX Deviations</strong></p> |
| <ul class="simple"> |
| <li><p>The values of <code class="docutils literal notranslate"><span class="pre">shm_perm.cuid</span></code>, <code class="docutils literal notranslate"><span class="pre">shm_perm.uid</span></code>, <code class="docutils literal notranslate"><span class="pre">shm_perm.cgid</span></code>, |
| and <code class="docutils literal notranslate"><span class="pre">shm_perm.gid</span></code> should be set equal to the effective user ID and |
| effective group ID, respectively, of the calling process. The NuttX |
| <code class="docutils literal notranslate"><span class="pre">ipc_perm</span></code> structure, however, does not support these fields |
| because user and group IDs are not yet supported by NuttX.</p></li> |
| </ul> |
| </dd></dl> |
| |
| <dl class="c function"> |
| <dt class="sig sig-object c" id="c.shmat"> |
| <span class="kt"><span class="pre">void</span></span><span class="w"> </span><span class="p"><span class="pre">*</span></span><span class="sig-name descname"><span class="n"><span class="pre">shmat</span></span></span><span class="sig-paren">(</span><span class="kt"><span class="pre">int</span></span><span class="w"> </span><span class="n"><span class="pre">shmid</span></span>, <span class="pre">FAR</span><span class="w"> </span><span class="k"><span class="pre">const</span></span><span class="w"> </span><span class="kt"><span class="pre">void</span></span><span class="w"> </span><span class="p"><span class="pre">*</span></span><span class="n"><span class="pre">shmaddr</span></span>, <span class="kt"><span class="pre">int</span></span><span class="w"> </span><span class="n"><span class="pre">shmflg</span></span><span class="sig-paren">)</span><a class="headerlink" href="#c.shmat" title="Permalink to this definition"></a><br /></dt> |
| <dd><p>Attaches the shared memory |
| segment associated with the shared memory identifier specified by |
| <code class="docutils literal notranslate"><span class="pre">shmid</span></code> to the address space of the calling process. The segment is |
| attached at the address specified by one of the following criteria:</p> |
| <ul class="simple"> |
| <li><p>If <code class="docutils literal notranslate"><span class="pre">shmaddr</span></code> is a null pointer, the segment is attached at the |
| first available address as selected by the system.</p></li> |
| <li><p>If <code class="docutils literal notranslate"><span class="pre">shmaddr</span></code> is not a null pointer and <code class="docutils literal notranslate"><span class="pre">(shmflg</span> <span class="pre">&</span> <span class="pre">SHM_RND)</span></code> is |
| non-zero, the segment is attached at the address given by |
| <code class="docutils literal notranslate"><span class="pre">(shmaddr</span> <span class="pre">-</span> <span class="pre">((uintptr_t)shmaddr</span> <span class="pre">%</span> <span class="pre">SHMLBA))</span></code>.</p></li> |
| <li><p>If <code class="docutils literal notranslate"><span class="pre">shmaddr</span></code> is not a null pointer and <code class="docutils literal notranslate"><span class="pre">(shmflg</span> <span class="pre">&</span> <span class="pre">SHM_RND)</span></code> is 0, |
| the segment is attached at the address given by <code class="docutils literal notranslate"><span class="pre">shmaddr</span></code>.</p></li> |
| <li><p>The segment is attached for reading if <code class="docutils literal notranslate"><span class="pre">(shmflg</span> <span class="pre">&</span> <span class="pre">SHM_RDONLY)</span></code> is |
| non-zero and the calling process has read permission; otherwise, if |
| it is 0 and the calling process has read and write permission, the |
| segment is attached for reading and writing.</p></li> |
| </ul> |
| <dl class="field-list simple"> |
| <dt class="field-odd">Parameters<span class="colon">:</span></dt> |
| <dd class="field-odd"><ul class="simple"> |
| <li><p><strong>shmid</strong> – Shared memory identifier</p></li> |
| <li><p><strong>smaddr</strong> – Determines mapping of the shared memory region</p></li> |
| <li><p><strong>shmflg</strong> – See <code class="docutils literal notranslate"><span class="pre">SHM_*</span></code> definitions in <code class="docutils literal notranslate"><span class="pre">include/sys/shm.h</span></code>. Only |
| <code class="docutils literal notranslate"><span class="pre">SHM_RDONLY</span></code> and <code class="docutils literal notranslate"><span class="pre">SHM_RND</span></code> are supported.</p></li> |
| </ul> |
| </dd> |
| <dt class="field-even">Returns<span class="colon">:</span></dt> |
| <dd class="field-even"><p><p>Upon successful completion, <code class="docutils literal notranslate"><span class="pre">shmat()</span></code> will |
| increment the value of <code class="docutils literal notranslate"><span class="pre">shm_nattch</span></code> in the data structure associated |
| with the shared memory ID of the attached shared memory segment and |
| return the segment’s start address. Otherwise, the shared memory segment |
| will not be attached, <code class="docutils literal notranslate"><span class="pre">shmat()</span></code> will return -1, and <code class="docutils literal notranslate"><span class="pre">errno</span></code> will be |
| set to indicate the error.</p> |
| <ul class="simple"> |
| <li><p><code class="docutils literal notranslate"><span class="pre">EACCES</span></code>. Operation permission is denied to the calling process</p></li> |
| <li><p><code class="docutils literal notranslate"><span class="pre">EINVAL</span></code>. The value of <code class="docutils literal notranslate"><span class="pre">shmid</span></code> is not a valid shared memory |
| identifier, the <code class="docutils literal notranslate"><span class="pre">shmaddr</span></code> is not a null pointer, and the value of |
| <code class="docutils literal notranslate"><span class="pre">(shmaddr</span> <span class="pre">-((uintptr_t)shmaddr</span> <span class="pre">%</span> <span class="pre">SHMLBA))</span></code> is an illegal address |
| for attaching shared memory; or the <code class="docutils literal notranslate"><span class="pre">shmaddr</span></code> is not a null |
| pointer, <code class="docutils literal notranslate"><span class="pre">(shmflg</span> <span class="pre">&</span> <span class="pre">SHM_RND)</span></code> is 0, and the value of <code class="docutils literal notranslate"><span class="pre">shmaddr</span></code> is |
| an illegal address for attaching shared memory.</p></li> |
| <li><p><code class="docutils literal notranslate"><span class="pre">EMFILE</span></code>. The number of shared memory segments attached to the |
| calling process would exceed the system-imposed limit.</p></li> |
| <li><p><code class="docutils literal notranslate"><span class="pre">ENOMEM</span></code>. The available data space is not large enough to |
| accommodate the shared memory segment.</p></li> |
| </ul> |
| </p> |
| </dd> |
| </dl> |
| </dd></dl> |
| |
| <dl class="c function"> |
| <dt class="sig sig-object c" id="c.shmctl"> |
| <span class="kt"><span class="pre">int</span></span><span class="w"> </span><span class="sig-name descname"><span class="n"><span class="pre">shmctl</span></span></span><span class="sig-paren">(</span><span class="kt"><span class="pre">int</span></span><span class="w"> </span><span class="n"><span class="pre">shmid</span></span>, <span class="kt"><span class="pre">int</span></span><span class="w"> </span><span class="n"><span class="pre">cmd</span></span>, <span class="pre">FAR</span><span class="w"> </span><span class="k"><span class="pre">struct</span></span><span class="w"> </span><span class="n"><span class="pre">shmid_ds</span></span><span class="w"> </span><span class="p"><span class="pre">*</span></span><span class="n"><span class="pre">buf</span></span><span class="sig-paren">)</span><a class="headerlink" href="#c.shmctl" title="Permalink to this definition"></a><br /></dt> |
| <dd><p>Provides a variety of shared |
| memory control operations as specified by <code class="docutils literal notranslate"><span class="pre">cmd</span></code>. The following values |
| for <code class="docutils literal notranslate"><span class="pre">cmd</span></code> are available:</p> |
| <ul class="simple"> |
| <li><p><code class="docutils literal notranslate"><span class="pre">IPC_STAT</span></code>. Place the current value of each member of the |
| <code class="docutils literal notranslate"><span class="pre">shmid_ds</span></code> data structure associated with <code class="docutils literal notranslate"><span class="pre">shmid</span></code> into the |
| structure pointed to by <code class="docutils literal notranslate"><span class="pre">buf</span></code>.</p></li> |
| <li><p><code class="docutils literal notranslate"><span class="pre">IPC_SET</span></code>. Set the value of the <code class="docutils literal notranslate"><span class="pre">shm_perm.mode</span></code> member of the |
| <code class="docutils literal notranslate"><span class="pre">shmid_ds</span></code> data structure associated with <code class="docutils literal notranslate"><span class="pre">shmid</span></code> to the |
| corresponding value found in the structure pointed to by <code class="docutils literal notranslate"><span class="pre">buf</span></code>.</p></li> |
| <li><p><code class="docutils literal notranslate"><span class="pre">IPC_RMID</span></code>. Remove the shared memory identifier specified by |
| <code class="docutils literal notranslate"><span class="pre">shmid</span></code> from the system and destroy the shared memory segment and |
| <code class="docutils literal notranslate"><span class="pre">shmid_ds</span></code> data structure associated with it.</p></li> |
| </ul> |
| <dl class="field-list simple"> |
| <dt class="field-odd">Parameters<span class="colon">:</span></dt> |
| <dd class="field-odd"><ul class="simple"> |
| <li><p><strong>shmid</strong> – Shared memory identifier</p></li> |
| <li><p><strong>cmd</strong> – <code class="docutils literal notranslate"><span class="pre">shmctl()</span></code> command</p></li> |
| <li><p><strong>buf</strong> – Data associated with the <code class="docutils literal notranslate"><span class="pre">shmctl()</span></code> command</p></li> |
| </ul> |
| </dd> |
| <dt class="field-even">Returns<span class="colon">:</span></dt> |
| <dd class="field-even"><p><p>Upon successful completion, <code class="docutils literal notranslate"><span class="pre">shmctl()</span></code> will return |
| 0; otherwise, it will return -1 and set <code class="docutils literal notranslate"><span class="pre">errno</span></code> to indicate the error.</p> |
| <ul class="simple"> |
| <li><p><code class="docutils literal notranslate"><span class="pre">EACCES</span></code>. The argument <code class="docutils literal notranslate"><span class="pre">cmd</span></code> is equal to <code class="docutils literal notranslate"><span class="pre">IPC_STAT</span></code> and the |
| calling process does not have read permission.</p></li> |
| <li><p><code class="docutils literal notranslate"><span class="pre">EINVAL</span></code>. The value of <code class="docutils literal notranslate"><span class="pre">shmid</span></code> is not a valid shared memory |
| identifier, or the value of <code class="docutils literal notranslate"><span class="pre">cmd</span></code>is not a valid command.</p></li> |
| <li><p><code class="docutils literal notranslate"><span class="pre">EPERM</span></code>. The argument <code class="docutils literal notranslate"><span class="pre">cmd</span></code> is equal to <code class="docutils literal notranslate"><span class="pre">IPC_RMID</span></code> or |
| <code class="docutils literal notranslate"><span class="pre">IPC_SET</span></code> and the effective user ID of the calling process is not |
| equal to that of a process with appropriate privileges and it is not |
| equal to the value of <code class="docutils literal notranslate"><span class="pre">shm_perm.cuid</span></code> or <code class="docutils literal notranslate"><span class="pre">shm_perm.uid</span></code> in the |
| data structure associated with <code class="docutils literal notranslate"><span class="pre">shmid</span></code>.</p></li> |
| <li><p><code class="docutils literal notranslate"><span class="pre">EOVERFLOW</span></code>. The <code class="docutils literal notranslate"><span class="pre">cmd</span></code> argument is <code class="docutils literal notranslate"><span class="pre">IPC_STAT</span></code> and the <code class="docutils literal notranslate"><span class="pre">gid</span></code> |
| or <code class="docutils literal notranslate"><span class="pre">uid</span></code> value is too large to be stored in the structure pointed |
| to by the <code class="docutils literal notranslate"><span class="pre">buf</span></code> argument.</p></li> |
| </ul> |
| </p> |
| </dd> |
| </dl> |
| <p><strong>POSIX Deviations</strong></p> |
| <ul class="simple"> |
| <li><p><code class="docutils literal notranslate"><span class="pre">IPC_SET</span></code>. Does not set the <code class="docutils literal notranslate"><span class="pre">shm_perm.uid</span></code> or |
| <code class="docutils literal notranslate"><span class="pre">shm_perm.gid</span></code>members of the <code class="docutils literal notranslate"><span class="pre">shmid_ds</span></code> data structure |
| associated with <code class="docutils literal notranslate"><span class="pre">shmid</span></code> because user and group IDs are not yet |
| supported by NuttX</p></li> |
| <li><p><code class="docutils literal notranslate"><span class="pre">IPC_SET</span></code>. Does not restrict the operation to processes with |
| appropriate privileges or matching user IDs in <code class="docutils literal notranslate"><span class="pre">shmid_ds</span></code> data |
| structure associated with <code class="docutils literal notranslate"><span class="pre">shmid</span></code>. Again because user IDs and |
| user/group privileges are are not yet supported by NuttX</p></li> |
| <li><p><code class="docutils literal notranslate"><span class="pre">IPC_RMID</span></code>. Does not restrict the operation to processes with |
| appropriate privileges or matching user IDs in <code class="docutils literal notranslate"><span class="pre">shmid_ds</span></code> data |
| structure associated with <code class="docutils literal notranslate"><span class="pre">shmid</span></code>. Again because user IDs and |
| user/group privileges are are not yet supported by NuttX</p></li> |
| </ul> |
| </dd></dl> |
| |
| <dl class="c function"> |
| <dt class="sig sig-object c" id="c.shmdt"> |
| <span class="kt"><span class="pre">int</span></span><span class="w"> </span><span class="sig-name descname"><span class="n"><span class="pre">shmdt</span></span></span><span class="sig-paren">(</span><span class="pre">FAR</span><span class="w"> </span><span class="k"><span class="pre">const</span></span><span class="w"> </span><span class="kt"><span class="pre">void</span></span><span class="w"> </span><span class="p"><span class="pre">*</span></span><span class="n"><span class="pre">shmaddr</span></span><span class="sig-paren">)</span><a class="headerlink" href="#c.shmdt" title="Permalink to this definition"></a><br /></dt> |
| <dd><p>Detaches the shared memory |
| segment located at the address specified by <code class="docutils literal notranslate"><span class="pre">shmaddr</span></code> from the address |
| space of the calling process.</p> |
| <dl class="field-list simple"> |
| <dt class="field-odd">Parameters<span class="colon">:</span></dt> |
| <dd class="field-odd"><ul class="simple"> |
| <li><p><strong>shmid</strong> – Shared memory identifier</p></li> |
| </ul> |
| </dd> |
| <dt class="field-even">Returns<span class="colon">:</span></dt> |
| <dd class="field-even"><p><p>Upon successful completion, <code class="docutils literal notranslate"><span class="pre">shmdt()</span></code> will |
| decrement the value of <code class="docutils literal notranslate"><span class="pre">shm_nattch</span></code> in the data structure associated |
| with the shared memory ID of the attached shared memory segment and |
| return 0.</p> |
| <p>Otherwise, the shared memory segment will not be detached, <code class="docutils literal notranslate"><span class="pre">shmdt()</span></code> |
| will return -1, and <code class="docutils literal notranslate"><span class="pre">errno</span></code> will be set to indicate the error.</p> |
| <ul class="simple"> |
| <li><p><code class="docutils literal notranslate"><span class="pre">EINVAL</span></code>. The value of <code class="docutils literal notranslate"><span class="pre">shmaddr</span></code> is not the data segment start |
| address of a shared memory segment.</p></li> |
| </ul> |
| </p> |
| </dd> |
| </dl> |
| </dd></dl> |
| |
| </section> |
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