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359 lines
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<title>shmat()-Attach Shared Memory Segment to Current Process</title>
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<a name="Top_Of_Page"></a>
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<h2>shmat()--Attach Shared Memory Segment to Current Process</h2>
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<div class="box" style="width: 70%;">
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<br>
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Syntax<br>
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<pre>
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#include <sys/shm.h>
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void *shmat(int <em>shmid</em>, const void <em>*shmaddr</em>,
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int <em>shmflg</em>);
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</pre>
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<br>
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Service Program Name: QP0ZUSHR<br>
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<!-- iddvc RMBR -->
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<br>
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Default Public Authority: *USE<br>
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<!-- iddvc RMBR -->
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<br>
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Threadsafe: Yes<br>
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<!-- iddvc RMBR -->
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<br>
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</div>
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<p>The <strong>shmat()</strong> function attaches to the shared memory segment
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specified by <em>shmid</em> and returns the address of the shared
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memory segment.</p>
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<p>The address specified by <em>shmaddr</em> is only used when <strong>
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shmat()</strong> is called from a program that uses data model *LLP64 and
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attaches to a teraspace shared memory segment. Otherwise the address specified
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by <em>shmaddr</em> is ignored and the actual shared memory segment address
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is returned regardless of the value of <em>shmaddr</em>.</p>
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<p>The system maintains status information about a shared memory segment which can
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be retrieved with the <a href="ipcshmct.htm">shmctl()</a> function. When a shared
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memory segment is successfully attached, the system sets the members of the
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<samp>shmid_ds</samp> structure as follows:<p>
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<ul>
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<li><samp>shm_nattch</samp> is incremented by 1.<p></li>
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<li><samp>shm_lpid</samp> is set to the process ID of the calling thread.<p></li>
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<li><samp>shm_atime</samp> is set to the current time.</li>
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</ul>
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<br>
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<h3>Parameters</h3>
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<dl>
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<dt><strong>shmid</strong></dt>
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<dd>(Input) Shared memory identifier, a positive integer. It is returned by the
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<a href="ipcshmgt.htm">shmget()</a> function and used to identify the shared memory
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segment.<br>
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<br>
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</dd>
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<dt><strong>shmaddr</strong></dt>
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<dd>(Input) Shared memory address. The address at which the calling thread
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would like the shared memory segment attached.<br>
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<br>
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</dd>
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<dt><strong>shmflg</strong></dt>
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<dd>(Input) Operations flags. The value of the <em>shmflg</em> parameter is either zero
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or is obtained by performing an OR operation on one or more of the
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following constants:<p>
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<dl>
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<dt><strong>SHM_RDONLY (0x00001000)</strong></dt>
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<dd>Attach the shared memory segment in read-only
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mode. This flag is valid only for teraspace shared memory segments.</dd>
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</dl>
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</dd>
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</dl>
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<br>
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<h3>Authorities</h3>
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<p><strong><a name="TBLAHAT1">Authorization Required for shmat()</a></strong></p>
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<table border cellpadding="5">
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<!-- cols="60 20 20" -->
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<tr>
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<th align="left" valign="bottom">Object Referred to</th>
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<th align="left" valign="bottom">Authority Required</th>
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<th align="left" valign="bottom">errno</th>
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</tr>
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<tr>
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<td align="left" valign="top">Shared memory segment to be attached in
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read/write memory</td>
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<td align="left" valign="top">Read and Write</td>
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<td align="left" valign="top">EACCES</td>
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</tr>
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<tr>
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<td align="left" valign="top">Shared memory segment to be attached in read-only
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memory in a process's teraspace.</td>
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<td align="left" valign="top">Read</td>
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<td align="left" valign="top">EACCES</td>
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</tr>
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</table>
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<br>
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<h3>Return Value</h3>
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<table cellpadding="5">
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<!-- cols="15 85" -->
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<tr>
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<td align="left" valign="top"><em>value</em></td>
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<td align="left" valign="top"><strong>shmat()</strong> was successful. The
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value returned is a pointer to the shared memory segment associated with the
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specified identifier.</td>
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</tr>
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<tr>
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<td align="left" valign="top"><em>NULL</em></td>
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<td align="left" valign="top"><strong>shmat()</strong> was not successful. The
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<em>errno</em> variable is set to indicate the error.</td>
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</tr>
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</table>
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<br>
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<h3>Error Conditions</h3>
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<p>If <strong>shmat()</strong> is not successful, <em>errno</em> usually
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indicates one of the following errors. Under some conditions, <em>errno</em>
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could indicate an error other than those listed here.</p>
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<dl>
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<dt><em>[EACCES]</em></dt>
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<dd>
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<p>Permission denied.</p>
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<p>An attempt was made to access an object in a way forbidden by its object
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access permissions.</p>
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<p>The thread does not have access to the specified file, directory, component,
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or path.</p>
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<p>Operation permission is denied to the calling thread.</p>
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<p>Shared memory operations are not permitted because the QSHRMEMCTL system
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value is set to 0.</p>
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<p>The shared memory segment is to be attached in read/write mode and the calling
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thread does not read and write permission to the shared memory segment.</p>
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<p>The shared memory segment is to be attached in read-only mode and the calling
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thread does not read permission to the shared memory segment.</p>
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</dd>
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<dt><em>[EADDRINUSE]</em></dt>
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<dd>
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<p>A damaged object was encountered.</p>
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<p>Address already in use.</p>
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<p>An attempt was made to attach to a teraspace shared memory segment with the
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SHM_MAP_FIXED_NP attribute and the address range is not available in the
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teraspace of the current job.</p>
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</dd>
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<dt><em>[EDAMAGE]</em></dt>
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<dd>
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<p>A damaged object was encountered.</p>
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<p>The shared memory segment has been damaged by a previous shared memory
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operation.</p>
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</dd>
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<dt><em>[EFAULT]</em></dt>
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<dd>
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<p>The address used for an argument is not correct.</p>
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<p>In attempting to use an argument in a call, the system detected an address
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that is not valid.</p>
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<p>While attempting to access a parameter passed to this function, the system
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detected an address that is not valid.</p>
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</dd>
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<dt><em>[EINVAL]</em></dt>
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<dd>
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<p>The value specified for the argument is not correct.</p>
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<p>A function was passed incorrect argument values, or an operation was
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attempted on an object and the operation specified is not supported for that
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type of object.</p>
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<p>An argument value is not valid, out of range, or NULL.</p>
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<p>The <em>shmid</em> parameter is not a valid shared memory identifier.</p>
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</dd>
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<dt><em>[EOPNOTSUPP]</em></dt>
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<dd>
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<p>Operation not supported.</p>
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<p>The operation, though supported in general, is not supported for the
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requested object or the requested arguments.</p>
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<p>The SHM_RDONLY flag is set in the <em>shmflg</em> parameter.
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Read-only shared memory segments are not supported for
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nonteraspace shared memory segments and for shared memory segments
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created with the SHM_MAP_FIXED_NP attribute.</p>
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</dd>
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<dt><em>[ENOMEM]</em></dt>
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<dd>
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<p>Storage allocation request failed.</p>
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<p>A function needed to allocate storage, but no storage is available.</p>
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<p>The available data space is not large enough to accommodate the shared
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memory segment.</p>
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</dd>
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<dt><em>[EUNKNOWN]</em></dt>
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<dd>
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<p>Unknown system state.</p>
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<p>The operation failed because of an unknown system state. See any messages in
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the job log and correct any errors that are indicated, then retry the
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operation.</p>
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</dd>
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</dl>
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<br>
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<h3>Error Messages</h3>
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<p>None.</p>
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<br>
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<h3><a name="USAGE_NOTES">Usage Notes</a></h3>
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<ol>
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<li>The only supported operation flag is SHM_RDONLY. This operation flag is
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supported only when you attach to a teraspace shared memory segment. If <em>
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shmflg</em> specifies SHM_RDONLY for a nonteraspace shared memory segment,
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then an [EOPNOTSUPP] error is returned. All other values for <em>shmflg</em>
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are ignored.<br>
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<br>
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</li>
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<li>A module that was not created with teraspace memory enabled should not
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attach to a teraspace shared memory segment. The call to <em>shmat()</em>
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will succeed and return a pointer. Any attempt, however, by a module not
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created with teraspace memory enabled to use the returned pointer will result
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in an MCH3601 (Pointer not set for location referenced) exception.<br>
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<br>
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</li>
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<li>When a job attaches to a shared memory segment that was created with the
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SHM_MAP_FIXED_NP attribute, an address range within the job's teraspace is used for
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the shared memory mapping. When a subsequent job attaches to the shared
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memory segment, the same address range within its teraspace must be available.
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If the address range is not available, the call to <em>shmat()</em> will fail
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with an [EADDRINUSE] error.<br>
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<br>
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</li>
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<li>The storage for a shared memory segment is allocated when the first job
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attaches to the shared memory segment. The storage is charged against the
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job's temporary storage limit. If the job does not have enough
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temporary storage to satisfy the request, the call to <em>shmat()</em> will
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fail with an [ENOMEM] error.</li>
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</ol>
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<br>
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<h3>Related Information</h3>
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<ul>
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<li>The <<strong>sys/shm.h</strong>> file (see <a href="unix13.htm">
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Header Files for UNIX-Type Functions</a>)<br>
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<br>
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</li>
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<li><a href="ipcshmat.htm">shmctl()</a>--Perform Shared Memory Control
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Operations<br>
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<br>
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</li>
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<li><a href="ipcshmgt.htm">shmget()</a>--Get ID of Shared Memory Segment with
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Key<br>
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<br>
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</li>
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<li><a href="ipcshmdt.htm">shmdt()</a>--Detach Shared Memory Segment from
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Calling Process</li>
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</ul>
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<br>
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<h3>Example</h3>
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<p>See <a href="../apiref/aboutapis.htm#codedisclaimer">Code disclaimer information</a>
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for information pertaining to code examples.</p>
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<p>For an example of using this function, see <a href="../apiref/apiexusmem.htm">
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Using Semaphores and Shared Memory</a> in Examples: APIs.</p>
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<br>
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<hr>
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API introduced: V3R6
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<hr>
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<td valign="middle" align="center"><a href="#Top_Of_Page">Top</a> | <a href=
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"unix.htm">UNIX-Type APIs</a> | <a href="aplist.htm">APIs by category</a></td>
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</tr>
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</table>
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</center>
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</body>
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</html>
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