191 lines
8.7 KiB
HTML
191 lines
8.7 KiB
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<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
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<html>
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<head>
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<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
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<meta name="Copyright" content="Copyright (c) 2006 by IBM Corporation">
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<title>Shared Memory Functions</title>
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<!-- All rights reserved. Licensed Materials Property of IBM -->
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<!-- Begin Header Records ========================================= -->
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</head>
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<body>
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<a name="Top_Of_Page"></a>
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<!-- Java sync-link -->
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<script type="text/javascript" language="Javascript" src="../rzahg/synch.js">
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</script>
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<h2><a name="sharedmemory">Shared Memory Functions</a></h2>
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<p>Processes and threads can communicate directly with one another by sharing
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parts of their memory space and then reading and writing the data stored in the
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<strong>shared memory</strong>. Synchronization of shared memory is the
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responsibility of the application program. Semaphores and mutexes provide ways
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to synchronize shared memory use across processes and threads.</p>
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<p>A thread gets a shared memory identifier by calling the <strong>
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shmget()</strong> function. Depending on the <em>key</em> and <em>shmflg</em>
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parameters passed in, either a new shared memory segment is created or an
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existing shared memory segment is accessed. The size of the shared memory
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segment is specified by the <em>size</em> parameter. When a new shared memory
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segment is created, a data structure is also created to contain information
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about the shared memory segment. This structure is defined in the <strong>
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<sys/shm.h></strong> header file as follows:</p>
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<pre>
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typedef struct shmid_ds {
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struct ipc_perm shm_perm; /* Operation permission struct*/
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int shm_segsz; /* Segment size */
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pid_t shm_lpid; /* Process id of last shmop */
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pid_t shm_cpid; /* Process id of creator */
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int shm_nattch; /* Current # attached */
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time_t shm_atime; /* Last shmat time */
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time_t shm_dtime; /* Last shmdt time */
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time_t shm_ctime; /* Last change time */
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} shmtablentry_t;
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</pre>
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<p>A process gets addressability to the shared memory segment by attaching to
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it using the <strong>shmat()</strong> function. The following parameters are
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passed in:</p>
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<ul>
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<li>Shared memory ID</li>
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<li>Pointer to an address</li>
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<li>Flag specifying how the shared memory segment is to be attached</li>
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</ul>
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<p>A process detaches a shared memory segment by calling the <strong>
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shmdt()</strong> function. The only parameter passed in is the shared memory
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segment address. The process implicitly detaches from the shared memory when
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the process ends.</p>
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<p>A thread removes a shared memory ID by calling the <strong>shmctl()</strong>
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function. The thread also can use the <strong>shmctl()</strong> function to
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change the data structure values associated with the shared memory ID or to
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retrieve the data structure values associated with the shared memory ID. The
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following parameters are passed in:</p>
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<ul>
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<li>Shared memory ID</li>
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<li>Command the thread wants to perform (remove ID, set data structure values,
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receive data structure values)</li>
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<li>Pointer to a buffer from which to set data structure values, or in which to
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receive data structure values.</li>
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</ul>
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<br>
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<h4>Shared Memory Differences and Restrictions</h4>
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<p>Shared memory segments are created as teraspace-shared memory
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segments or as nonteraspace-shared memory segments. A teraspace shared memory
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segment is accessed by adding the shared memory segment to a process's
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teraspace. A teraspace is a space that has a much larger capacity than other
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i5/OS spaces and is addressable from only one process. A nonteraspace shared
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memory segment creates shared memory using i5/OS space objects.</p>
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<p>A teraspace shared memory segment is created if SHM_TS_NP is specified on
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the <em>shmflag</em> parameter of <strong>shmget()</strong> or if a shared
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memory segment is created from a program that was compiled using the
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TERASPACE(*YES *TSIFC) option of CRTBNDC or CRTCMOD. The following capabilities
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and restrictions apply for teraspace shared memory segments.</p>
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<ul>
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<li>Teraspace shared memory objects may be attached in read-only mode.<br><br></li>
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<li>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 it is not possible to
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specify the address in teraspace at which the shared memory is to be mapped.
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The <em>shmaddr</em> parameter on the <strong>shmat()</strong> function is
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ignored.<br><br></li>
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<li>After a teraspace shared memory segment is detached, it cannot be addressed
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through a pointer saved by the process.<br><br></li>
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<li>The maximum size of a teraspace shared memory segment is
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4 294 967 295 bytes (4 GB minus 1).<br><br></li>
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<li>The maximum number of shared memory segments that can be created
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(system-wide) is 2 147 483 646.<br><br></li>
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<li>A teraspace shared memory segment may be created such that its size can be
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changed after it is created. The maximum size of this type of shared memory
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segment is 268 435 456 bytes (256 MB).</li>
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</ul>
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<p>The i5/OS nonteraspace shared memory differs from the shared memory definition
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in the Single UNIX Specification in the following
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ways:</p>
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<ul>
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<li>The nonteraspace shared memory segments are i5/OS space objects and can be
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attached only in read/write mode, not in the read-only mode that the
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Single UNIX Specification
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allows. If the SHM_RDONLY flag is specified in the <em>
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shmflg</em> parameter on a <strong>shmget()</strong> call, the call fails and
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the <em>errno</em> variable is set to [EOPNOTSUPP].<br><br></li>
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<li>A nonteraspace shared memory segment can be attached only at the actual
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address of the i5/OS space object, not at an address specified by the thread.
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The <em>shmaddr</em> parameter on the <strong>shmat()</strong> function is
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ignored.<br><br></li>
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<li>After a nonteraspace shared memory segment is detached from a process, it
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still can be addressed through a pointer saved by the process. For nonteraspace
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shared memory segments, i5/OS does not "map" and "unmap" regions of storage to
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the address space of a process.<br><br></li>
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<li>The maximum size of a nonteraspace shared memory segment is
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16 776 960 bytes. Although the maximum size of a shared memory
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segment is 16 776 960 bytes, shared memory segments larger than
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16 773 120 bytes should be created as teraspace shared memory
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segments. When the operating system accesses a nonteraspace shared memory
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segment that has a size larger than 16 773 120 bytes, a performance
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degradation may be observed.<br><br></li>
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<li>The maximum number of shared memory segments that can be created
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(system-wide) is 2 147 483 646.<br><br></li>
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<li>The size of a nonteraspace shared memory segment may be changed using the
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SHM_RESIZE command of <strong>shmctl()</strong>, up to a maximum size of
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16 773 120 bytes.</li>
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</ul>
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<p>The shared memory functions are:</p>
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<!--***************API BEGIN PASTE***************-->
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<ul>
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<li><A HREF="ipcshmat.htm">shmat()</A> (Attach Shared Memory Segment to Current Process) returns the address of the shared memory segment associated with the specified shared memory identifier.</li>
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<li><A HREF="ipcshmct.htm">shmctl()</A> (Perform Shared Memory Control Operations) provides shared memory control operations as specified by cmd on the shared memory segment specified by shmid.</li>
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<li><A HREF="ipcshmdt.htm">shmdt()</A> (Detach Shared Memory Segment from Calling Process) detaches the shared memory segment specified by shmaddr from the calling process.</li>
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<li><A HREF="ipcshmgt.htm">shmget()</A> (Get ID of Shared Memory Segment with Key) returns the shared memory ID associated with the specified shared memory key.</li>
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</ul>
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<!--***************API END PASTE***************-->
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<p>See also <a href="unix3a4.htm">IPC Key Generation Functions</a> for additional shared memory functions.</p>
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<br>
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<hr>
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<center>
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<table cellpadding="2" cellspacing="2">
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<tr align="center">
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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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