266 lines
9.8 KiB
HTML
266 lines
9.8 KiB
HTML
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<title>semget</title>
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<basefont size="3"> <a name="semget"></a> <a name="tag_03_639"></a><!-- semget -->
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<!--header start-->
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<center><font size="2">The Open Group Base Specifications Issue 6<br>
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IEEE Std 1003.1-2001<br>
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Copyright © 2001 The IEEE and The Open Group, All Rights reserved.</font></center>
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<!--header end-->
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<hr size="2" noshade>
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<h4><a name="tag_03_639_01"></a>NAME</h4>
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<blockquote>semget - get set of XSI semaphores</blockquote>
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<h4><a name="tag_03_639_02"></a>SYNOPSIS</h4>
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<blockquote class="synopsis">
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<div class="box"><code><tt><sup>[<a href="javascript:open_code('XSI')">XSI</a>]</sup> <img src="../images/opt-start.gif" alt=
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"[Option Start]" border="0"> #include <<a href="../basedefs/sys/sem.h.html">sys/sem.h</a>><br>
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<br>
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int semget(key_t</tt> <i>key</i><tt>, int</tt> <i>nsems</i><tt>, int</tt> <i>semflg</i><tt>); <img src="../images/opt-end.gif"
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alt="[Option End]" border="0"></tt></code></div>
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<tt><br>
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</tt></blockquote>
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<h4><a name="tag_03_639_03"></a>DESCRIPTION</h4>
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<blockquote>
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<p>The <i>semget</i>() function operates on XSI semaphores (see the Base Definitions volume of IEEE Std 1003.1-2001, <a
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href="../basedefs/xbd_chap04.html#tag_04_15">Section 4.15, Semaphore</a>). It is unspecified whether this function interoperates
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with the realtime interprocess communication facilities defined in <a href="xsh_chap02_08.html#tag_02_08"><i>Realtime</i></a> .</p>
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<p>The <i>semget</i>() function shall return the semaphore identifier associated with <i>key</i>.</p>
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<p>A semaphore identifier with its associated <b>semid_ds</b> data structure and its associated set of <i>nsems</i> semaphores (see
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<a href="../basedefs/sys/sem.h.html"><i><sys/sem.h></i></a>) is created for <i>key</i> if one of the following is true:</p>
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<ul>
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<li>
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<p>The argument <i>key</i> is equal to IPC_PRIVATE.</p>
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</li>
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<li>
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<p>The argument <i>key</i> does not already have a semaphore identifier associated with it and (<i>semflg</i> &IPC_CREAT) is
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non-zero.</p>
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</li>
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</ul>
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<p>Upon creation, the <b>semid_ds</b> data structure associated with the new semaphore identifier is initialized as follows:</p>
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<ul>
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<li>
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<p>In the operation permissions structure <i>sem_perm.cuid</i>, <i>sem_perm.uid</i>, <i>sem_perm.cgid</i>, and <i>sem_perm.gid</i>
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shall be set equal to the effective user ID and effective group ID, respectively, of the calling process.</p>
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</li>
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<li>
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<p>The low-order 9 bits of <i>sem_perm.mode</i> shall be set equal to the low-order 9 bits of <i>semflg</i>.</p>
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</li>
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<li>
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<p>The variable <i>sem_nsems</i> shall be set equal to the value of <i>nsems</i>.</p>
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</li>
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<li>
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<p>The variable <i>sem_otime</i> shall be set equal to 0 and <i>sem_ctime</i> shall be set equal to the current time.</p>
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</li>
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<li>
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<p>The data structure associated with each semaphore in the set shall not be initialized. The <a href=
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"../functions/semctl.html"><i>semctl</i>()</a> function with the command SETVAL or SETALL can be used to initialize each
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semaphore.</p>
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</li>
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</ul>
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</blockquote>
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<h4><a name="tag_03_639_04"></a>RETURN VALUE</h4>
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<blockquote>
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<p>Upon successful completion, <i>semget</i>() shall return a non-negative integer, namely a semaphore identifier; otherwise, it
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shall return -1 and set <i>errno</i> to indicate the error.</p>
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</blockquote>
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<h4><a name="tag_03_639_05"></a>ERRORS</h4>
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<blockquote>
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<p>The <i>semget</i>() function shall fail if:</p>
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<dl compact>
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<dt>[EACCES]</dt>
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<dd>A semaphore identifier exists for <i>key</i>, but operation permission as specified by the low-order 9 bits of <i>semflg</i>
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would not be granted; see <a href="xsh_chap02_07.html#tag_02_07"><i>XSI Interprocess Communication</i></a> .</dd>
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<dt>[EEXIST]</dt>
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<dd>A semaphore identifier exists for the argument <i>key</i> but ((<i>semflg</i> &IPC_CREAT) &&(<i>semflg</i>
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&IPC_EXCL)) is non-zero.</dd>
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<dt>[EINVAL]</dt>
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<dd>The value of <i>nsems</i> is either less than or equal to 0 or greater than the system-imposed limit, or a semaphore identifier
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exists for the argument <i>key</i>, but the number of semaphores in the set associated with it is less than <i>nsems</i> and
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<i>nsems</i> is not equal to 0.</dd>
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<dt>[ENOENT]</dt>
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<dd>A semaphore identifier does not exist for the argument <i>key</i> and (<i>semflg</i> &IPC_CREAT) is equal to 0.</dd>
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<dt>[ENOSPC]</dt>
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<dd>A semaphore identifier is to be created but the system-imposed limit on the maximum number of allowed semaphores system-wide
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would be exceeded.</dd>
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</dl>
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</blockquote>
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<hr>
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<div class="box"><em>The following sections are informative.</em></div>
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<h4><a name="tag_03_639_06"></a>EXAMPLES</h4>
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<blockquote>
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<h5><a name="tag_03_639_06_01"></a>Creating a Semaphore Identifier</h5>
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<p>The following example gets a unique semaphore key using the <a href="../functions/ftok.html"><i>ftok</i>()</a> function, then
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gets a semaphore ID associated with that key using the <i>semget</i>() function (the first call also tests to make sure the
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semaphore exists). If the semaphore does not exist, the program creates it, as shown by the second call to <i>semget</i>(). In
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creating the semaphore for the queuing process, the program attempts to create one semaphore with read/write permission for all. It
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also uses the IPC_EXCL flag, which forces <i>semget</i>() to fail if the semaphore already exists.</p>
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<p>After creating the semaphore, the program uses a call to <a href="../functions/semop.html"><i>semop</i>()</a> to initialize it
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to the values in the <i>sbuf</i> array. The number of processes that can execute concurrently without queuing is initially set to
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2. The final call to <i>semget</i>() creates a semaphore identifier that can be used later in the program.</p>
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<pre>
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<tt>#include <sys/types.h>
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#include <stdio.h>
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#include <sys/ipc.h>
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#include <sys/sem.h>
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#include <sys/stat.h>
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#include <errno.h>
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#include <unistd.h>
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#include <stdlib.h>
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#include <pwd.h>
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#include <fcntl.h>
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#include <limits.h>
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...
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key_t semkey;
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int semid, pfd, fv;
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struct sembuf sbuf;
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char *lgn;
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char filename[PATH_MAX+1];
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struct stat outstat;
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struct passwd *pw;
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...
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/* Get unique key for semaphore. */
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if ((semkey = ftok("/tmp", 'a')) == (key_t) -1) {
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perror("IPC error: ftok"); exit(1);
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}
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<br>
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/* Get semaphore ID associated with this key. */
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if ((semid = semget(semkey, 0, 0)) == -1) {
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<br>
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/* Semaphore does not exist - Create. */
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if ((semid = semget(semkey, 1, IPC_CREAT | IPC_EXCL | S_IRUSR |
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S_IWUSR | S_IRGRP | S_IWGRP | S_IROTH | S_IWOTH)) != -1)
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{
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/* Initialize the semaphore. */
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sbuf.sem_num = 0;
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sbuf.sem_op = 2; /* This is the number of runs
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without queuing. */
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sbuf.sem_flg = 0;
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if (semop(semid, &sbuf, 1) == -1) {
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perror("IPC error: semop"); exit(1);
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}
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}
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else if (errno == EEXIST) {
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if ((semid = semget(semkey, 0, 0)) == -1) {
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perror("IPC error 1: semget"); exit(1);
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}
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}
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else {
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perror("IPC error 2: semget"); exit(1);
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}
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}
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...
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</tt>
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</pre>
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</blockquote>
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<h4><a name="tag_03_639_07"></a>APPLICATION USAGE</h4>
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<blockquote>
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<p>The POSIX Realtime Extension defines alternative interfaces for interprocess communication. Application developers who need to
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use IPC should design their applications so that modules using the IPC routines described in <a href=
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"xsh_chap02_07.html#tag_02_07"><i>XSI Interprocess Communication</i></a> can be easily modified to use the alternative
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interfaces.</p>
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</blockquote>
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<h4><a name="tag_03_639_08"></a>RATIONALE</h4>
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<blockquote>
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<p>None.</p>
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</blockquote>
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<h4><a name="tag_03_639_09"></a>FUTURE DIRECTIONS</h4>
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<blockquote>
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<p>None.</p>
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</blockquote>
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<h4><a name="tag_03_639_10"></a>SEE ALSO</h4>
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<blockquote>
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<p><a href="xsh_chap02_07.html#tag_02_07"><i>XSI Interprocess Communication</i></a> , <a href=
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"xsh_chap02_08.html#tag_02_08"><i>Realtime</i></a> , <a href="semctl.html"><i>semctl</i>()</a> , <a href=
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"semop.html"><i>semop</i>()</a> , <a href="sem_close.html"><i>sem_close</i>()</a> , <a href=
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"sem_destroy.html"><i>sem_destroy</i>()</a> , <a href="sem_getvalue.html"><i>sem_getvalue</i>()</a> , <a href=
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"sem_init.html"><i>sem_init</i>()</a> , <a href="sem_open.html"><i>sem_open</i>()</a> , <a href=
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"sem_post.html"><i>sem_post</i>()</a> , <a href="sem_unlink.html"><i>sem_unlink</i>()</a> , <a href=
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"sem_wait.html"><i>sem_wait</i>()</a> , the Base Definitions volume of IEEE Std 1003.1-2001, <a href=
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"../basedefs/sys/sem.h.html"><i><sys/sem.h></i></a></p>
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</blockquote>
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<h4><a name="tag_03_639_11"></a>CHANGE HISTORY</h4>
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<blockquote>
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<p>First released in Issue 2. Derived from Issue 2 of the SVID.</p>
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</blockquote>
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<h4><a name="tag_03_639_12"></a>Issue 5</h4>
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<blockquote>
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<p>The note about use of POSIX Realtime Extension IPC routines has been moved from FUTURE DIRECTIONS to a new APPLICATION USAGE
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section.</p>
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</blockquote>
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<div class="box"><em>End of informative text.</em></div>
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<hr>
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<center><font size="2"><!--footer start-->
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UNIX ® is a registered Trademark of The Open Group.<br>
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POSIX ® is a registered Trademark of The IEEE.<br>
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