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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"><html><head><meta name="generator" content="HTML Tidy, see www.w3.org"><meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1"><link type="text/css" rel="stylesheet" href="style.css"><!-- Generated by The Open Group's rhtm tool v1.2.1 --><!-- Copyright (c) 2001-2004 IEEE and The Open Group, All Rights Reserved --><title>semget</title></head><body bgcolor="white"><script type="text/javascript" language="JavaScript" src="../jscript/codes.js"></script><basefont size="3"> <a name="semget"></a> <a name="tag_03_639"></a><!-- semget --> <!--header start--><center><font size="2">The Open Group Base Specifications Issue 6<br>IEEE Std 1003.1, 2004 Edition<br>Copyright © 2001-2004 The IEEE and The Open Group, All Rights reserved.</font></center><!--header end--><hr size="2" noshade><h4><a name="tag_03_639_01"></a>NAME</h4><blockquote>semget - get set of XSI semaphores</blockquote><h4><a name="tag_03_639_02"></a>SYNOPSIS</h4><blockquote class="synopsis"><div class="box"><code><tt><sup>[<a href="javascript:open_code('XSI')">XSI</a>]</sup> <img src="../images/opt-start.gif" alt="[Option Start]" border="0"> #include <<a href="../basedefs/sys/sem.h.html">sys/sem.h</a>><br><br> 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"alt="[Option End]" border="0"></tt></code></div><tt><br></tt></blockquote><h4><a name="tag_03_639_03"></a>DESCRIPTION</h4><blockquote><p>The <i>semget</i>() function operates on XSI semaphores (see the Base Definitions volume of IEEE Std 1003.1-2001, <ahref="../basedefs/xbd_chap04.html#tag_04_15">Section 4.15, Semaphore</a>). It is unspecified whether this function interoperateswith the realtime interprocess communication facilities defined in <a href="xsh_chap02_08.html#tag_02_08"><i>Realtime</i></a>.</p><p>The <i>semget</i>() function shall return the semaphore identifier associated with <i>key</i>.</p><p>A semaphore identifier with its associated <b>semid_ds</b> data structure and its associated set of <i>nsems</i> semaphores (see<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><ul><li><p>The argument <i>key</i> is equal to IPC_PRIVATE.</p></li><li><p>The argument <i>key</i> does not already have a semaphore identifier associated with it and (<i>semflg</i> &IPC_CREAT) isnon-zero.</p></li></ul><p>Upon creation, the <b>semid_ds</b> data structure associated with the new semaphore identifier is initialized as follows:</p><ul><li><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>shall be set equal to the effective user ID and effective group ID, respectively, of the calling process.</p></li><li><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></li><li><p>The variable <i>sem_nsems</i> shall be set equal to the value of <i>nsems</i>.</p></li><li><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></li><li><p>The data structure associated with each semaphore in the set need not be initialized. The <a href="../functions/semctl.html"><i>semctl</i>()</a> function with the command SETVAL or SETALL can be used to initialize eachsemaphore.</p></li></ul></blockquote><h4><a name="tag_03_639_04"></a>RETURN VALUE</h4><blockquote><p>Upon successful completion, <i>semget</i>() shall return a non-negative integer, namely a semaphore identifier; otherwise, itshall return -1 and set <i>errno</i> to indicate the error.</p></blockquote><h4><a name="tag_03_639_05"></a>ERRORS</h4><blockquote><p>The <i>semget</i>() function shall fail if:</p><dl compact><dt>[EACCES]</dt><dd>A semaphore identifier exists for <i>key</i>, but operation permission as specified by the low-order 9 bits of <i>semflg</i>would not be granted; see <a href="xsh_chap02_07.html#tag_02_07"><i>XSI Interprocess Communication</i></a>.</dd><dt>[EEXIST]</dt><dd>A semaphore identifier exists for the argument <i>key</i> but ((<i>semflg</i> &IPC_CREAT) &&(<i>semflg</i>&IPC_EXCL)) is non-zero.</dd><dt>[EINVAL]</dt><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 identifierexists for the argument <i>key</i>, but the number of semaphores in the set associated with it is less than <i>nsems</i> and<i>nsems</i> is not equal to 0.</dd><dt>[ENOENT]</dt><dd>A semaphore identifier does not exist for the argument <i>key</i> and (<i>semflg</i> &IPC_CREAT) is equal to 0.</dd><dt>[ENOSPC]</dt><dd>A semaphore identifier is to be created but the system-imposed limit on the maximum number of allowed semaphores system-widewould be exceeded.</dd></dl></blockquote><hr><div class="box"><em>The following sections are informative.</em></div><h4><a name="tag_03_639_06"></a>EXAMPLES</h4><blockquote><h5><a name="tag_03_639_06_01"></a>Creating a Semaphore Identifier</h5><p>The following example gets a unique semaphore key using the <a href="../functions/ftok.html"><i>ftok</i>()</a> function, thengets a semaphore ID associated with that key using the <i>semget</i>() function (the first call also tests to make sure thesemaphore exists). If the semaphore does not exist, the program creates it, as shown by the second call to <i>semget</i>(). Increating the semaphore for the queuing process, the program attempts to create one semaphore with read/write permission for all. Italso uses the IPC_EXCL flag, which forces <i>semget</i>() to fail if the semaphore already exists.</p><p>After creating the semaphore, the program uses a call to <a href="../functions/semop.html"><i>semop</i>()</a> to initialize itto the values in the <i>sbuf</i> array. The number of processes that can execute concurrently without queuing is initially set to2. The final call to <i>semget</i>() creates a semaphore identifier that can be used later in the program.</p><pre><tt>#include <sys/types.h>#include <stdio.h>#include <sys/ipc.h>#include <sys/sem.h>#include <sys/stat.h>#include <errno.h>#include <unistd.h>#include <stdlib.h>#include <pwd.h>#include <fcntl.h>#include <limits.h>...key_t semkey;int semid, pfd, fv;struct sembuf sbuf;char *lgn;char filename[PATH_MAX+1];struct stat outstat;struct passwd *pw;.../* Get unique key for semaphore. */if ((semkey = ftok("/tmp", 'a')) == (key_t) -1) { perror("IPC error: ftok"); exit(1);}<br>/* Get semaphore ID associated with this key. */if ((semid = semget(semkey, 0, 0)) == -1) {<br> /* Semaphore does not exist - Create. */ if ((semid = semget(semkey, 1, IPC_CREAT | IPC_EXCL | S_IRUSR | S_IWUSR | S_IRGRP | S_IWGRP | S_IROTH | S_IWOTH)) != -1) { /* Initialize the semaphore. */ sbuf.sem_num = 0; sbuf.sem_op = 2; /* This is the number of runs without queuing. */ sbuf.sem_flg = 0; if (semop(semid, &sbuf, 1) == -1) { perror("IPC error: semop"); exit(1); } } else if (errno == EEXIST) { if ((semid = semget(semkey, 0, 0)) == -1) { perror("IPC error 1: semget"); exit(1); } } else { perror("IPC error 2: semget"); exit(1); }}...</tt></pre></blockquote><h4><a name="tag_03_639_07"></a>APPLICATION USAGE</h4><blockquote><p>The POSIX Realtime Extension defines alternative interfaces for interprocess communication. Application developers who need touse IPC should design their applications so that modules using the IPC routines described in <a href="xsh_chap02_07.html#tag_02_07"><i>XSI Interprocess Communication</i></a> can be easily modified to use the alternativeinterfaces.</p></blockquote><h4><a name="tag_03_639_08"></a>RATIONALE</h4><blockquote><p>None.</p></blockquote><h4><a name="tag_03_639_09"></a>FUTURE DIRECTIONS</h4><blockquote><p>None.</p></blockquote><h4><a name="tag_03_639_10"></a>SEE ALSO</h4><blockquote><p><a href="xsh_chap02_07.html#tag_02_07"><i>XSI Interprocess Communication</i></a>, <a href="xsh_chap02_08.html#tag_02_08"><i>Realtime</i></a>, <a href="semctl.html"><i>semctl</i>()</a>, <a href="semop.html"><i>semop</i>()</a>, <a href="sem_close.html"><i>sem_close</i>()</a>, <a href="sem_destroy.html"><i>sem_destroy</i>()</a>, <a href="sem_getvalue.html"><i>sem_getvalue</i>()</a>, <a href="sem_init.html"><i>sem_init</i>()</a>, <a href="sem_open.html"><i>sem_open</i>()</a>, <a href="sem_post.html"><i>sem_post</i>()</a>, <a href="sem_unlink.html"><i>sem_unlink</i>()</a>, <a href="sem_wait.html"><i>sem_wait</i>()</a>, the Base Definitions volume of IEEE Std 1003.1-2001, <a href="../basedefs/sys/sem.h.html"><i><sys/sem.h></i></a></p></blockquote><h4><a name="tag_03_639_11"></a>CHANGE HISTORY</h4><blockquote><p>First released in Issue 2. Derived from Issue 2 of the SVID.</p></blockquote><h4><a name="tag_03_639_12"></a>Issue 5</h4><blockquote><p>The note about use of POSIX Realtime Extension IPC routines has been moved from FUTURE DIRECTIONS to a new APPLICATION USAGEsection.</p></blockquote><h4><a name="tag_03_639_13"></a>Issue 6</h4><blockquote><p>IEEE Std 1003.1-2001/Cor 2-2004, item XSH/TC2/D6/122 is applied, updating the DESCRIPTION from ``each semaphorein the set shall not be initialized'' to ``each semaphore in the set need not be initialized''.</p></blockquote><div class="box"><em>End of informative text.</em></div><hr size="2" noshade><center><font size="2"><!--footer start-->UNIX ® is a registered Trademark of The Open Group.<br>POSIX ® is a registered Trademark of The IEEE.<br>[ <a href="../mindex.html">Main Index</a> | <a href="../basedefs/contents.html">XBD</a> | <a href="../utilities/contents.html">XCU</a> | <a href="../functions/contents.html">XSH</a> | <a href="../xrat/contents.html">XRAT</a>]</font></center><!--footer end--><hr size="2" noshade></body></html>⌨️ 快捷键说明
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