semop.html

IEEE 1003.1-2003, Single Unix Specification v3

<dl compact>
<dt>[E2BIG]</dt>

<dd>The value of <i>nsops</i> is greater than the system-imposed maximum.</dd>

<dt>[EACCES]</dt>

<dd>Operation permission is denied to the calling process; see <a href="xsh_chap02_07.html#tag_02_07"><i>XSI Interprocess
Communication</i></a> .</dd>

<dt>[EAGAIN]</dt>

<dd>The operation would result in suspension of the calling process but (<i>sem_flg</i> &amp;IPC_NOWAIT) is non-zero.</dd>

<dt>[EFBIG]</dt>

<dd>The value of <i>sem_num</i> is less than 0 or greater than or equal to the number of semaphores in the set associated with
<i>semid</i>.</dd>

<dt>[EIDRM]</dt>

<dd>The semaphore identifier <i>semid</i> is removed from the system.</dd>

<dt>[EINTR]</dt>

<dd>The <i>semop</i>() function was interrupted by a signal.</dd>

<dt>[EINVAL]</dt>

<dd>The value of <i>semid</i> is not a valid semaphore identifier, or the number of individual semaphores for which the calling
process requests a SEM_UNDO would exceed the system-imposed limit.</dd>

<dt>[ENOSPC]</dt>

<dd>The limit on the number of individual processes requesting a SEM_UNDO would be exceeded.</dd>

<dt>[ERANGE]</dt>

<dd>An operation would cause a <i>semval</i> to overflow the system-imposed limit, or an operation would cause a <i>semadj</i>
value to overflow the system-imposed limit.</dd>
</dl>
</blockquote>

<hr>
<div class="box"><em>The following sections are informative.</em></div>

<h4><a name="tag_03_640_06"></a>EXAMPLES</h4>

<blockquote>
<h5><a name="tag_03_640_06_01"></a>Setting Values in Semaphores</h5>

<p>The following example sets the values of the two semaphores associated with the <i>semid</i> identifier to the values contained
in the <i>sb</i> array.</p>

<pre>
<tt>#include &lt;sys/sem.h&gt;
...
int semid;
struct sembuf sb[2];
int nsops = 2;
int result;
<br>
/* Adjust value of semaphore in the semaphore array semid. */
sb[0].sem_num = 0;
sb[0].sem_op = -1;
sb[0].sem_flg = SEM_UNDO | IPC_NOWAIT;
sb[1].sem_num = 1;
sb[1].sem_op =  1;
sb[1].sem_flg = 0;
<br>
result = semop(semid, sb, nsops);
</tt>
</pre>

<h5><a name="tag_03_640_06_02"></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, then
gets a semaphore ID associated with that key using the <a href="../functions/semget.html"><i>semget</i>()</a> function (the first
call also tests to make sure the semaphore exists). If the semaphore does not exist, the program creates it, as shown by the second
call to <a href="../functions/semget.html"><i>semget</i>()</a>. In creating the semaphore for the queuing process, the program
attempts to create one semaphore with read/write permission for all. It also uses the IPC_EXCL flag, which forces <a href=
"../functions/semget.html"><i>semget</i>()</a> to fail if the semaphore already exists.</p>

<p>After creating the semaphore, the program uses a call to <i>semop</i>() to initialize it to the values in the <i>sbuf</i> array.
The number of processes that can execute concurrently without queuing is initially set to 2. The final call to <a href=
"../functions/semget.html"><i>semget</i>()</a> creates a semaphore identifier that can be used later in the program.</p>

<p>The final call to <i>semop</i>() acquires the semaphore and waits until it is free; the SEM_UNDO option releases the semaphore
when the process exits, waiting until there are less than two processes running concurrently.</p>

<pre>
<tt>#include &lt;sys/types.h&gt;
#include &lt;stdio.h&gt;
#include &lt;sys/ipc.h&gt;
#include &lt;sys/sem.h&gt;
#include &lt;sys/stat.h&gt;
#include &lt;errno.h&gt;
#include &lt;unistd.h&gt;
#include &lt;stdlib.h&gt;
#include &lt;pwd.h&gt;
#include &lt;fcntl.h&gt;
#include &lt;limits.h&gt;
...
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, &amp;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);
    }
}
...
sbuf.sem_num = 0;
sbuf.sem_op = -1;
sbuf.sem_flg = SEM_UNDO;
if (semop(semid, &amp;sbuf, 1) == -1) {
    perror("IPC Error: semop"); exit(1);
}
</tt>
</pre>
</blockquote>

<h4><a name="tag_03_640_07"></a>APPLICATION USAGE</h4>

<blockquote>
<p>The POSIX Realtime Extension defines alternative interfaces for interprocess communication. Application developers who need to
use 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 alternative
interfaces.</p>
</blockquote>

<h4><a name="tag_03_640_08"></a>RATIONALE</h4>

<blockquote>
<p>None.</p>
</blockquote>

<h4><a name="tag_03_640_09"></a>FUTURE DIRECTIONS</h4>

<blockquote>
<p>None.</p>
</blockquote>

<h4><a name="tag_03_640_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="exec.html"><i><a href="../functions/exec.html">exec</a></i>()</a> ,
<a href="exit.html"><i>exit</i>()</a> , <a href="fork.html"><i>fork</i>()</a> , <a href="semctl.html"><i>semctl</i>()</a> , <a
href="semget.html"><i>semget</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&nbsp;Std&nbsp;1003.1-2001, <a href=
"../basedefs/sys/ipc.h.html"><i>&lt;sys/ipc.h&gt;</i></a>, <a href="../basedefs/sys/sem.h.html"><i>&lt;sys/sem.h&gt;</i></a>, <a
href="../basedefs/sys/types.h.html"><i>&lt;sys/types.h&gt;</i></a></p>
</blockquote>

<h4><a name="tag_03_640_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_640_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 USAGE
section.</p>
</blockquote>

<div class="box"><em>End of informative text.</em></div>

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