225 lines
9.8 KiB
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225 lines
9.8 KiB
HTML
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<title>sem_open</title>
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<basefont size="3"> <a name="sem_open"></a> <a name="tag_03_632"></a><!-- sem_open -->
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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_632_01"></a>NAME</h4>
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<blockquote>sem_open - initialize and open a named semaphore (<b>REALTIME</b>)</blockquote>
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<h4><a name="tag_03_632_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('SEM')">SEM</a>]</sup> <img src="../images/opt-start.gif" alt=
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"[Option Start]" border="0"> #include <<a href="../basedefs/semaphore.h.html">semaphore.h</a>><br>
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<br>
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sem_t *sem_open(const char *</tt><i>name</i><tt>, int</tt> <i>oflag</i><tt>, ...); <img src="../images/opt-end.gif" alt=
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"[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_632_03"></a>DESCRIPTION</h4>
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<blockquote>
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<p>The <i>sem_open</i>() function shall establish a connection between a named semaphore and a process. Following a call to
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<i>sem_open</i>() with semaphore name <i>name</i>, the process may reference the semaphore associated with <i>name</i> using the
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address returned from the call. This semaphore may be used in subsequent calls to <a href=
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"../functions/sem_wait.html"><i>sem_wait</i>()</a>, <a href="../functions/sem_trywait.html"><i>sem_trywait</i>()</a>, <a href=
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"../functions/sem_post.html"><i>sem_post</i>()</a>, and <a href="../functions/sem_close.html"><i>sem_close</i>()</a>. The semaphore
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remains usable by this process until the semaphore is closed by a successful call to <a href=
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"../functions/sem_close.html"><i>sem_close</i>()</a>, <a href="../functions/_exit.html"><i>_exit</i>()</a>, or one of the <i><a
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href="../functions/exec.html">exec</a></i> functions.</p>
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<p>The <i>oflag</i> argument controls whether the semaphore is created or merely accessed by the call to <i>sem_open</i>(). The
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following flag bits may be set in <i>oflag</i>:</p>
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<dl compact>
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<dt>O_CREAT</dt>
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<dd>This flag is used to create a semaphore if it does not already exist. If O_CREAT is set and the semaphore already exists, then
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O_CREAT has no effect, except as noted under O_EXCL. Otherwise, <i>sem_open</i>() creates a named semaphore. The O_CREAT flag
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requires a third and a fourth argument: <i>mode</i>, which is of type <b>mode_t</b>, and <i>value</i>, which is of type
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<b>unsigned</b>. The semaphore is created with an initial value of <i>value</i>. Valid initial values for semaphores are less than
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or equal to {SEM_VALUE_MAX}.
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<p>The user ID of the semaphore is set to the effective user ID of the process; the group ID of the semaphore is set to a system
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default group ID or to the effective group ID of the process. The permission bits of the semaphore are set to the value of the
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<i>mode</i> argument except those set in the file mode creation mask of the process. When bits in <i>mode</i> other than the file
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permission bits are specified, the effect is unspecified.</p>
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<p>After the semaphore named <i>name</i> has been created by <i>sem_open</i>() with the O_CREAT flag, other processes can connect
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to the semaphore by calling <i>sem_open</i>() with the same value of <i>name</i>.</p>
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</dd>
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<dt>O_EXCL</dt>
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<dd>If O_EXCL and O_CREAT are set, <i>sem_open</i>() fails if the semaphore <i>name</i> exists. The check for the existence of the
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semaphore and the creation of the semaphore if it does not exist are atomic with respect to other processes executing
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<i>sem_open</i>() with O_EXCL and O_CREAT set. If O_EXCL is set and O_CREAT is not set, the effect is undefined.
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<p>If flags other than O_CREAT and O_EXCL are specified in the <i>oflag</i> parameter, the effect is unspecified.</p>
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</dd>
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</dl>
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<p>The <i>name</i> argument points to a string naming a semaphore object. It is unspecified whether the name appears in the file
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system and is visible to functions that take pathnames as arguments. The <i>name</i> argument conforms to the construction rules
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for a pathname. If <i>name</i> begins with the slash character, then processes calling <i>sem_open</i>() with the same value of
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<i>name</i> shall refer to the same semaphore object, as long as that name has not been removed. If <i>name</i> does not begin with
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the slash character, the effect is implementation-defined. The interpretation of slash characters other than the leading slash
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character in <i>name</i> is implementation-defined.</p>
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<p>If a process makes multiple successful calls to <i>sem_open</i>() with the same value for <i>name</i>, the same semaphore
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address shall be returned for each such successful call, provided that there have been no calls to <a href=
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"../functions/sem_unlink.html"><i>sem_unlink</i>()</a> for this semaphore.</p>
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<p>References to copies of the semaphore produce undefined results.</p>
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</blockquote>
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<h4><a name="tag_03_632_04"></a>RETURN VALUE</h4>
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<blockquote>
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<p>Upon successful completion, the <i>sem_open</i>() function shall return the address of the semaphore. Otherwise, it shall return
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a value of SEM_FAILED and set <i>errno</i> to indicate the error. The symbol SEM_FAILED is defined in the <a href=
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"../basedefs/semaphore.h.html"><i><semaphore.h></i></a> header. No successful return from <i>sem_open</i>() shall return the
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value SEM_FAILED.</p>
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</blockquote>
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<h4><a name="tag_03_632_05"></a>ERRORS</h4>
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<blockquote>
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<p>If any of the following conditions occur, the <i>sem_open</i>() function shall return SEM_FAILED and set <i>errno</i> to the
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corresponding value:</p>
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<dl compact>
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<dt>[EACCES]</dt>
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<dd>The named semaphore exists and the permissions specified by <i>oflag</i> are denied, or the named semaphore does not exist and
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permission to create the named semaphore is denied.</dd>
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<dt>[EEXIST]</dt>
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<dd>O_CREAT and O_EXCL are set and the named semaphore already exists.</dd>
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<dt>[EINTR]</dt>
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<dd>The <i>sem_open</i>() operation was interrupted by a signal.</dd>
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<dt>[EINVAL]</dt>
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<dd>The <i>sem_open</i>() operation is not supported for the given name, or O_CREAT was specified in <i>oflag</i> and <i>value</i>
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was greater than {SEM_VALUE_MAX}.</dd>
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<dt>[EMFILE]</dt>
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<dd>Too many semaphore descriptors or file descriptors are currently in use by this process.</dd>
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<dt>[ENAMETOOLONG]</dt>
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<dd>
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The length of the <i>name</i> argument exceeds {PATH_MAX} or a pathname component is longer than {NAME_MAX}.</dd>
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<dt>[ENFILE]</dt>
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<dd>Too many semaphores are currently open in the system.</dd>
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<dt>[ENOENT]</dt>
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<dd>O_CREAT is not set and the named semaphore does not exist.</dd>
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<dt>[ENOSPC]</dt>
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<dd>There is insufficient space for the creation of the new named semaphore.</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_632_06"></a>EXAMPLES</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_632_07"></a>APPLICATION USAGE</h4>
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<blockquote>
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<p>The <i>sem_open</i>() function is part of the Semaphores option and need not be available on all implementations.</p>
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</blockquote>
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<h4><a name="tag_03_632_08"></a>RATIONALE</h4>
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<blockquote>
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<p>Early drafts required an error return value of -1 with the type <b>sem_t *</b> for the <i>sem_open</i>() function, which is not
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guaranteed to be portable across implementations. The revised text provides the symbolic error code SEM_FAILED to eliminate the
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type conflict.</p>
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</blockquote>
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<h4><a name="tag_03_632_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_632_10"></a>SEE ALSO</h4>
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<blockquote>
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<p><a href="semctl.html"><i>semctl</i>()</a> , <a href="semget.html"><i>semget</i>()</a> , <a href="semop.html"><i>semop</i>()</a>
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, <a href="sem_close.html"><i>sem_close</i>()</a> , <a href="sem_post.html"><i>sem_post</i>()</a> , <a href=
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"sem_timedwait.html"><i>sem_timedwait</i>()</a> , <a href="sem_trywait.html"><i>sem_trywait</i>()</a> , <a href=
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"sem_unlink.html"><i>sem_unlink</i>()</a> , <a href="sem_wait.html"><i>sem_wait</i>()</a> , the Base Definitions volume of
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IEEE Std 1003.1-2001, <a href="../basedefs/semaphore.h.html"><i><semaphore.h></i></a></p>
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</blockquote>
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<h4><a name="tag_03_632_11"></a>CHANGE HISTORY</h4>
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<blockquote>
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<p>First released in Issue 5. Included for alignment with the POSIX Realtime Extension.</p>
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</blockquote>
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<h4><a name="tag_03_632_12"></a>Issue 6</h4>
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<blockquote>
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<p>The <i>sem_open</i>() function is marked as part of the Semaphores option.</p>
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<p>The [ENOSYS] error condition has been removed as stubs need not be provided if an implementation does not support the Semaphores
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option.</p>
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<p>The <a href="../functions/sem_timedwait.html"><i>sem_timedwait</i>()</a> function is added to the SEE ALSO section for alignment
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with IEEE Std 1003.1d-1999.</p>
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</blockquote>
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<div class="box"><em>End of informative text.</em></div>
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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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