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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 3.2 Final//EN"><html><head><!-- Copyright 1997 The Open Group, All Rights Reserved --><title>sem_open</title></head><body bgcolor=white><center><font size=2>The Single UNIX ® Specification, Version 2<br>Copyright © 1997 The Open Group</font></center><hr size=2 noshade><h4><a name = "tag_000_008_306"> </a>NAME</h4><blockquote>sem_open - initialise and open a named semaphore(<b>REALTIME</b>)</blockquote><h4><a name = "tag_000_008_307"> </a>SYNOPSIS</h4><blockquote><pre><code>#include <<a href="semaphore.h.html">semaphore.h</a>>sem_t *sem_open(const char *<i>name</i>, int <i>oflag</i>, ...);</code></pre></blockquote><h4><a name = "tag_000_008_308"> </a>DESCRIPTION</h4><blockquote>The<i>sem_open()</i>function establishes a connection between a named semaphore and a process.Following a call to<i>sem_open()</i>with semaphore name<i>name</i>,the process may reference the semaphore associated with<i>name</i>using the address returned from the call.This semaphore may be used in subsequent calls to<i><a href="sem_wait.html">sem_wait()</a></i>,<i><a href="sem_trywait.html">sem_trywait()</a></i>,<i><a href="sem_post.html">sem_post()</a></i>,and<i><a href="sem_close.html">sem_close()</a></i>.The semaphore remains usable by this process until the semaphore is closedby a successful call to<i><a href="sem_close.html">sem_close()</a></i>,<i><a href="_exit.html">_exit()</a></i>,or one of the<i>exec</i>functions.<p>The<i>oflag</i>argument controls whether the semaphore is createdor merely accessed by the call to<i>sem_open()</i>.The following flag bits may be set in<i>oflag</i>:<dl compact><dt>O_CREAT<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, thenO_CREAT has no effect, except as noted under O_EXCL.Otherwise,<i>sem_open()</i>creates a named semaphore.The O_CREAT flag 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<b>unsigned int</b>.The semaphore is created with an initial value of<i>value</i>.Valid initial values for semaphores are less than or equal toSEM_VALUE_MAX.The user ID of the semaphore is setto the effective user ID of the process;the group ID of the semaphore is setto a system 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<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 permission bits are specified, the effect is unspecified.After the semaphore named<i>name</i>has been created by<i>sem_open()</i>with the O_CREAT flag, other processes can connect to the semaphore by calling<i>sem_open()</i>with the same value of<i>name</i>.<dt>O_EXCL<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 semaphoreand the creation of the semaphore if it does not existare atomic with respect to other processes executing<i>sem_open()</i>with O_EXCL and O_CREAT set.If O_EXCL is set and O_CREATis not set, the effect is undefined.If flags other than O_CREAT and O_EXCLare specified in the<i>oflag</i>parameter, the effect is unspecified.</dl><p>The<i>name</i>argument points to a string naming a semaphore object.It is unspecified whether the name appears in the file systemand is visible to functions that take pathnames as arguments.The<i>name</i>argument conforms to the construction rules for a pathname.If<i>name</i>begins with the slash character, then processes calling<i>sem_open()</i>with the same value of<i>name</i>will refer to the same semaphore object,as long as that name has not been removed.If<i>name</i>does not begin with the slash character,the effect is implementation-dependent.The interpretation of slashcharacters other than the leading slash character in<i>name</i>is implementation-dependent.<p>If a process makes multiple successful calls to<i>sem_open()</i>with the same value for<i>name</i>,the same semaphore address is returned for each such successful call,provided that there have been no calls to<i><a href="sem_unlink.html">sem_unlink()</a></i>for this semaphore.<p>References to copies of the semaphoreproduce undefined results.</blockquote><h4><a name = "tag_000_008_309"> </a>RETURN VALUE</h4><blockquote>Upon successful completion,the function returns the address of the semaphore.Otherwise, it will return a value of SEM_FAILED and set<i>errno</i>to indicate the error. The symbol SEM_FAILED is defined in the header<i><a href="semaphore.h.html"><semaphore.h></a></i>.No successful return from<i>sem_open()</i>will return the value SEM_FAILED.</blockquote><h4><a name = "tag_000_008_310"> </a>ERRORS</h4><blockquote>If any of the following conditions occur, the <i>sem_open()</i>function will return SEM_FAILED and set<i>errno</i>to the corresponding value:<dl compact><dt>[EACCES]<dd>The named semaphoreexists and the permissions specified by<i>oflag</i>are denied, orthe named semaphoredoes not exist and permission to createthe named semaphoreis denied.<dt>[EEXIST]<dd>O_CREAT and O_EXCL are set and the named semaphorealready exists.<dt>[EINTR]<dd>The<i>sem_open()</i>operation was interrupted by a signal.<dt>[EINVAL]<dd>The<i>sem_open()</i>operation is not supported for the given name, orO_CREAT was specified in<i>oflag</i>and<i>value</i>was greater than SEM_VALUE_MAX.<dt>[EMFILE]<dd>Too many semaphore descriptors orfile descriptors are currently in use by this process.<dt>[ENAMETOOLONG]<dd>The length of the<i>name</i>string exceeds PATH_MAX,or a pathname component is longer thanNAME_MAX while _POSIX_NO_TRUNC is in effect.<dt>[ENFILE]<dd>Too many semaphores are currently open in the system.<dt>[ENOENT]<dd>O_CREAT is not set and the named semaphoredoes not exist.<dt>[ENOSPC]<dd>There is insufficient space for the creation of the newnamed semaphore.<dt>[ENOSYS]<dd>The function<i>sem_open()</i>is not supported by this implementation.</dl></blockquote><h4><a name = "tag_000_008_311"> </a>EXAMPLES</h4><blockquote>None.</blockquote><h4><a name = "tag_000_008_312"> </a>APPLICATION USAGE</h4><blockquote>None.</blockquote><h4><a name = "tag_000_008_313"> </a>FUTURE DIRECTIONS</h4><blockquote>None.</blockquote><h4><a name = "tag_000_008_314"> </a>SEE ALSO</h4><blockquote><i><a href="semctl.html">semctl()</a></i>,<i><a href="semget.html">semget()</a></i>,<i><a href="semop.html">semop()</a></i>,<i><a href="sem_close.html">sem_close()</a></i>,<i><a href="sem_post.html">sem_post()</a></i>,<i><a href="sem_trywait.html">sem_trywait()</a></i>,<i><a href="sem_unlink.html">sem_unlink()</a></i>,<i><a href="sem_wait.html">sem_wait()</a></i>,<i><a href="semaphore.h.html"><semaphore.h></a></i>.</blockquote><h4>DERIVATION</h4><blockquote>Derived from the POSIX Realtime Extension (1003.1b-1993/1003.1i-1995)</blockquote><hr size=2 noshade><center><font size=2>UNIX ® is a registered Trademark of The Open Group.<br>Copyright © 1997 The Open Group<br> [ <a href="../index.html">Main Index</a> | <a href="../xshix.html">XSH</a> | <a href="../xcuix.html">XCU</a> | <a href="../xbdix.html">XBD</a> | <a href="../cursesix.html">XCURSES</a> | <a href="../xnsix.html">XNS</a> ]</font></center><hr size=2 noshade></body></html>⌨️ 快捷键说明
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