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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) 2001-2003 The Open Group, All Rights Reserved --><title>lockf</title></head><body bgcolor="white"><script type="text/javascript" language="JavaScript" src="../jscript/codes.js"></script><basefont size="3"> <a name="lockf"></a> <a name="tag_03_342"></a><!-- lockf --> <!--header start--><center><font size="2">The Open Group Base Specifications Issue 6<br>IEEE Std 1003.1, 2003 Edition<br>Copyright © 2001-2003 The IEEE and The Open Group, All Rights reserved.</font></center><!--header end--><hr size="2" noshade><h4><a name="tag_03_342_01"></a>NAME</h4><blockquote>lockf - record locking on files</blockquote><h4><a name="tag_03_342_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/unistd.h.html">unistd.h</a>><br><br> int lockf(int</tt> <i>fildes</i><tt>, int</tt> <i>function</i><tt>, off_t</tt> <i>size</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_342_03"></a>DESCRIPTION</h4><blockquote><p>The <i>lockf</i>() function shall lock sections of a file with advisory-mode locks. Calls to <i>lockf</i>() from other threadswhich attempt to lock the locked file section shall either return an error value or block until the section becomes unlocked. Allthe locks for a process are removed when the process terminates. Record locking with <i>lockf</i>() shall be supported for regularfiles and may be supported for other files.</p><p>The <i>fildes</i> argument is an open file descriptor. To establish a lock with this function, the file descriptor shall beopened with write-only permission (O_WRONLY) or with read/write permission (O_RDWR).</p><p>The <i>function</i> argument is a control value which specifies the action to be taken. The permissible values for<i>function</i> are defined in <a href="../basedefs/unistd.h.html"><i><unistd.h></i></a> as follows:</p><center><table border="1" cellpadding="3" align="center"><tr valign="top"><th align="center"><p class="tent"><b>Function</b></p></th><th align="center"><p class="tent"><b>Description</b></p></th></tr><tr valign="top"><td align="left"><p class="tent">F_ULOCK</p></td><td align="left"><p class="tent">Unlock locked sections.</p></td></tr><tr valign="top"><td align="left"><p class="tent">F_LOCK</p></td><td align="left"><p class="tent">Lock a section for exclusive use.</p></td></tr><tr valign="top"><td align="left"><p class="tent">F_TLOCK</p></td><td align="left"><p class="tent">Test and lock a section for exclusive use.</p></td></tr><tr valign="top"><td align="left"><p class="tent">F_TEST</p></td><td align="left"><p class="tent">Test a section for locks by other processes.</p></td></tr></table></center><p>F_TEST shall detect if a lock by another process is present on the specified section.</p><p>F_LOCK and F_TLOCK shall both lock a section of a file if the section is available.</p><p>F_ULOCK shall remove locks from a section of the file.</p><p>The <i>size</i> argument is the number of contiguous bytes to be locked or unlocked. The section to be locked or unlocked startsat the current offset in the file and extends forward for a positive size or backward for a negative size (the preceding bytes upto but not including the current offset). If <i>size</i> is 0, the section from the current offset through the largest possiblefile offset shall be locked (that is, from the current offset through the present or any future end-of-file). An area need not beallocated to the file to be locked because locks may exist past the end-of-file.</p><p>The sections locked with F_LOCK or F_TLOCK may, in whole or in part, contain or be contained by a previously locked section forthe same process. When this occurs, or if adjacent locked sections would occur, the sections shall be combined into a single lockedsection. If the request would cause the number of locks to exceed a system-imposed limit, the request shall fail.</p><p>F_LOCK and F_TLOCK requests differ only by the action taken if the section is not available. F_LOCK shall block the callingthread until the section is available. F_TLOCK shall cause the function to fail if the section is already locked by anotherprocess.</p><p>File locks shall be released on first close by the locking process of any file descriptor for the file.</p><p>F_ULOCK requests may release (wholly or in part) one or more locked sections controlled by the process. Locked sections shall beunlocked starting at the current file offset through <i>size</i> bytes or to the end-of-file if <i>size</i> is (<b>off_t</b>)0.When all of a locked section is not released (that is, when the beginning or end of the area to be unlocked falls within a lockedsection), the remaining portions of that section shall remain locked by the process. Releasing the center portion of a lockedsection shall cause the remaining locked beginning and end portions to become two separate locked sections. If the request wouldcause the number of locks in the system to exceed a system-imposed limit, the request shall fail.</p><p>A potential for deadlock occurs if the threads of a process controlling a locked section are blocked by accessing anotherprocess' locked section. If the system detects that deadlock would occur, <i>lockf</i>() shall fail with an [EDEADLK] error.</p><p>The interaction between <a href="../functions/fcntl.html"><i>fcntl</i>()</a> and <i>lockf</i>() locks is unspecified.</p><p>Blocking on a section shall be interrupted by any signal.</p><p>An F_ULOCK request in which <i>size</i> is non-zero and the offset of the last byte of the requested section is the maximumvalue for an object of type <b>off_t</b>, when the process has an existing lock in which <i>size</i> is 0 and which includes thelast byte of the requested section, shall be treated as a request to unlock from the start of the requested section with a sizeequal to 0. Otherwise, an F_ULOCK request shall attempt to unlock only the requested section.</p><p>Attempting to lock a section of a file that is associated with a buffered stream produces unspecified results.</p></blockquote><h4><a name="tag_03_342_04"></a>RETURN VALUE</h4><blockquote><p>Upon successful completion, <i>lockf</i>() shall return 0. Otherwise, it shall return -1, set <i>errno</i> to indicate an error,and existing locks shall not be changed.</p></blockquote><h4><a name="tag_03_342_05"></a>ERRORS</h4><blockquote><p>The <i>lockf</i>() function shall fail if:</p><dl compact><dt>[EBADF]</dt><dd>The <i>fildes</i> argument is not a valid open file descriptor; or <i>function</i> is F_LOCK or F_TLOCK and <i>fildes</i> isnot a valid file descriptor open for writing.</dd><dt>[EACCES] or [EAGAIN]</dt><dd><br>The <i>function</i> argument is F_TLOCK or F_TEST and the section is already locked by another process.</dd><dt>[EDEADLK]</dt><dd>The <i>function</i> argument is F_LOCK and a deadlock is detected.</dd><dt>[EINTR]</dt><dd>A signal was caught during execution of the function.</dd><dt>[EINVAL]</dt><dd>The <i>function</i> argument is not one of F_LOCK, F_TLOCK, F_TEST, or F_ULOCK; or <i>size</i> plus the current file offset isless than 0.</dd><dt>[EOVERFLOW]</dt><dd>The offset of the first, or if <i>size</i> is not 0 then the last, byte in the requested section cannot be representedcorrectly in an object of type <b>off_t</b>.</dd></dl><p>The <i>lockf</i>() function may fail if:</p><dl compact><dt>[EAGAIN]</dt><dd>The <i>function</i> argument is F_LOCK or F_TLOCK and the file is mapped with <a href="../functions/mmap.html"><i>mmap</i>()</a>.</dd><dt>[EDEADLK] or [ENOLCK]</dt><dd><br>The <i>function</i> argument is F_LOCK, F_TLOCK, or F_ULOCK, and the request would cause the number of locks to exceed asystem-imposed limit.</dd><dt>[EOPNOTSUPP] or [EINVAL]</dt><dd><br>The implementation does not support the locking of files of the type indicated by the <i>fildes</i> argument.</dd></dl></blockquote><hr><div class="box"><em>The following sections are informative.</em></div><h4><a name="tag_03_342_06"></a>EXAMPLES</h4><blockquote><h5><a name="tag_03_342_06_01"></a>Locking a Portion of a File</h5><p>In the following example, a file named <b>/home/cnd/mod1</b> is being modified. Other processes that use locking are preventedfrom changing it during this process. Only the first 10000 bytes are locked, and the lock call fails if another process has anypart of this area locked already.</p><pre><tt>#include <fcntl.h>#include <unistd.h><br>int fildes;int status;...fildes = open("/home/cnd/mod1", O_RDWR);status = lockf(fildes, F_TLOCK, (off_t)10000);</tt></pre></blockquote><h4><a name="tag_03_342_07"></a>APPLICATION USAGE</h4><blockquote><p>Record-locking should not be used in combination with the <a href="../functions/fopen.html"><i>fopen</i>()</a>, <a href="../functions/fread.html"><i>fread</i>()</a>, <a href="../functions/fwrite.html"><i>fwrite</i>()</a>, and other <i>stdio</i>functions. Instead, the more primitive, non-buffered functions (such as <a href="../functions/open.html"><i>open</i>()</a>) shouldbe used. Unexpected results may occur in processes that do buffering in the user address space. The process may later read/writedata which is/was locked. The <i>stdio</i> functions are the most common source of unexpected buffering.</p><p>The <a href="../functions/alarm.html"><i>alarm</i>()</a> function may be used to provide a timeout facility in applicationsrequiring it.</p></blockquote><h4><a name="tag_03_342_08"></a>RATIONALE</h4><blockquote><p>None.</p></blockquote><h4><a name="tag_03_342_09"></a>FUTURE DIRECTIONS</h4><blockquote><p>None.</p></blockquote><h4><a name="tag_03_342_10"></a>SEE ALSO</h4><blockquote><p><a href="alarm.html"><i>alarm</i>()</a> , <a href="chmod.html"><i>chmod</i>()</a> , <a href="close.html"><i>close</i>()</a> , <ahref="creat.html"><i>creat</i>()</a> , <a href="fcntl.html"><i>fcntl</i>()</a> , <a href="fopen.html"><i>fopen</i>()</a> , <a href="mmap.html"><i>mmap</i>()</a> , <a href="open.html"><i>open</i>()</a> , <a href="read.html"><i>read</i>()</a> , <a href="write.html"><i>write</i>()</a> , the Base Definitions volume of IEEE Std 1003.1-2001, <a href="../basedefs/unistd.h.html"><i><unistd.h></i></a></p></blockquote><h4><a name="tag_03_342_11"></a>CHANGE HISTORY</h4><blockquote><p>First released in Issue 4, Version 2.</p></blockquote><h4><a name="tag_03_342_12"></a>Issue 5</h4><blockquote><p>Moved from X/OPEN UNIX extension to BASE.</p><p>Large File Summit extensions are added. In particular, the description of [EINVAL] is clarified and moved from optional tomandatory status.</p><p>A note is added to the DESCRIPTION indicating the effects of attempting to lock a section of a file that is associated with abuffered stream.</p></blockquote><h4><a name="tag_03_342_13"></a>Issue 6</h4><blockquote><p>The DESCRIPTION is updated to avoid use of the term "must" for application requirements.</p></blockquote><div class="box"><em>End of informative text.</em></div><hr><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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