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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>lockf</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_007_399"> </a>NAME</h4><blockquote>lockf - record locking on files</blockquote><h4><a name = "tag_000_007_400"> </a>SYNOPSIS</h4><blockquote><pre><code>#include <<a href="unistd.h.html">unistd.h</a>>int lockf(int <i>fildes</i>, int <i>function</i>, off_t <i>size</i>);</code></pre></blockquote><h4><a name = "tag_000_007_401"> </a>DESCRIPTION</h4><blockquote>The<i>lockf()</i>function allows sections of a file to be locked with advisory-modelocks. Calls to<i>lockf()</i>from other threads which attempt to lock the locked file section will eitherreturn an error value or block until the section becomes unlocked. All thelocks for a process are removed when the process terminates.Record locking with<i>lockf()</i>is supported for regular files and may be supported for other files.<p>The <i>fildes</i> argument is an open file descriptor. The file descriptormust have been opened with write-only permission (O_WRONLY) or with read/writepermission (O_RDWR) to establish a lock with this function.<p>The <i>function</i> argument is a control value which specifies the action tobe taken. The permissible values for <i>function</i> are defined in<i><a href="unistd.h.html"><unistd.h></a></i>as follows:<p><table bordercolor=#000000 border=1 align=center><tr valign=top><th align=center><b>Function</b><th align=center><b>Description</b><tr valign=top><td align=left>F_ULOCK<td align=left>unlock locked sections<tr valign=top><td align=left>F_LOCK<td align=left>lock a section for exclusive use<tr valign=top><td align=left>F_TLOCK<td align=left>test and lock a section for exclusive use<tr valign=top><td align=left>F_TEST<td align=left>test a section for locks by other processes</table><p>F_TEST detects if a lock by another process is present on the specifiedsection; F_LOCK and F_TLOCK both lock a section of a file if the section isavailable; F_ULOCK removes locks from a section of the file.<p>The <i>size</i> argument is the number of contiguous bytes to be lockedor unlocked.The section to be locked or unlocked starts at the current offset in the fileand extends forward for a positive size or backward for a negative size (thepreceding bytes up to but not including the current offset).If <i>size</i> is 0, the section from the current offset through the largestpossible file offset is locked (that is, from the current offset through thepresent or any future end-of-file). An area need not be allocated to the fileto be locked because locks may exist past the end-of-file.<p>The sections locked with F_LOCK or F_TLOCK may, in whole or in part, containor be contained by a previously locked section for the same process. Whenthis occurs, or if adjacent locked sections would occur, the sections arecombined into a single locked section. If the request would cause the numberof locks to exceed a system-imposed limit, the request will fail.<p>F_LOCK and F_TLOCK requests differ only by the action taken if the section isnot available. F_LOCK blocks the calling thread until the section isavailable. F_TLOCK makes the function fail if the section is already lockedby another process.<p>File locks are released on first close by the locking process of anyfile descriptor for the file.<p>F_ULOCK requests may release (wholly or in part) one or more lockedsections controlled by the process.Locked sections will be unlocked starting at the current file offsetthrough <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 thearea to be unlocked falls within a locked section),the remaining portions of that section are still locked by the process.Releasing the center portion of a locked section will cause theremaining locked beginning and end portions to become two separatelocked sections. If the request would cause the number of locks in the systemto exceed a system-imposed limit, the request will fail.<p>A potential for deadlock occurs if the threads of a process controlling a locked section are blocked by accessing another process' locked section.If the system detects that deadlock would occur,<i>lockf()</i>will fail with an [EDEADLK] error.<p>The interaction between<i><a href="fcntl.html">fcntl()</a></i>and<i>lockf()</i>locks is unspecified.<p>Blocking on a section is interrupted by any signal.<p>An F_ULOCK request in which <i>size</i>is non-zero and the offset of thelast byte of the requested section is the maximum value for an objectof type<b>off_t</b>,when the process has an existing lock in which <i>size</i>is 0 and which includes the last byte of the requested section, will betreated as a request to unlock from the start of the requested sectionwith a size equal to 0. Otherwise an F_ULOCK request will attempt tounlock only the requested section. <p>Attempting to lock a section of a file that is associated with a bufferedstream produces unspecified results.</blockquote><h4><a name = "tag_000_007_402"> </a>RETURN VALUE</h4><blockquote>Upon successful completion,<i>lockf()</i>returns 0. Otherwise, it returns -1, sets<i>errno</i> to indicate an error, and existing locks are not changed.</blockquote><h4><a name = "tag_000_007_403"> </a>ERRORS</h4><blockquote>The<i>lockf()</i>function will fail if:<dl compact><dt>[EBADF]<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> is not a valid filedescriptor open for writing.<dt>[EACCES] or [EAGAIN]<dd>The <i>function</i> argument is F_TLOCK or F_TEST and thesection is already locked by another process.<dt>[EDEADLK]<dd>The <i>function</i> argument is F_LOCK and a deadlock is detected.<dt>[EINTR]<dd>A signal was caught during execution of the function.<dt>[EINVAL]<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 is less than 0. <dt>[EOVERFLOW]<dd>The offset of the first, or if<i>size</i>is not 0 then the last, byte in the requested section cannot be represented correctly in an object oftype<b>off_t</b>.</dl><p>The<i>lockf()</i>function may fail if:<dl compact><dt>[EAGAIN]<dd>The <i>function</i> argument is F_LOCK or F_TLOCK and the file is mapped with<i><a href="mmap.html">mmap()</a></i>.<dt>[EDEADLK] or [ENOLCK]<dd>The <i>function</i> argument is F_LOCK, F_TLOCK, or F_ULOCK, and therequest would cause the number of locks to exceed a system-imposed limit.<dt>[EOPNOTSUPP] or [EINVAL]<dd>The implementation does not support the locking of filesof the type indicated by the <i>fildes</i> argument.</dl></blockquote><h4><a name = "tag_000_007_404"> </a>EXAMPLES</h4><blockquote>None.</blockquote><h4><a name = "tag_000_007_405"> </a>APPLICATION USAGE</h4><blockquote>Record-locking should not be used in combination with the<i><a href="fopen.html">fopen()</a></i>,<i><a href="fread.html">fread()</a></i>,<i><a href="fwrite.html">fwrite()</a></i>and other<i>stdio</i>functions. Instead, the more primitive, non-buffered functions (such as<i><a href="open.html">open()</a></i>)should be used. Unexpected results may occur in processes that do bufferingin the user address space. The process may later read/write data which is/waslocked. The<i>stdio</i>functions are the most common source of unexpected buffering.<p>The<i><a href="alarm.html">alarm()</a></i>function may be used to provide a timeout facility in applications requiringit.</blockquote><h4><a name = "tag_000_007_406"> </a>FUTURE DIRECTIONS</h4><blockquote>None.</blockquote><h4><a name = "tag_000_007_407"> </a>SEE ALSO</h4><blockquote><i><a href="alarm.html">alarm()</a></i>,<i><a href="chmod.html">chmod()</a></i>,<i><a href="close.html">close()</a></i>,<i><a href="creat.html">creat()</a></i>,<i><a href="fcntl.html">fcntl()</a></i>,<i><a href="fopen.html">fopen()</a></i>,<i><a href="mmap.html">mmap()</a></i>,<i><a href="open.html">open()</a></i>,<i><a href="read.html">read()</a></i>,<i><a href="write.html">write()</a></i>,<i><a href="unistd.h.html"><unistd.h></a></i>.</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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