1.\" Copyright (c) 1983, 1991 The Regents of the University of California. 2.\" All rights reserved. 3.\" 4.\" Redistribution and use in source and binary forms, with or without 5.\" modification, are permitted provided that the following conditions 6.\" are met: 7.\" 1. Redistributions of source code must retain the above copyright 8.\" notice, this list of conditions and the following disclaimer. 9.\" 2. Redistributions in binary form must reproduce the above copyright 10.\" notice, this list of conditions and the following disclaimer in the 11.\" documentation and/or other materials provided with the distribution. 12.\" 3. All advertising materials mentioning features or use of this software 13.\" must display the following acknowledgement: 14.\" This product includes software developed by the University of 15.\" California, Berkeley and its contributors. 16.\" 4. Neither the name of the University nor the names of its contributors 17.\" may be used to endorse or promote products derived from this software 18.\" without specific prior written permission. 19.\" 20.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND 21.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 22.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 23.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE 24.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 25.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 26.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 27.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 28.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 29.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 30.\" SUCH DAMAGE. 31.\" 32.\" from: @(#)flock.2 6.7 (Berkeley) 3/10/91 33.\" $Id: flock.2,v 1.4 1993/11/29 21:25:06 jtc Exp $ 34.\" 35.Dd March 10, 1991 36.Dt FLOCK 2 37.Os BSD 4.2 38.Sh NAME 39.Nm flock 40.Nd "apply or remove an advisory lock on an open file" 41.Sh SYNOPSIS 42.Fd #include <sys/file.h> 43.Fd #define LOCK_SH 1 /* shared lock */ 44.Fd #define LOCK_EX 2 /* exclusive lock */ 45.Fd #define LOCK_NB 4 /* don't block when locking */ 46.Fd #define LOCK_UN 8 /* unlock */ 47.Ft int 48.Fn flock "int fd" "int operation" 49.Sh DESCRIPTION 50.Fn Flock 51applies or removes an 52.Em advisory 53lock on the file associated with the file descriptor 54.Fa fd . 55A lock is applied by specifying an 56.Fa operation 57parameter that is the inclusive or of 58.Dv LOCK_SH 59or 60.Dv LOCK_EX 61and, possibly, 62.Dv LOCK_NB . 63To unlock 64an existing lock 65.Dv operation 66should be 67.Dv LOCK_UN . 68.Pp 69Advisory locks allow cooperating processes to perform 70consistent operations on files, but do not guarantee 71consistency (i.e., processes may still access files 72without using advisory locks possibly resulting in 73inconsistencies). 74.Pp 75The locking mechanism allows two types of locks: 76.Em shared 77locks and 78.Em exclusive 79locks. 80At any time multiple shared locks may be applied to a file, 81but at no time are multiple exclusive, or both shared and exclusive, 82locks allowed simultaneously on a file. 83.Pp 84A shared lock may be 85.Em upgraded 86to an exclusive lock, and vice versa, simply by specifying 87the appropriate lock type; this results in the previous 88lock being released and the new lock applied (possibly 89after other processes have gained and released the lock). 90.Pp 91Requesting a lock on an object that is already locked 92normally causes the caller to be blocked until the lock may be 93acquired. If 94.Dv LOCK_NB 95is included in 96.Fa operation , 97then this will not happen; instead the call will fail and 98the error 99.Er EWOULDBLOCK 100will be returned. 101.Sh NOTES 102Locks are on files, not file descriptors. That is, file descriptors 103duplicated through 104.Xr dup 2 105or 106.Xr fork 2 107do not result in multiple instances of a lock, but rather multiple 108references to a single lock. If a process holding a lock on a file 109forks and the child explicitly unlocks the file, the parent will 110lose its lock. 111.Pp 112Processes blocked awaiting a lock may be awakened by signals. 113.Sh RETURN VALUES 114Zero is returned if the operation was successful; 115on an error a -1 is returned and an error code is left in 116the global location 117.Va errno . 118.Sh ERRORS 119The 120.Fn flock 121call fails if: 122.Bl -tag -width Er 123.It Bq Er EWOULDBLOCK 124The file is locked and the 125.Dv LOCK_NB 126option was specified. 127.It Bq Er EBADF 128The argument 129.Fa fd 130is an invalid descriptor. 131.It Bq Er EINVAL 132The argument 133.Fa fd 134refers to an object other than a file. 135.El 136.Sh SEE ALSO 137.Xr open 2 , 138.Xr close 2 , 139.Xr dup 2 , 140.Xr execve 2 , 141.Xr fork 2 142.Sh HISTORY 143The 144.Fn flock 145function call appeared in 146.Bx 4.2 . 147