1.\" Copyright (c) 1983, 1991, 1993 2.\" The Regents of the University of California. 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. Neither the name of the University nor the names of its contributors 13.\" may be used to endorse or promote products derived from this software 14.\" without specific prior written permission. 15.\" 16.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND 17.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 18.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 19.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE 20.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 21.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 22.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 23.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 24.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 25.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 26.\" SUCH DAMAGE. 27.\" 28.\" @(#)flock.2 8.2 (Berkeley) 12/11/93 29.\" $FreeBSD: src/lib/libc/sys/flock.2,v 1.8.2.8 2001/12/14 18:34:00 ru Exp $ 30.\" $DragonFly: src/lib/libc/sys/flock.2,v 1.3 2007/12/03 20:13:10 swildner Exp $ 31.\" 32.Dd December 11, 1993 33.Dt FLOCK 2 34.Os 35.Sh NAME 36.Nm flock 37.Nd "apply or remove an advisory lock on an open file" 38.Sh LIBRARY 39.Lb libc 40.Sh SYNOPSIS 41.In sys/file.h 42.Fd "#define LOCK_SH 0x01 /* shared file lock */" 43.Fd "#define LOCK_EX 0x02 /* exclusive file lock */" 44.Fd "#define LOCK_NB 0x04 /* don't block when locking */" 45.Fd "#define LOCK_UN 0x08 /* unlock file */" 46.Ft int 47.Fn flock "int fd" "int operation" 48.Sh DESCRIPTION 49.Fn Flock 50applies or removes an 51.Em advisory 52lock on the file associated with the file descriptor 53.Fa fd . 54A lock is applied by specifying an 55.Fa operation 56parameter that is one of 57.Dv LOCK_SH 58or 59.Dv LOCK_EX 60with the optional addition of 61.Dv LOCK_NB . 62To unlock 63an existing lock 64.Fa operation 65should be 66.Dv LOCK_UN . 67.Pp 68Advisory locks allow cooperating processes to perform 69consistent operations on files, but do not guarantee 70consistency (i.e., processes may still access files 71without using advisory locks possibly resulting in 72inconsistencies). 73.Pp 74The locking mechanism allows two types of locks: 75.Em shared 76locks and 77.Em exclusive 78locks. 79At any time multiple shared locks may be applied to a file, 80but at no time are multiple exclusive, or both shared and exclusive, 81locks allowed simultaneously on a file. 82.Pp 83A shared lock may be 84.Em upgraded 85to an exclusive lock, and vice versa, simply by specifying 86the appropriate lock type; this results in the previous 87lock being released and the new lock applied (possibly 88after other processes have gained and released the lock). 89.Pp 90Requesting a lock on an object that is already locked 91normally causes the caller to be blocked until the lock may be 92acquired. If 93.Dv LOCK_NB 94is included in 95.Fa operation , 96then this will not happen; instead the call will fail and 97the error 98.Er EWOULDBLOCK 99will be returned. 100.Sh NOTES 101Locks are on files, not file descriptors. That is, file descriptors 102duplicated through 103.Xr dup 2 104or 105.Xr fork 2 106do not result in multiple instances of a lock, but rather multiple 107references to a single lock. If a process holding a lock on a file 108forks and the child explicitly unlocks the file, the parent will 109lose its lock. 110.Pp 111Processes blocked awaiting a lock may be awakened by signals. 112.Sh RETURN VALUES 113.Rv -std flock 114.Sh ERRORS 115The 116.Fn flock 117call fails if: 118.Bl -tag -width Er 119.It Bq Er EWOULDBLOCK 120The file is locked and the 121.Dv LOCK_NB 122option was specified. 123.It Bq Er EBADF 124The argument 125.Fa fd 126is an invalid descriptor. 127.It Bq Er EINVAL 128The argument 129.Fa fd 130refers to an object other than a file. 131.It Bq Er EOPNOTSUPP 132The argument 133.Fa fd 134refers to an object that does not support file locking. 135.El 136.Sh SEE ALSO 137.Xr close 2 , 138.Xr dup 2 , 139.Xr execve 2 , 140.Xr fork 2 , 141.Xr open 2 142.Sh HISTORY 143The 144.Fn flock 145function call appeared in 146.Bx 4.2 . 147