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.\" 31.Dd December 11, 1993 32.Dt FLOCK 2 33.Os 34.Sh NAME 35.Nm flock 36.Nd "apply or remove an advisory lock on an open file" 37.Sh LIBRARY 38.Lb libc 39.Sh SYNOPSIS 40.In sys/file.h 41.Fd "#define LOCK_SH 0x01 /* shared file lock */" 42.Fd "#define LOCK_EX 0x02 /* exclusive file lock */" 43.Fd "#define LOCK_NB 0x04 /* don't block when locking */" 44.Fd "#define LOCK_UN 0x08 /* unlock file */" 45.Ft int 46.Fn flock "int fd" "int operation" 47.Sh DESCRIPTION 48.Fn Flock 49applies or removes an 50.Em advisory 51lock on the file associated with the file descriptor 52.Fa fd . 53A lock is applied by specifying an 54.Fa operation 55parameter that is one of 56.Dv LOCK_SH 57or 58.Dv LOCK_EX 59with the optional addition of 60.Dv LOCK_NB . 61To unlock 62an existing lock 63.Fa operation 64should be 65.Dv LOCK_UN . 66.Pp 67Advisory locks allow cooperating processes to perform 68consistent operations on files, but do not guarantee 69consistency (i.e., processes may still access files 70without using advisory locks possibly resulting in 71inconsistencies). 72.Pp 73The locking mechanism allows two types of locks: 74.Em shared 75locks and 76.Em exclusive 77locks. 78At any time multiple shared locks may be applied to a file, 79but at no time are multiple exclusive, or both shared and exclusive, 80locks allowed simultaneously on a file. 81.Pp 82A shared lock may be 83.Em upgraded 84to an exclusive lock, and vice versa, simply by specifying 85the appropriate lock type; this results in the previous 86lock being released and the new lock applied (possibly 87after other processes have gained and released the lock). 88.Pp 89Requesting a lock on an object that is already locked 90normally causes the caller to be blocked until the lock may be 91acquired. If 92.Dv LOCK_NB 93is included in 94.Fa operation , 95then this will not happen; instead the call will fail and 96the error 97.Er EWOULDBLOCK 98will be returned. 99.Sh NOTES 100Locks are on files, not file descriptors. That is, file descriptors 101duplicated through 102.Xr dup 2 103or 104.Xr fork 2 105do not result in multiple instances of a lock, but rather multiple 106references to a single lock. If a process holding a lock on a file 107forks and the child explicitly unlocks the file, the parent will 108lose its lock. 109.Pp 110Processes blocked awaiting a lock may be awakened by signals. 111.Sh RETURN VALUES 112.Rv -std flock 113.Sh ERRORS 114The 115.Fn flock 116call fails if: 117.Bl -tag -width Er 118.It Bq Er EWOULDBLOCK 119The file is locked and the 120.Dv LOCK_NB 121option was specified. 122.It Bq Er EBADF 123The argument 124.Fa fd 125is an invalid descriptor. 126.It Bq Er EINVAL 127The argument 128.Fa fd 129refers to an object other than a file. 130.It Bq Er EOPNOTSUPP 131The argument 132.Fa fd 133refers to an object that does not support file locking. 134.El 135.Sh SEE ALSO 136.Xr close 2 , 137.Xr dup 2 , 138.Xr execve 2 , 139.Xr fork 2 , 140.Xr open 2 141.Sh HISTORY 142The 143.Fn flock 144function call appeared in 145.Bx 4.2 . 146