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 February 11, 2020 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 fcntl.h 41.Ft int 42.Fn flock "int fd" "int operation" 43.Sh DESCRIPTION 44.Fn Flock 45applies or removes an 46.Em advisory 47lock on the file associated with the file descriptor 48.Fa fd . 49A lock is applied by specifying an 50.Fa operation 51parameter that is one of 52.Dv LOCK_SH 53or 54.Dv LOCK_EX 55with the optional addition of 56.Dv LOCK_NB . 57To unlock 58an existing lock 59.Fa operation 60should be 61.Dv LOCK_UN . 62.Pp 63Advisory locks allow cooperating processes to perform 64consistent operations on files, but do not guarantee 65consistency (i.e., processes may still access files 66without using advisory locks possibly resulting in 67inconsistencies). 68.Pp 69The locking mechanism allows two types of locks: 70.Em shared 71locks and 72.Em exclusive 73locks. 74At any time multiple shared locks may be applied to a file, 75but at no time are multiple exclusive, or both shared and exclusive, 76locks allowed simultaneously on a file. 77.Pp 78A shared lock may be 79.Em upgraded 80to an exclusive lock, and vice versa, simply by specifying 81the appropriate lock type; this results in the previous 82lock being released and the new lock applied (possibly 83after other processes have gained and released the lock). 84.Pp 85Requesting a lock on an object that is already locked 86normally causes the caller to be blocked until the lock may be 87acquired. If 88.Dv LOCK_NB 89is included in 90.Fa operation , 91then this will not happen; instead the call will fail and 92the error 93.Er EWOULDBLOCK 94will be returned. 95.Sh NOTES 96Locks are on files, not file descriptors. That is, file descriptors 97duplicated through 98.Xr dup 2 99or 100.Xr fork 2 101do not result in multiple instances of a lock, but rather multiple 102references to a single lock. If a process holding a lock on a file 103forks and the child explicitly unlocks the file, the parent will 104lose its lock. 105.Pp 106Processes blocked awaiting a lock may be awakened by signals. 107.Sh RETURN VALUES 108.Rv -std flock 109.Sh ERRORS 110The 111.Fn flock 112call fails if: 113.Bl -tag -width Er 114.It Bq Er EWOULDBLOCK 115The file is locked and the 116.Dv LOCK_NB 117option was specified. 118.It Bq Er EBADF 119The argument 120.Fa fd 121is an invalid descriptor. 122.It Bq Er EINVAL 123The argument 124.Fa fd 125refers to an object other than a file. 126.It Bq Er EOPNOTSUPP 127The argument 128.Fa fd 129refers to an object that does not support file locking. 130.El 131.Sh SEE ALSO 132.Xr close 2 , 133.Xr dup 2 , 134.Xr execve 2 , 135.Xr fork 2 , 136.Xr open 2 137.Sh HISTORY 138The 139.Fn flock 140function call appeared in 141.Bx 4.2 . 142