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. 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.\" @(#)flock.2 8.2 (Berkeley) 12/11/93 33.\" $FreeBSD: src/lib/libc/sys/flock.2,v 1.8.2.8 2001/12/14 18:34:00 ru Exp $ 34.\" 35.Dd December 11, 1993 36.Dt FLOCK 2 37.Os 38.Sh NAME 39.Nm flock 40.Nd "apply or remove an advisory lock on an open file" 41.Sh LIBRARY 42.Lb libc 43.Sh SYNOPSIS 44.In sys/file.h 45.Fd "#define LOCK_SH 0x01 /* shared file lock */" 46.Fd "#define LOCK_EX 0x02 /* exclusive file lock */" 47.Fd "#define LOCK_NB 0x04 /* don't block when locking */" 48.Fd "#define LOCK_UN 0x08 /* unlock file */" 49.Ft int 50.Fn flock "int fd" "int operation" 51.Sh DESCRIPTION 52.Fn Flock 53applies or removes an 54.Em advisory 55lock on the file associated with the file descriptor 56.Fa fd . 57A lock is applied by specifying an 58.Fa operation 59parameter that is one of 60.Dv LOCK_SH 61or 62.Dv LOCK_EX 63with the optional addition of 64.Dv LOCK_NB . 65To unlock 66an existing lock 67.Dv operation 68should be 69.Dv LOCK_UN . 70.Pp 71Advisory locks allow cooperating processes to perform 72consistent operations on files, but do not guarantee 73consistency (i.e., processes may still access files 74without using advisory locks possibly resulting in 75inconsistencies). 76.Pp 77The locking mechanism allows two types of locks: 78.Em shared 79locks and 80.Em exclusive 81locks. 82At any time multiple shared locks may be applied to a file, 83but at no time are multiple exclusive, or both shared and exclusive, 84locks allowed simultaneously on a file. 85.Pp 86A shared lock may be 87.Em upgraded 88to an exclusive lock, and vice versa, simply by specifying 89the appropriate lock type; this results in the previous 90lock being released and the new lock applied (possibly 91after other processes have gained and released the lock). 92.Pp 93Requesting a lock on an object that is already locked 94normally causes the caller to be blocked until the lock may be 95acquired. If 96.Dv LOCK_NB 97is included in 98.Fa operation , 99then this will not happen; instead the call will fail and 100the error 101.Er EWOULDBLOCK 102will be returned. 103.Sh NOTES 104Locks are on files, not file descriptors. That is, file descriptors 105duplicated through 106.Xr dup 2 107or 108.Xr fork 2 109do not result in multiple instances of a lock, but rather multiple 110references to a single lock. If a process holding a lock on a file 111forks and the child explicitly unlocks the file, the parent will 112lose its lock. 113.Pp 114Processes blocked awaiting a lock may be awakened by signals. 115.Sh RETURN VALUES 116.Rv -std flock 117.Sh ERRORS 118The 119.Fn flock 120call fails if: 121.Bl -tag -width Er 122.It Bq Er EWOULDBLOCK 123The file is locked and the 124.Dv LOCK_NB 125option was specified. 126.It Bq Er EBADF 127The argument 128.Fa fd 129is an invalid descriptor. 130.It Bq Er EINVAL 131The argument 132.Fa fd 133refers to an object other than a file. 134.It Bq Er EOPNOTSUPP 135The argument 136.Fa fd 137refers to an object that does not support file locking. 138.El 139.Sh SEE ALSO 140.Xr close 2 , 141.Xr dup 2 , 142.Xr execve 2 , 143.Xr fork 2 , 144.Xr open 2 145.Sh HISTORY 146The 147.Fn flock 148function call appeared in 149.Bx 4.2 . 150