1.\" $OpenBSD: flock.2,v 1.12 2000/10/18 05:12:09 aaron Exp $ 2.\" $NetBSD: flock.2,v 1.5 1995/02/27 12:32:32 cgd Exp $ 3.\" 4.\" Copyright (c) 1983, 1991, 1993 5.\" The Regents of the University of California. All rights reserved. 6.\" 7.\" Redistribution and use in source and binary forms, with or without 8.\" modification, are permitted provided that the following conditions 9.\" are met: 10.\" 1. Redistributions of source code must retain the above copyright 11.\" notice, this list of conditions and the following disclaimer. 12.\" 2. Redistributions in binary form must reproduce the above copyright 13.\" notice, this list of conditions and the following disclaimer in the 14.\" documentation and/or other materials provided with the distribution. 15.\" 3. All advertising materials mentioning features or use of this software 16.\" must display the following acknowledgement: 17.\" This product includes software developed by the University of 18.\" California, Berkeley and its contributors. 19.\" 4. Neither the name of the University nor the names of its contributors 20.\" may be used to endorse or promote products derived from this software 21.\" without specific prior written permission. 22.\" 23.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND 24.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 25.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 26.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE 27.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 28.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 29.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 30.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 31.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 32.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 33.\" SUCH DAMAGE. 34.\" 35.\" @(#)flock.2 8.2 (Berkeley) 12/11/93 36.\" 37.Dd December 11, 1993 38.Dt FLOCK 2 39.Os 40.Sh NAME 41.Nm flock 42.Nd "apply or remove an advisory lock on an open file" 43.Sh SYNOPSIS 44.Fd #include <fcntl.h> 45 46.Fd #define LOCK_SH 1 /* shared lock */ 47.Fd #define LOCK_EX 2 /* exclusive lock */ 48.Fd #define LOCK_NB 4 /* don't block when locking */ 49.Fd #define LOCK_UN 8 /* unlock */ 50.Ft int 51.Fn flock "int fd" "int operation" 52.Sh DESCRIPTION 53.Fn flock 54applies or removes an 55.Em advisory 56lock on the file associated with the file descriptor 57.Fa fd . 58A lock is applied by specifying an 59.Fa operation 60parameter that is one of 61.Dv LOCK_SH 62or 63.Dv LOCK_EX 64with the optional addition of 65.Dv LOCK_NB . 66To unlock an 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 normally causes 94the caller to be blocked until the lock may be acquired. 95If 96.Dv LOCK_NB 97is included in 98.Fa operation , 99then this will not happen; instead the call will fail and the error 100.Er EWOULDBLOCK 101will be returned. 102.Sh NOTES 103Locks are on files, not file descriptors. 104That is, file descriptors duplicated through 105.Xr dup 2 106or 107.Xr fork 2 108do not result in multiple instances of a lock, but rather multiple 109references to a single lock. 110If a process holding a lock on a file forks and the child explicitly 111unlocks the file, the parent will lose its lock. 112.Pp 113Processes blocked awaiting a lock may be awakened by signals. 114.Sh RETURN VALUES 115Zero is returned if the operation was successful; 116on an error a \-1 is returned and an error code is left in 117the global location 118.Va errno . 119.Sh ERRORS 120The 121.Fn flock 122call fails if: 123.Bl -tag -width Er 124.It Bq Er EWOULDBLOCK 125The file is locked and the 126.Dv LOCK_NB 127option was specified. 128.It Bq Er EBADF 129The argument 130.Fa fd 131is an invalid descriptor. 132.It Bq Er EINVAL 133The argument 134.Fa operation 135have an invalid value. 136.It Bq Er EOPNOTSUPP 137The referenced descriptor is not of the correct type. 138.El 139.Sh SEE ALSO 140.Xr close 2 , 141.Xr dup 2 , 142.Xr execve 2 , 143.Xr fcntl 2 , 144.Xr fork 2 , 145.Xr open 2 146.Sh HISTORY 147The 148.Fn flock 149function call appeared in 150.Bx 4.2 . 151