1.\" $NetBSD: flock.2,v 1.13 2001/09/16 02:13:27 wiz Exp $ 2.\" 3.\" Copyright (c) 1983, 1991, 1993 4.\" The Regents of the University of California. All rights reserved. 5.\" 6.\" Redistribution and use in source and binary forms, with or without 7.\" modification, are permitted provided that the following conditions 8.\" are met: 9.\" 1. Redistributions of source code must retain the above copyright 10.\" notice, this list of conditions and the following disclaimer. 11.\" 2. Redistributions in binary form must reproduce the above copyright 12.\" notice, this list of conditions and the following disclaimer in the 13.\" documentation and/or other materials provided with the distribution. 14.\" 3. All advertising materials mentioning features or use of this software 15.\" must display the following acknowledgement: 16.\" This product includes software developed by the University of 17.\" California, Berkeley and its contributors. 18.\" 4. Neither the name of the University nor the names of its contributors 19.\" may be used to endorse or promote products derived from this software 20.\" without specific prior written permission. 21.\" 22.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND 23.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 24.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 25.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE 26.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 27.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 28.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 29.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 30.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 31.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 32.\" SUCH DAMAGE. 33.\" 34.\" @(#)flock.2 8.2 (Berkeley) 12/11/93 35.\" 36.Dd December 11, 1993 37.Dt FLOCK 2 38.Os 39.Sh NAME 40.Nm flock 41.Nd "apply or remove an advisory lock on an open file" 42.Sh LIBRARY 43.Lb libc 44.Sh SYNOPSIS 45.Fd #include <fcntl.h> 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 67an existing lock 68.Dv operation 69should be 70.Dv LOCK_UN . 71.Pp 72Advisory locks allow cooperating processes to perform 73consistent operations on files, but do not guarantee 74consistency (i.e., processes may still access files 75without using advisory locks possibly resulting in 76inconsistencies). 77.Pp 78The locking mechanism allows two types of locks: 79.Em shared 80locks and 81.Em exclusive 82locks. 83At any time multiple shared locks may be applied to a file, 84but at no time are multiple exclusive, or both shared and exclusive, 85locks allowed simultaneously on a file. 86.Pp 87A shared lock may be 88.Em upgraded 89to an exclusive lock, and vice versa, simply by specifying 90the appropriate lock type; this results in the previous 91lock being released and the new lock applied (possibly 92after other processes have gained and released the lock). 93.Pp 94Requesting a lock on an object that is already locked 95normally causes the caller to be blocked until the lock may be 96acquired. If 97.Dv LOCK_NB 98is included in 99.Fa operation , 100then this will not happen; instead the call will fail and 101the error 102.Er EAGAIN 103will be returned. 104.Sh NOTES 105Locks are on files, not file descriptors. That is, file descriptors 106duplicated through 107.Xr dup 2 108or 109.Xr fork 2 110do not result in multiple instances of a lock, but rather multiple 111references to a single lock. If a process holding a lock on a file 112forks and the child explicitly unlocks the file, the parent will 113lose its lock. 114.Pp 115Processes blocked awaiting a lock may be awakened by signals. 116.Sh RETURN VALUES 117Zero is returned if the operation was successful; 118on an error a -1 is returned and an error code is left in 119the global location 120.Va errno . 121.Sh ERRORS 122The 123.Fn flock 124call fails if: 125.Bl -tag -width Er 126.It Bq Er EAGAIN 127The file is locked and the 128.Dv LOCK_NB 129option was specified. 130.It Bq Er EBADF 131The argument 132.Fa fd 133is an invalid descriptor. 134.It Bq Er EOPNOTSUPP 135The argument 136.Fa fd 137refers to an object other than a file. 138.It Bq Er EINVAL 139The argument 140.Fa operation 141does not include one of 142.Dv LOCK_EX , 143.Dv LOCK_SH 144or 145.Dv LOCK_UN . 146.El 147.Sh SEE ALSO 148.Xr close 2 , 149.Xr dup 2 , 150.Xr execve 2 , 151.Xr fork 2 , 152.Xr open 2 153.Sh HISTORY 154The 155.Fn flock 156function call appeared in 157.Bx 4.2 . 158