1.\" $NetBSD: flock.2,v 1.5 1995/02/27 12:32:32 cgd 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 BSD 4.2 39.Sh NAME 40.Nm flock 41.Nd "apply or remove an advisory lock on an open file" 42.Sh SYNOPSIS 43.Fd #include <sys/file.h> 44.Fd #define LOCK_SH 1 /* shared lock */ 45.Fd #define LOCK_EX 2 /* exclusive lock */ 46.Fd #define LOCK_NB 4 /* don't block when locking */ 47.Fd #define LOCK_UN 8 /* unlock */ 48.Ft int 49.Fn flock "int fd" "int operation" 50.Sh DESCRIPTION 51.Fn Flock 52applies or removes an 53.Em advisory 54lock on the file associated with the file descriptor 55.Fa fd . 56A lock is applied by specifying an 57.Fa operation 58parameter that is one of 59.Dv LOCK_SH 60or 61.Dv LOCK_EX 62with the optional addition of 63.Dv LOCK_NB . 64To unlock 65an existing lock 66.Dv operation 67should be 68.Dv LOCK_UN . 69.Pp 70Advisory locks allow cooperating processes to perform 71consistent operations on files, but do not guarantee 72consistency (i.e., processes may still access files 73without using advisory locks possibly resulting in 74inconsistencies). 75.Pp 76The locking mechanism allows two types of locks: 77.Em shared 78locks and 79.Em exclusive 80locks. 81At any time multiple shared locks may be applied to a file, 82but at no time are multiple exclusive, or both shared and exclusive, 83locks allowed simultaneously on a file. 84.Pp 85A shared lock may be 86.Em upgraded 87to an exclusive lock, and vice versa, simply by specifying 88the appropriate lock type; this results in the previous 89lock being released and the new lock applied (possibly 90after other processes have gained and released the lock). 91.Pp 92Requesting a lock on an object that is already locked 93normally causes the caller to be blocked until the lock may be 94acquired. If 95.Dv LOCK_NB 96is included in 97.Fa operation , 98then this will not happen; instead the call will fail and 99the error 100.Er EWOULDBLOCK 101will be returned. 102.Sh NOTES 103Locks are on files, not file descriptors. That is, file descriptors 104duplicated 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. If a process holding a lock on a file 110forks and the child explicitly unlocks the file, the parent will 111lose 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 fd 135refers to an object other than a file. 136.El 137.Sh SEE ALSO 138.Xr open 2 , 139.Xr close 2 , 140.Xr dup 2 , 141.Xr execve 2 , 142.Xr fork 2 143.Sh HISTORY 144The 145.Fn flock 146function call appeared in 147.Bx 4.2 . 148