1 .\" Copyright (c) 1983, 1991, 1993
2 .\" The Regents of the University of California. All rights reserved.
4 .\" Redistribution and use in source and binary forms, with or without
5 .\" modification, are permitted provided that the following conditions
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.
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
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 .\" $DragonFly: src/lib/libc/sys/flock.2,v 1.3 2007/12/03 20:13:10 swildner Exp $
37 .Nd "apply or remove an advisory lock on an open file"
42 .Fd "#define LOCK_SH 0x01 /* shared file lock */"
43 .Fd "#define LOCK_EX 0x02 /* exclusive file lock */"
44 .Fd "#define LOCK_NB 0x04 /* don't block when locking */"
45 .Fd "#define LOCK_UN 0x08 /* unlock file */"
47 .Fn flock "int fd" "int operation"
52 lock on the file associated with the file descriptor
54 A lock is applied by specifying an
56 parameter that is one of
60 with the optional addition of
68 Advisory locks allow cooperating processes to perform
69 consistent operations on files, but do not guarantee
70 consistency (i.e., processes may still access files
71 without using advisory locks possibly resulting in
74 The locking mechanism allows two types of locks:
79 At any time multiple shared locks may be applied to a file,
80 but at no time are multiple exclusive, or both shared and exclusive,
81 locks allowed simultaneously on a file.
85 to an exclusive lock, and vice versa, simply by specifying
86 the appropriate lock type; this results in the previous
87 lock being released and the new lock applied (possibly
88 after other processes have gained and released the lock).
90 Requesting a lock on an object that is already locked
91 normally causes the caller to be blocked until the lock may be
96 then this will not happen; instead the call will fail and
101 Locks are on files, not file descriptors. That is, file descriptors
106 do not result in multiple instances of a lock, but rather multiple
107 references to a single lock. If a process holding a lock on a file
108 forks and the child explicitly unlocks the file, the parent will
111 Processes blocked awaiting a lock may be awakened by signals.
119 .It Bq Er EWOULDBLOCK
120 The file is locked and the
122 option was specified.
126 is an invalid descriptor.
130 refers to an object other than a file.
134 refers to an object that does not support file locking.
145 function call appeared in