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. 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.
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
32 .\" @(#)flock.2 8.2 (Berkeley) 12/11/93
40 .Nd "apply or remove an advisory lock on an open file"
45 .Fd "#define LOCK_SH 0x01 /* shared file lock */"
46 .Fd "#define LOCK_EX 0x02 /* exclusive file lock */"
47 .Fd "#define LOCK_NB 0x04 /* do not block when locking */"
48 .Fd "#define LOCK_UN 0x08 /* unlock file */"
50 .Fn flock "int fd" "int operation"
54 system call applies or removes an
56 lock on the file associated with the file descriptor
58 A lock is applied by specifying an
60 argument that is one of
64 with the optional addition of
72 Advisory locks allow cooperating processes to perform
73 consistent operations on files, but do not guarantee
74 consistency (i.e., processes may still access files
75 without using advisory locks possibly resulting in
78 The locking mechanism allows two types of locks:
83 At any time multiple shared locks may be applied to a file,
84 but at no time are multiple exclusive, or both shared and exclusive,
85 locks allowed simultaneously on a file.
89 to an exclusive lock, and vice versa, simply by specifying
90 the appropriate lock type; this results in the previous
91 lock being released and the new lock applied (possibly
92 after other processes have gained and released the lock).
94 Requesting a lock on an object that is already locked
95 normally causes the caller to be blocked until the lock may be
101 then this will not happen; instead the call will fail and
106 Locks are on files, not file descriptors.
107 That is, file descriptors
112 do not result in multiple instances of a lock, but rather multiple
113 references to a single lock.
114 If a process holding a lock on a file
115 forks and the child explicitly unlocks the file, the parent will
123 locks are compatible.
124 Processes using different locking interfaces can cooperate
125 over the same file safely.
126 However, only one of such interfaces should be used within
128 If a file is locked by a process through
130 any record within the file will be seen as locked
131 from the viewpoint of another process using
137 Processes blocked awaiting a lock may be awakened by signals.
143 system call fails if:
145 .It Bq Er EWOULDBLOCK
146 The file is locked and the
148 option was specified.
152 is an invalid descriptor.
156 refers to an object other than a file.
160 refers to an object that does not support file locking.
174 system call appeared in