xref: /dragonfly/lib/libc/sys/flock.2 (revision e8c03636) 
1.\" Copyright (c) 1983, 1991, 1993
2.\"	The Regents of the University of California.  All rights reserved.
3.\"
4.\" Redistribution and use in source and binary forms, with or without
5.\" modification, are permitted provided that the following conditions
6.\" are met:
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.
15.\"
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
26.\" SUCH DAMAGE.
27.\"
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 $
31.\"
32.Dd December 11, 1993
33.Dt FLOCK 2
34.Os
35.Sh NAME
36.Nm flock
37.Nd "apply or remove an advisory lock on an open file"
38.Sh LIBRARY
39.Lb libc
40.Sh SYNOPSIS
41.In sys/file.h
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 */"
46.Ft int
47.Fn flock "int fd" "int operation"
48.Sh DESCRIPTION
49.Fn Flock
50applies or removes an
51.Em advisory
52lock on the file associated with the file descriptor
53.Fa fd .
54A lock is applied by specifying an
55.Fa operation
56parameter that is one of
57.Dv LOCK_SH
58or
59.Dv LOCK_EX
60with the optional addition of
61.Dv LOCK_NB .
62To unlock
63an existing lock
64.Fa operation
65should be
66.Dv LOCK_UN .
67.Pp
68Advisory locks allow cooperating processes to perform
69consistent operations on files, but do not guarantee
70consistency (i.e., processes may still access files
71without using advisory locks possibly resulting in
72inconsistencies).
73.Pp
74The locking mechanism allows two types of locks:
75.Em shared
76locks and
77.Em exclusive
78locks.
79At any time multiple shared locks may be applied to a file,
80but at no time are multiple exclusive, or both shared and exclusive,
81locks allowed simultaneously on a file.
82.Pp
83A shared lock may be
84.Em upgraded
85to an exclusive lock, and vice versa, simply by specifying
86the appropriate lock type; this results in the previous
87lock being released and the new lock applied (possibly
88after other processes have gained and released the lock).
89.Pp
90Requesting a lock on an object that is already locked
91normally causes the caller to be blocked until the lock may be
92acquired.  If
93.Dv LOCK_NB
94is included in
95.Fa operation ,
96then this will not happen; instead the call will fail and
97the error
98.Er EWOULDBLOCK
99will be returned.
100.Sh NOTES
101Locks are on files, not file descriptors.  That is, file descriptors
102duplicated through
103.Xr dup 2
104or
105.Xr fork 2
106do not result in multiple instances of a lock, but rather multiple
107references to a single lock.  If a process holding a lock on a file
108forks and the child explicitly unlocks the file, the parent will
109lose its lock.
110.Pp
111Processes blocked awaiting a lock may be awakened by signals.
112.Sh RETURN VALUES
113.Rv -std flock
114.Sh ERRORS
115The
116.Fn flock
117call fails if:
118.Bl -tag -width Er
119.It Bq Er EWOULDBLOCK
120The file is locked and the
121.Dv LOCK_NB
122option was specified.
123.It Bq Er EBADF
124The argument
125.Fa fd
126is an invalid descriptor.
127.It Bq Er EINVAL
128The argument
129.Fa fd
130refers to an object other than a file.
131.It Bq Er EOPNOTSUPP
132The argument
133.Fa fd
134refers to an object that does not support file locking.
135.El
136.Sh SEE ALSO
137.Xr close 2 ,
138.Xr dup 2 ,
139.Xr execve 2 ,
140.Xr fork 2 ,
141.Xr open 2
142.Sh HISTORY
143The
144.Fn flock
145function call appeared in
146.Bx 4.2 .
147