1.\"	$OpenBSD: lockf.3,v 1.13 2013/06/05 03:39:22 tedu Exp $
2.\"	$NetBSD: lockf.3,v 1.1 1997/12/20 20:23:17 kleink Exp $
3.\"
4.\" Copyright (c) 1997 The NetBSD Foundation, Inc.
5.\" All rights reserved.
6.\"
7.\" This code is derived from software contributed to The NetBSD Foundation
8.\" by Klaus Klein and S.P. Zeidler.
9.\"
10.\" Redistribution and use in source and binary forms, with or without
11.\" modification, are permitted provided that the following conditions
12.\" are met:
13.\" 1. Redistributions of source code must retain the above copyright
14.\"    notice, this list of conditions and the following disclaimer.
15.\" 2. Redistributions in binary form must reproduce the above copyright
16.\"    notice, this list of conditions and the following disclaimer in the
17.\"    documentation and/or other materials provided with the distribution.
18.\"
19.\" THIS SOFTWARE IS PROVIDED BY THE NETBSD FOUNDATION, INC. AND CONTRIBUTORS
20.\" ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED
21.\" TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
22.\" PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL THE FOUNDATION OR CONTRIBUTORS
23.\" BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
24.\" CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
25.\" SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
26.\" INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
27.\" CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
28.\" ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
29.\" POSSIBILITY OF SUCH DAMAGE.
30.\"
31.Dd $Mdocdate: June 5 2013 $
32.Dt LOCKF 3
33.Os
34.Sh NAME
35.Nm lockf
36.Nd record locking on files
37.Sh SYNOPSIS
38.In unistd.h
39.Ft int
40.Fn lockf "int filedes" "int function" "off_t size"
41.Sh DESCRIPTION
42The
43.Fn lockf
44function allows sections of a file to be locked with advisory-mode locks.
45Calls to
46.Fn lockf
47from other processes which attempt to lock the locked file section will
48either return an error value or block until the section becomes unlocked.
49All the locks for a process are removed when the process terminates.
50.Pp
51The argument
52.Fa filedes
53is an open file descriptor.
54The file descriptor must have been opened either for write-only
55.Pq Dv O_WRONLY
56or read/write
57.Pq Dv O_RDWR
58operation.
59.Pp
60The
61.Fa function
62argument is a control value which specifies the action to be taken.
63The permissible values for
64.Fa function
65are as follows:
66.Pp
67.Bl -tag -width F_ULOCKXX -compact -offset indent
68.It Sy Function
69.Sy Description
70.It Dv F_ULOCK
71Unlock locked sections.
72.It Dv F_LOCK
73Lock a section for exclusive use.
74.It Dv F_TLOCK
75Test and lock a section for exclusive use.
76.It Dv F_TEST
77Test a section for locks by other processes.
78.El
79.Pp
80The
81.Dv F_ULOCK
82function removes locks from a section of the file;
83.Dv F_LOCK
84and
85.Dv F_TLOCK
86both lock a section of a file if the section is available;
87.Dv F_TEST
88detects if a lock by another process is present on the specified section.
89.Pp
90The
91.Fa size
92argument is the number of contiguous bytes to be locked or unlocked.
93The section to be locked or unlocked starts at the current
94offset in the file and extends forward for a positive size or backward
95for a negative size (the preceding bytes up to but not including the
96current offset).
97However, it is not permitted to lock a section that
98starts or extends before the beginning of the file.
99If
100.Fa size
101is 0, the section from the current offset through the largest possible
102file offset is locked (that is, from the current offset through the
103present or any future end-of-file).
104.Pp
105The sections locked with
106.Dv F_LOCK
107or
108.Dv F_TLOCK
109may, in whole or in part, contain or be contained by a previously
110locked section for the same process.
111When this occurs, or if adjacent
112locked sections would occur, the sections are combined into a single
113locked section.
114If the request would cause the number of locks to
115exceed a system-imposed limit, the request will fail.
116.Pp
117The
118.Dv F_LOCK
119and
120.Dv F_TLOCK
121requests differ only by the action taken if the section is not
122available.
123.Dv F_LOCK
124blocks the calling process until the section is available.
125.Dv F_TLOCK
126makes the function fail if the section is already locked by another
127process.
128.Pp
129File locks are released on first close by the locking process of any
130file descriptor for the file.
131.Pp
132.Dv F_ULOCK
133requests release (wholly or in part) of one or more locked sections
134controlled by the process.
135Locked sections will be unlocked starting
136at the current file offset through
137.Fa size
138bytes or to the end of the file if
139.Fa size
140is 0.
141When all of a locked section
142is not released (that is, when the beginning or end of the area to be
143unlocked falls within a locked section), the remaining portions of
144that section are still locked by the process.
145Releasing the center
146portion of a locked section will cause the remaining locked beginning
147and end portions to become two separate locked sections.
148If the
149request would cause the number of locks in the system to exceed a
150system-imposed limit, the request will fail.
151.Pp
152An
153.Dv F_ULOCK
154request in which
155.Fa size
156is non-zero and the offset of the last byte of
157the requested section is the maximum value for an object of type
158.Li off_t ,
159when the process has an existing lock in which size is 0 and
160which includes the last byte of the requested section, will be treated
161as a request to unlock from the start of the requested section with a
162size equal to 0.
163Otherwise an
164.Dv F_ULOCK
165request will attempt to unlock only the requested section.
166.Pp
167A potential for deadlock occurs if a process controlling a locked
168region is put to sleep by attempting to lock the locked region of
169another process.
170This implementation detects that sleeping until a
171locked region is unlocked would cause a deadlock and fails with an
172.Er EDEADLK
173error.
174.Pp
175.Fn lockf ,
176.Xr fcntl 2 ,
177and
178.Xr flock 2
179locks may be safely used concurrently.
180.Pp
181Blocking on a section is interrupted by any signal.
182.Sh RETURN VALUES
183If successful, the
184.Fn lockf
185function returns 0.
186Otherwise, it returns \-1, sets the global variable
187.Va errno
188to indicate an error, and existing locks are not changed.
189.Sh ERRORS
190.Fn lockf
191will fail if:
192.Bl -tag -width Er
193.It Bq Er EAGAIN
194The argument
195.Fa function
196is
197.Dv F_TLOCK
198or
199.Dv F_TEST
200and the section is already locked by another process.
201.It Bq Er EBADF
202The argument
203.Fa filedes
204is not a valid open file descriptor.
205.Pp
206The argument
207.Fa function
208is
209.Dv F_LOCK
210or
211.Dv F_TLOCK ,
212and
213.Fa filedes
214is not a valid file descriptor open for writing.
215.It Bq Er EDEADLK
216The argument
217.Fa function
218is
219.Dv F_LOCK
220and a deadlock is detected.
221.It Bq Er EINTR
222The argument
223.Fa function
224is F_LOCK
225and
226.Fn lockf
227was interrupted by the delivery of a signal.
228.It Bq Er EINVAL
229The argument
230.Fa function
231is not one of
232.Dv F_ULOCK ,
233.Dv F_LOCK ,
234.Dv F_TLOCK ,
235or
236.Dv F_TEST .
237.Pp
238The argument
239.Fa filedes
240refers to a file that does not support locking.
241.It Bq Er ENOLCK
242The argument
243.Fa function
244is
245.Dv F_ULOCK ,
246.Dv F_LOCK ,
247or
248.Dv F_TLOCK ,
249and satisfying the lock or unlock request would result in the number
250of locked regions in the system exceeding a system-imposed limit.
251.El
252.Sh SEE ALSO
253.Xr fcntl 2 ,
254.Xr flock 2
255.Sh STANDARDS
256The
257.Fn lockf
258function conforms to
259.St -xpg4.2 .
260