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1 .\" $NetBSD: lockf.3,v 1.2 1998/02/05 18:47:28 perry Exp $
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3 .\" Copyright (c) 1997 The NetBSD Foundation, Inc.
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6 .\" This code is derived from software contributed to The NetBSD Foundation
7 .\" by Klaus Klein and S.P. Zeidler.
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37 .\" $FreeBSD: src/lib/libc/gen/lockf.3,v 1.5.2.5 2003/03/13 18:05:37 trhodes Exp $
38 .\" $DragonFly: src/lib/libc/gen/lockf.3,v 1.2 2003/06/17 04:26:42 dillon Exp $
39 .\"
40 .Dd December 19, 1997
41 .Dt LOCKF 3
42 .Os
43 .Sh NAME
44 .Nm lockf
45 .Nd record locking on files
46 .Sh LIBRARY
47 .Lb libc
48 .Sh SYNOPSIS
49 .In unistd.h
50 .Ft int
51 .Fn lockf "int filedes" "int function" "off_t size"
52 .Sh DESCRIPTION
53 The
54 .Fn lockf
55 function allows sections of a file to be locked with advisory-mode locks.
56 Calls to
57 .Fn lockf
58 from other processes which attempt to lock the locked file section will
59 either return an error value or block until the section becomes unlocked.
60 All the locks for a process are removed when the process terminates.
61 .Pp
62 The argument
63 .Fa filedes
64 is an open file descriptor.
65 The file descriptor must have been opened either for write-only
66 .Dv ( O_WRONLY )
67 or read/write
68 .Dv ( O_RDWR )
69 operation.
70 .Pp
71 The
72 .Fa function
73 argument is a control value which specifies the action to be taken.
74 The permissible values for
75 .Fa function
76 are as follows:
77 .Bl -tag -width F_ULOCKXX -compact -offset indent
78 .It Sy Function
79 .Sy Description
80 .It Dv F_ULOCK
81 unlock locked sections
82 .It Dv F_LOCK
83 lock a section for exclusive use
84 .It Dv F_TLOCK
85 test and lock a section for exclusive use
86 .It Dv F_TEST
87 test a section for locks by other processes
88 .El
89 .Pp
90 .Dv F_ULOCK
91 removes locks from a section of the file;
92 .Dv F_LOCK
93 and
94 .Dv F_TLOCK
95 both lock a section of a file if the section is available;
96 .Dv F_TEST
97 detects if a lock by another process is present on the specified section.
98 .Pp
99 The
100 .Fa size
101 argument is the number of contiguous bytes to be locked or
102 unlocked.
103 The section to be locked or unlocked starts at the current
104 offset in the file and extends forward for a positive size or backward
105 for a negative size (the preceding bytes up to but not including the
106 current offset). However, it is not permitted to lock a section that
107 starts or extends before the beginning of the file.
108 If
109 .Fa size
110 is 0, the section from the current offset through the largest possible
111 file offset is locked (that is, from the current offset through the
112 present or any future end-of-file).
113 .Pp
114 The sections locked with
115 .Dv F_LOCK
116 or
117 .Dv F_TLOCK
118 may, in whole or in part, contain or be contained by a previously
119 locked section for the same process.
120 When this occurs, or if adjacent
121 locked sections would occur, the sections are combined into a single
122 locked section.
123 If the request would cause the number of locks to
124 exceed a system-imposed limit, the request will fail.
125 .Pp
126 .Dv F_LOCK
127 and
128 .Dv F_TLOCK
129 requests differ only by the action taken if the section is not
130 available.
131 .Dv F_LOCK
132 blocks the calling process until the section is available.
133 .Dv F_TLOCK
134 makes the function fail if the section is already locked by another
135 process.
136 .Pp
137 File locks are released on first close by the locking process of any
138 file descriptor for the file.
139 .Pp
140 .Dv F_ULOCK
141 requests release (wholly or in part) one or more locked sections
142 controlled by the process.
143 Locked sections will be unlocked starting
144 at the current file offset through
145 .Fa size
146 bytes or to the end of file if size is 0. When all of a locked section
147 is not released (that is, when the beginning or end of the area to be
148 unlocked falls within a locked section), the remaining portions of
149 that section are still locked by the process.
150 Releasing the center
151 portion of a locked section will cause the remaining locked beginning
152 and end portions to become two separate locked sections.
153 If the
154 request would cause the number of locks in the system to exceed a
155 system-imposed limit, the request will fail.
156 .Pp
157 An
158 .Dv F_ULOCK
159 request in which size is non-zero and the offset of the last byte of
160 the requested section is the maximum value for an object of type
161 off_t, when the process has an existing lock in which size is 0 and
162 which includes the last byte of the requested section, will be treated
163 as a request to unlock from the start of the requested section with a
164 size equal to 0. Otherwise an
165 .Dv F_ULOCK
166 request will attempt to unlock only the requested section.
167 .Pp
168 A potential for deadlock occurs if a process controlling a locked
169 region is put to sleep by attempting to lock the locked region of
170 another process. This implementation detects that sleeping until a
171 locked region is unlocked would cause a deadlock and fails with an
172 .Er EDEADLK
173 error.
174 .Pp
175 The
176 .Fn lockf ,
177 .Xr fcntl 2
178 and
179 .Xr flock 2
180 locks may be safely used concurrently.
181 .Pp
182 Blocking on a section is interrupted by any signal.
183 .Sh RETURN VALUES
184 .Rv -std lockf
185 In the case of a failure, existing locks are not changed.
186 .Sh ERRORS
187 The
188 .Fn lockf
189 function
190 will fail if:
191 .Bl -tag -width Er
192 .It Bq Er EAGAIN
193 The argument
194 .Fa function
195 is
196 .Dv F_TLOCK
197 or
198 .Dv F_TEST
199 and the section is already locked by another process.
200 .It Bq Er EBADF
201 The argument
202 .Fa filedes
203 is not a valid open file descriptor.
204 .Pp
205 The argument
206 .Fa function
207 is
208 .Dv F_LOCK
209 or
210 .Dv F_TLOCK ,
211 and
212 .Fa filedes
213 is not a valid file descriptor open for writing.
214 .It Bq Er EDEADLK
215 The argument
216 .Fa function
217 is
218 .Dv F_LOCK
219 and a deadlock is detected.
220 .It Bq Er EINTR
221 The argument
222 .Fa function
223 is F_LOCK
224 and
225 .Fn lockf
226 was interrupted by the delivery of a signal.
227 .It Bq Er EINVAL
228 The argument
229 .Fa function
230 is not one of
231 .Dv F_ULOCK ,
232 .Dv F_LOCK ,
233 .Dv F_TLOCK
234 or
235 .Dv F_TEST .
236 .Pp
237 The argument
238 .Fa filedes
239 refers to a file that does not support locking.
240 .It Bq Er ENOLCK
241 The argument
242 .Fa function
243 is
244 .Dv F_ULOCK ,
245 .Dv F_LOCK
246 or
247 .Dv F_TLOCK ,
248 and satisfying the lock or unlock request would result in the number
249 of locked regions in the system exceeding a system-imposed limit.
250 .El
251 .Sh SEE ALSO
252 .Xr fcntl 2 ,
253 .Xr flock 2
254 .Sh STANDARDS
255 The
256 .Fn lockf
257 function conforms to
258 .St -xpg4.2 .