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1 .\" $NetBSD: lockf.3,v 1.9 2003/04/16 13:34:38 wiz Exp $
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3 .\" Copyright (c) 1997 The NetBSD Foundation, Inc.
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30 .Dd December 19, 1997
31 .Dt LOCKF 3
32 .Os
33 .Sh NAME
34 .Nm lockf
35 .Nd record locking on files
36 .Sh LIBRARY
37 .Lb libc
38 .Sh SYNOPSIS
39 .In unistd.h
40 .Ft int
41 .Fn lockf "int filedes" "int function" "off_t size"
42 .Sh DESCRIPTION
43 The
44 .Fn lockf
45 function allows sections of a file to be locked with advisory-mode locks.
46 Calls to
47 .Fn lockf
48 from other processes which attempt to lock the locked file section will
49 either return an error value or block until the section becomes unlocked.
50 All the locks for a process are removed when the process terminates.
51 .Pp
52 The argument
53 .Fa filedes
54 is an open file descriptor.
55 The file descriptor must have been opened either for write-only
56 .Dv ( O_WRONLY )
57 or read/write
58 .Dv ( O_RDWR )
59 operation.
60 .Pp
61 The
62 .Fa function
63 argument is a control value which specifies the action to be taken.
64 The permissible values for
65 .Fa function
66 are as follows:
67 .Bl -tag -width F_ULOCKXX -compact -offset indent
68 .It Sy Function
69 .Sy Description
70 .It Dv F_ULOCK
71 unlock locked sections
72 .It Dv F_LOCK
73 lock a section for exclusive use
74 .It Dv F_TLOCK
75 test and lock a section for exclusive use
76 .It Dv F_TEST
77 test a section for locks by other processes
78 .El
79 .Pp
80 .Dv F_ULOCK
81 removes locks from a section of the file;
82 .Dv F_LOCK
83 and
84 .Dv F_TLOCK
85 both lock a section of a file if the section is available;
86 .Dv F_TEST
87 detects if a lock by another process is present on the specified section.
88 .Pp
89 The
90 .Fa size
91 argument is the number of contiguous bytes to be locked or
92 unlocked.
93 The section to be locked or unlocked starts at the current
94 offset in the file and extends forward for a positive size or backward
95 for a negative size (the preceding bytes up to but not including the
96 current offset).
97 However, it is not permitted to lock a section that
98 starts or extends before the beginning of the file.
99 If
100 .Fa size
101 is 0, the section from the current offset through the largest possible
102 file offset is locked (that is, from the current offset through the
103 present or any future end-of-file).
104 .Pp
105 The sections locked with
106 .Dv F_LOCK
107 or
108 .Dv F_TLOCK
109 may, in whole or in part, contain or be contained by a previously
110 locked section for the same process.
111 When this occurs, or if adjacent
112 locked sections would occur, the sections are combined into a single
113 locked section.
114 If the request would cause the number of locks to
115 exceed a system-imposed limit, the request will fail.
116 .Pp
117 .Dv F_LOCK
118 and
119 .Dv F_TLOCK
120 requests differ only by the action taken if the section is not
121 available.
122 .Dv F_LOCK
123 blocks the calling process until the section is available.
124 .Dv F_TLOCK
125 makes the function fail if the section is already locked by another
126 process.
127 .Pp
128 File locks are released on first close by the locking process of any
129 file descriptor for the file.
130 .Pp
131 .Dv F_ULOCK
132 requests release (wholly or in part) one or more locked sections
133 controlled by the process.
134 Locked sections will be unlocked starting
135 at the current file offset through
136 .Fa size
137 bytes or to the end of file if size is 0.
138 When all of a locked section
139 is not released (that is, when the beginning or end of the area to be
140 unlocked falls within a locked section), the remaining portions of
141 that section are still locked by the process.
142 Releasing the center
143 portion of a locked section will cause the remaining locked beginning
144 and end portions to become two separate locked sections.
145 If the
146 request would cause the number of locks in the system to exceed a
147 system-imposed limit, the request will fail.
148 .Pp
149 An
150 .Dv F_ULOCK
151 request in which size is non-zero and the offset of the last byte of
152 the requested section is the maximum value for an object of type
153 off_t, when the process has an existing lock in which size is 0 and
154 which includes the last byte of the requested section, will be treated
155 as a request to unlock from the start of the requested section with a
156 size equal to 0.
157 Otherwise an
158 .Dv F_ULOCK
159 request will attempt to unlock only the requested section.
160 .Pp
161 A potential for deadlock occurs if a process controlling a locked
162 region is put to sleep by attempting to lock the locked region of
163 another process.
164 This implementation detects that sleeping until a
165 locked region is unlocked would cause a deadlock and fails with an
166 .Er EDEADLK
167 error.
168 .Pp
169 .Fn lockf ,
170 .Xr fcntl 2
171 and
172 .Xr flock 2
173 locks may be safely used concurrently.
174 .Pp
175 Blocking on a section is interrupted by any signal.
176 .Sh RETURN VALUES
177 If successful, the
178 .Fn lockf
179 function returns 0.
180 Otherwise, it returns \-1, sets
181 .Va errno
182 to indicate an error, and existing locks are not changed.
183 .Sh ERRORS
184 .Fn lockf
185 will fail if:
186 .Bl -tag -width Er
187 .It Bq Er EAGAIN
188 The argument
189 .Fa function
190 is
191 .Dv F_TLOCK
192 or
193 .Dv F_TEST
194 and the section is already locked by another process.
195 .It Bq Er EBADF
196 The argument
197 .Fa filedes
198 is not a valid open file descriptor.
199 .Pp
200 The argument
201 .Fa function
202 is
203 .Dv F_LOCK
204 or
205 .Dv F_TLOCK ,
206 and
207 .Fa filedes
208 is not a valid file descriptor open for writing.
209 .It Bq Er EDEADLK
210 The argument
211 .Fa function
212 is
213 .Dv F_LOCK
214 and a deadlock is detected.
215 .It Bq Er EINTR
216 The argument
217 .Fa function
218 is F_LOCK
219 and
220 .Fn lockf
221 was interrupted by the delivery of a signal.
222 .It Bq Er EINVAL
223 The argument
224 .Fa function
225 is not one of
226 .Dv F_ULOCK ,
227 .Dv F_LOCK ,
228 .Dv F_TLOCK
229 or
230 .Dv F_TEST .
231 .Pp
232 The argument
233 .Fa filedes
234 refers to a file that does not support locking.
235 .It Bq Er ENOLCK
236 The argument
237 .Fa function
238 is
239 .Dv F_ULOCK ,
240 .Dv F_LOCK
241 or
242 .Dv F_TLOCK ,
243 and satisfying the lock or unlock request would result in the number
244 of locked regions in the system exceeding a system-imposed limit.
245 .El
246 .Sh SEE ALSO
247 .Xr fcntl 2 ,
248 .Xr flock 2
249 .Sh STANDARDS
250 The
251 .Fn lockf
252 function conforms to
253 .St -xpg4.2 .