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c13182ef | 1 | .\" Copyright 1993 Rickard E. Faith (faith@cs.unc.edu) and |
6883b3e7 | 2 | .\" and Copyright 2002 Michael Kerrisk |
fea681da | 3 | .\" |
9ad94e1a | 4 | .\" %%%LICENSE_START(VERBATIM) |
fea681da MK |
5 | .\" Permission is granted to make and distribute verbatim copies of this |
6 | .\" manual provided the copyright notice and this permission notice are | |
7 | .\" preserved on all copies. | |
8 | .\" | |
9 | .\" Permission is granted to copy and distribute modified versions of this | |
10 | .\" manual under the conditions for verbatim copying, provided that the | |
11 | .\" entire resulting derived work is distributed under the terms of a | |
12 | .\" permission notice identical to this one. | |
c13182ef | 13 | .\" |
fea681da MK |
14 | .\" Since the Linux kernel and libraries are constantly changing, this |
15 | .\" manual page may be incorrect or out-of-date. The author(s) assume no | |
16 | .\" responsibility for errors or omissions, or for damages resulting from | |
17 | .\" the use of the information contained herein. The author(s) may not | |
18 | .\" have taken the same level of care in the production of this manual, | |
19 | .\" which is licensed free of charge, as they might when working | |
a6760fe6 | 20 | .\" professionally. |
c13182ef | 21 | .\" |
fea681da MK |
22 | .\" Formatted or processed versions of this manual, if unaccompanied by |
23 | .\" the source, must acknowledge the copyright and authors of this work. | |
9ad94e1a | 24 | .\" %%%LICENSE_END |
fea681da MK |
25 | .\" |
26 | .\" Modified Fri Jan 31 16:26:07 1997 by Eric S. Raymond <esr@thyrsus.com> | |
27 | .\" Modified Fri Dec 11 17:57:27 1998 by Jamie Lokier <jamie@imbolc.ucc.ie> | |
c11b1abf | 28 | .\" Modified 24 Apr 2002 by Michael Kerrisk <mtk.manpages@gmail.com> |
fea681da | 29 | .\" Substantial rewrites and additions |
6a916f1c | 30 | .\" 2005-05-10 mtk, noted that lock conversions are not atomic. |
fea681da | 31 | .\" |
bea08fec MK |
32 | .\" FIXME Maybe document LOCK_MAND, LOCK_RW, LOCK_READ, LOCK_WRITE |
33 | .\" which only have effect for SAMBA. | |
4784c377 | 34 | .\" |
4b8c67d9 | 35 | .TH FLOCK 2 2017-09-15 "Linux" "Linux Programmer's Manual" |
fea681da MK |
36 | .SH NAME |
37 | flock \- apply or remove an advisory lock on an open file | |
38 | .SH SYNOPSIS | |
39 | .B #include <sys/file.h> | |
68e4db0a | 40 | .PP |
964d4d9c | 41 | .BI "int flock(int " fd ", int " operation ); |
fea681da MK |
42 | .SH DESCRIPTION |
43 | Apply or remove an advisory lock on the open file specified by | |
44 | .IR fd . | |
c4bb193f | 45 | The argument |
fea681da MK |
46 | .I operation |
47 | is one of the following: | |
54f73b77 MK |
48 | .RS 4 |
49 | .TP 9 | |
fea681da MK |
50 | .B LOCK_SH |
51 | Place a shared lock. | |
52 | More than one process may hold a shared lock for a given file | |
53 | at a given time. | |
54 | .TP | |
55 | .B LOCK_EX | |
56 | Place an exclusive lock. | |
57 | Only one process may hold an exclusive lock for a given | |
58 | file at a given time. | |
59 | .TP | |
60 | .B LOCK_UN | |
61 | Remove an existing lock held by this process. | |
fea681da | 62 | .RE |
54f73b77 | 63 | .PP |
fea681da MK |
64 | A call to |
65 | .BR flock () | |
66 | may block if an incompatible lock is held by another process. | |
ff40dbb3 | 67 | To make a nonblocking request, include |
fea681da | 68 | .B LOCK_NB |
cebca1bd | 69 | (by ORing) |
fea681da | 70 | with any of the above operations. |
efeece04 | 71 | .PP |
fea681da | 72 | A single file may not simultaneously have both shared and exclusive locks. |
efeece04 | 73 | .PP |
fea681da MK |
74 | Locks created by |
75 | .BR flock () | |
61a37c81 MK |
76 | are associated with an open file description (see |
77 | .BR open (2)). | |
fea681da | 78 | This means that duplicate file descriptors (created by, for example, |
c13182ef | 79 | .BR fork (2) |
35e21ba7 MK |
80 | or |
81 | .BR dup (2)) | |
fea681da | 82 | refer to the same lock, and this lock may be modified |
d9cb0d7d | 83 | or released using any of these file descriptors. |
fea681da MK |
84 | Furthermore, the lock is released either by an explicit |
85 | .B LOCK_UN | |
d9cb0d7d MK |
86 | operation on any of these duplicate file descriptors, or when all |
87 | such file descriptors have been closed. | |
efeece04 | 88 | .PP |
d04e1109 MK |
89 | If a process uses |
90 | .BR open (2) | |
d9cb0d7d MK |
91 | (or similar) to obtain more than one file descriptor for the same file, |
92 | these file descriptors are treated independently by | |
d04e1109 MK |
93 | .BR flock (). |
94 | An attempt to lock the file using one of these file descriptors | |
95 | may be denied by a lock that the calling process has | |
d9cb0d7d | 96 | already placed via another file descriptor. |
efeece04 | 97 | .PP |
33a0ccb2 | 98 | A process may hold only one type of lock (shared or exclusive) |
fea681da MK |
99 | on a file. |
100 | Subsequent | |
101 | .BR flock () | |
102 | calls on an already locked file will convert an existing lock to the new | |
103 | lock mode. | |
efeece04 | 104 | .PP |
fea681da MK |
105 | Locks created by |
106 | .BR flock () | |
107 | are preserved across an | |
108 | .BR execve (2). | |
efeece04 | 109 | .PP |
fea681da MK |
110 | A shared or exclusive lock can be placed on a file regardless of the |
111 | mode in which the file was opened. | |
47297adb | 112 | .SH RETURN VALUE |
c13182ef MK |
113 | On success, zero is returned. |
114 | On error, \-1 is returned, and | |
fea681da MK |
115 | .I errno |
116 | is set appropriately. | |
117 | .SH ERRORS | |
118 | .TP | |
119 | .B EBADF | |
120 | .I fd | |
0f7bde2b | 121 | is not an open file descriptor. |
fea681da MK |
122 | .TP |
123 | .B EINTR | |
124 | While waiting to acquire a lock, the call was interrupted by | |
01538d0d MK |
125 | delivery of a signal caught by a handler; see |
126 | .BR signal (7). | |
fea681da MK |
127 | .TP |
128 | .B EINVAL | |
129 | .I operation | |
130 | is invalid. | |
131 | .TP | |
132 | .B ENOLCK | |
133 | The kernel ran out of memory for allocating lock records. | |
134 | .TP | |
135 | .B EWOULDBLOCK | |
136 | The file is locked and the | |
137 | .B LOCK_NB | |
138 | flag was selected. | |
47297adb | 139 | .SH CONFORMING TO |
fea681da | 140 | 4.4BSD (the |
2777b1ca | 141 | .BR flock () |
fea681da MK |
142 | call first appeared in 4.2BSD). |
143 | A version of | |
2777b1ca | 144 | .BR flock (), |
fea681da MK |
145 | possibly implemented in terms of |
146 | .BR fcntl (2), | |
008f1ecc | 147 | appears on most UNIX systems. |
fea681da | 148 | .SH NOTES |
b1b70920 MK |
149 | Since kernel 2.0, |
150 | .BR flock () | |
151 | is implemented as a system call in its own right rather | |
152 | than being emulated in the GNU C library as a call to | |
153 | .BR fcntl (2). | |
9cd6730c | 154 | With this implementation, |
b1b70920 MK |
155 | there is no interaction between the types of lock |
156 | placed by | |
157 | .BR flock () | |
158 | and | |
159 | .BR fcntl (2), | |
160 | and | |
161 | .BR flock () | |
162 | does not detect deadlock. | |
9cd6730c | 163 | (Note, however, that on some systems, such as the modern BSDs, |
05a0fb66 MK |
164 | .\" E.g., according to the flock(2) man page, FreeBSD since at least 5.3 |
165 | .BR flock () | |
166 | and | |
167 | .BR fcntl (2) | |
168 | locks | |
169 | .I do | |
170 | interact with one another.) | |
b1b70920 | 171 | .PP |
2777b1ca | 172 | .BR flock () |
fea681da MK |
173 | places advisory locks only; given suitable permissions on a file, |
174 | a process is free to ignore the use of | |
2777b1ca | 175 | .BR flock () |
fea681da MK |
176 | and perform I/O on the file. |
177 | .PP | |
2777b1ca | 178 | .BR flock () |
fea681da MK |
179 | and |
180 | .BR fcntl (2) | |
181 | locks have different semantics with respect to forked processes and | |
182 | .BR dup (2). | |
d04e1109 MK |
183 | On systems that implement |
184 | .BR flock () | |
185 | using | |
0bfa087b | 186 | .BR fcntl (2), |
d04e1109 MK |
187 | the semantics of |
188 | .BR flock () | |
189 | will be different from those described in this manual page. | |
6a916f1c MK |
190 | .PP |
191 | Converting a lock | |
192 | (shared to exclusive, or vice versa) is not guaranteed to be atomic: | |
193 | the existing lock is first removed, and then a new lock is established. | |
194 | Between these two steps, | |
195 | a pending lock request by another process may be granted, | |
c13182ef | 196 | with the result that the conversion either blocks, or fails if |
6a916f1c MK |
197 | .B LOCK_NB |
198 | was specified. | |
d9bfdb9c | 199 | (This is the original BSD behavior, |
6a916f1c MK |
200 | and occurs on many other implementations.) |
201 | .\" Kernel 2.5.21 changed things a little: during lock conversion | |
202 | .\" it is now the highest priority process that will get the lock -- mtk | |
50bfd0b3 MK |
203 | .SS NFS details |
204 | In Linux kernels up to 2.6.11, | |
205 | .BR flock () | |
206 | does not lock files over NFS | |
207 | (i.e., the scope of locks was limited to the local system). | |
208 | Instead, one could use | |
209 | .BR fcntl (2) | |
210 | byte-range locking, which does work over NFS, | |
211 | given a sufficiently recent version of | |
212 | Linux and a server which supports locking. | |
3b53a603 | 213 | .PP |
50bfd0b3 MK |
214 | Since Linux 2.6.12, NFS clients support |
215 | .BR flock () | |
3b53a603 MK |
216 | locks by emulating them as |
217 | .BR fcntl (2) | |
218 | byte-range locks on the entire file. | |
50bfd0b3 MK |
219 | This means that |
220 | .BR fcntl (2) | |
221 | and | |
222 | .BR flock () | |
223 | locks | |
224 | .I do | |
225 | interact with one another over NFS. | |
ff8bc614 MK |
226 | It also means that in order to place an exclusive lock, |
227 | the file must be opened for writing. | |
657a14bf | 228 | .PP |
50bfd0b3 MK |
229 | Since Linux 2.6.37, |
230 | .\" commit 5eebde23223aeb0ad2d9e3be6590ff8bbfab0fc2 | |
231 | the kernel supports a compatibility mode that allows | |
232 | .BR flock () | |
233 | locks (and also | |
234 | .BR fcntl (2) | |
235 | byte region locks) to be treated as local; | |
236 | see the discussion of the | |
237 | .I "local_lock" | |
238 | option in | |
239 | .BR nfs (5). | |
47297adb | 240 | .SH SEE ALSO |
278742f7 | 241 | .BR flock (1), |
fea681da MK |
242 | .BR close (2), |
243 | .BR dup (2), | |
244 | .BR execve (2), | |
245 | .BR fcntl (2), | |
246 | .BR fork (2), | |
247 | .BR open (2), | |
0d7ae27d MK |
248 | .BR lockf (3), |
249 | .BR lslocks (8) | |
efeece04 | 250 | .PP |
fa9efa86 | 251 | .I Documentation/filesystems/locks.txt |
173fe7e7 | 252 | in the Linux kernel source tree |
de544d7f | 253 | .RI ( Documentation/locks.txt |
173fe7e7 | 254 | in older kernels) |