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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 | .\" |
5fbde956 | 4 | .\" SPDX-License-Identifier: Linux-man-pages-copyleft |
fea681da MK |
5 | .\" |
6 | .\" Modified Fri Jan 31 16:26:07 1997 by Eric S. Raymond <esr@thyrsus.com> | |
7 | .\" Modified Fri Dec 11 17:57:27 1998 by Jamie Lokier <jamie@imbolc.ucc.ie> | |
c11b1abf | 8 | .\" Modified 24 Apr 2002 by Michael Kerrisk <mtk.manpages@gmail.com> |
fea681da | 9 | .\" Substantial rewrites and additions |
6a916f1c | 10 | .\" 2005-05-10 mtk, noted that lock conversions are not atomic. |
fea681da | 11 | .\" |
bea08fec MK |
12 | .\" FIXME Maybe document LOCK_MAND, LOCK_RW, LOCK_READ, LOCK_WRITE |
13 | .\" which only have effect for SAMBA. | |
4784c377 | 14 | .\" |
4c1c5274 | 15 | .TH flock 2 (date) "Linux man-pages (unreleased)" |
fea681da MK |
16 | .SH NAME |
17 | flock \- apply or remove an advisory lock on an open file | |
2369f403 AC |
18 | .SH LIBRARY |
19 | Standard C library | |
8fc3b2cf | 20 | .RI ( libc ", " \-lc ) |
fea681da | 21 | .SH SYNOPSIS |
c7db92b9 | 22 | .nf |
fea681da | 23 | .B #include <sys/file.h> |
c6d039a3 | 24 | .P |
964d4d9c | 25 | .BI "int flock(int " fd ", int " operation ); |
c7db92b9 | 26 | .fi |
fea681da MK |
27 | .SH DESCRIPTION |
28 | Apply or remove an advisory lock on the open file specified by | |
29 | .IR fd . | |
c4bb193f | 30 | The argument |
fea681da MK |
31 | .I operation |
32 | is one of the following: | |
54f73b77 MK |
33 | .RS 4 |
34 | .TP 9 | |
fea681da MK |
35 | .B LOCK_SH |
36 | Place a shared lock. | |
37 | More than one process may hold a shared lock for a given file | |
38 | at a given time. | |
39 | .TP | |
40 | .B LOCK_EX | |
41 | Place an exclusive lock. | |
42 | Only one process may hold an exclusive lock for a given | |
43 | file at a given time. | |
44 | .TP | |
45 | .B LOCK_UN | |
46 | Remove an existing lock held by this process. | |
fea681da | 47 | .RE |
c6d039a3 | 48 | .P |
fea681da MK |
49 | A call to |
50 | .BR flock () | |
51 | may block if an incompatible lock is held by another process. | |
ff40dbb3 | 52 | To make a nonblocking request, include |
fea681da | 53 | .B LOCK_NB |
cebca1bd | 54 | (by ORing) |
fea681da | 55 | with any of the above operations. |
c6d039a3 | 56 | .P |
fea681da | 57 | A single file may not simultaneously have both shared and exclusive locks. |
c6d039a3 | 58 | .P |
fea681da MK |
59 | Locks created by |
60 | .BR flock () | |
61a37c81 MK |
61 | are associated with an open file description (see |
62 | .BR open (2)). | |
fea681da | 63 | This means that duplicate file descriptors (created by, for example, |
c13182ef | 64 | .BR fork (2) |
35e21ba7 MK |
65 | or |
66 | .BR dup (2)) | |
fea681da | 67 | refer to the same lock, and this lock may be modified |
d9cb0d7d | 68 | or released using any of these file descriptors. |
fea681da MK |
69 | Furthermore, the lock is released either by an explicit |
70 | .B LOCK_UN | |
d9cb0d7d MK |
71 | operation on any of these duplicate file descriptors, or when all |
72 | such file descriptors have been closed. | |
c6d039a3 | 73 | .P |
d04e1109 MK |
74 | If a process uses |
75 | .BR open (2) | |
d9cb0d7d MK |
76 | (or similar) to obtain more than one file descriptor for the same file, |
77 | these file descriptors are treated independently by | |
d04e1109 MK |
78 | .BR flock (). |
79 | An attempt to lock the file using one of these file descriptors | |
80 | may be denied by a lock that the calling process has | |
d9cb0d7d | 81 | already placed via another file descriptor. |
c6d039a3 | 82 | .P |
33a0ccb2 | 83 | A process may hold only one type of lock (shared or exclusive) |
fea681da MK |
84 | on a file. |
85 | Subsequent | |
86 | .BR flock () | |
87 | calls on an already locked file will convert an existing lock to the new | |
88 | lock mode. | |
c6d039a3 | 89 | .P |
fea681da MK |
90 | Locks created by |
91 | .BR flock () | |
92 | are preserved across an | |
93 | .BR execve (2). | |
c6d039a3 | 94 | .P |
fea681da MK |
95 | A shared or exclusive lock can be placed on a file regardless of the |
96 | mode in which the file was opened. | |
47297adb | 97 | .SH RETURN VALUE |
c13182ef MK |
98 | On success, zero is returned. |
99 | On error, \-1 is returned, and | |
fea681da | 100 | .I errno |
f6a4078b | 101 | is set to indicate the error. |
fea681da MK |
102 | .SH ERRORS |
103 | .TP | |
104 | .B EBADF | |
105 | .I fd | |
0f7bde2b | 106 | is not an open file descriptor. |
fea681da MK |
107 | .TP |
108 | .B EINTR | |
109 | While waiting to acquire a lock, the call was interrupted by | |
01538d0d MK |
110 | delivery of a signal caught by a handler; see |
111 | .BR signal (7). | |
fea681da MK |
112 | .TP |
113 | .B EINVAL | |
114 | .I operation | |
115 | is invalid. | |
116 | .TP | |
117 | .B ENOLCK | |
118 | The kernel ran out of memory for allocating lock records. | |
119 | .TP | |
120 | .B EWOULDBLOCK | |
121 | The file is locked and the | |
122 | .B LOCK_NB | |
123 | flag was selected. | |
4131356c | 124 | .SH VERSIONS |
b324e17d | 125 | Since Linux 2.0, |
b1b70920 MK |
126 | .BR flock () |
127 | is implemented as a system call in its own right rather | |
128 | than being emulated in the GNU C library as a call to | |
129 | .BR fcntl (2). | |
9cd6730c | 130 | With this implementation, |
b1b70920 MK |
131 | there is no interaction between the types of lock |
132 | placed by | |
133 | .BR flock () | |
134 | and | |
135 | .BR fcntl (2), | |
136 | and | |
137 | .BR flock () | |
138 | does not detect deadlock. | |
9cd6730c | 139 | (Note, however, that on some systems, such as the modern BSDs, |
05a0fb66 MK |
140 | .\" E.g., according to the flock(2) man page, FreeBSD since at least 5.3 |
141 | .BR flock () | |
142 | and | |
143 | .BR fcntl (2) | |
144 | locks | |
145 | .I do | |
146 | interact with one another.) | |
4131356c AC |
147 | .SS CIFS details |
148 | Up to Linux 5.4, | |
2777b1ca | 149 | .BR flock () |
4131356c AC |
150 | is not propagated over SMB. |
151 | A file with such locks will not appear locked for remote clients. | |
c6d039a3 | 152 | .P |
4131356c | 153 | Since Linux 5.5, |
2777b1ca | 154 | .BR flock () |
4131356c AC |
155 | locks are emulated with SMB byte-range locks on the entire file. |
156 | Similarly to NFS, this means that | |
fea681da | 157 | .BR fcntl (2) |
4131356c | 158 | and |
d04e1109 | 159 | .BR flock () |
4131356c AC |
160 | locks interact with one another. |
161 | Another important side-effect is that the locks are not advisory anymore: | |
162 | any IO on a locked file will always fail with | |
163 | .B EACCES | |
164 | when done from a separate file descriptor. | |
165 | This difference originates from the design of locks in the SMB protocol, | |
166 | which provides mandatory locking semantics. | |
c6d039a3 | 167 | .P |
4131356c AC |
168 | Remote and mandatory locking semantics may vary with |
169 | SMB protocol, mount options and server type. | |
170 | See | |
171 | .BR mount.cifs (8) | |
172 | for additional information. | |
173 | .SH STANDARDS | |
174 | BSD. | |
175 | .SH HISTORY | |
176 | 4.4BSD (the | |
177 | .BR flock () | |
178 | call first appeared in 4.2BSD). | |
179 | A version of | |
180 | .BR flock (), | |
181 | possibly implemented in terms of | |
182 | .BR fcntl (2), | |
183 | appears on most UNIX systems. | |
50bfd0b3 | 184 | .SS NFS details |
b324e17d | 185 | Up to Linux 2.6.11, |
50bfd0b3 MK |
186 | .BR flock () |
187 | does not lock files over NFS | |
188 | (i.e., the scope of locks was limited to the local system). | |
189 | Instead, one could use | |
190 | .BR fcntl (2) | |
191 | byte-range locking, which does work over NFS, | |
192 | given a sufficiently recent version of | |
193 | Linux and a server which supports locking. | |
c6d039a3 | 194 | .P |
50bfd0b3 MK |
195 | Since Linux 2.6.12, NFS clients support |
196 | .BR flock () | |
3b53a603 MK |
197 | locks by emulating them as |
198 | .BR fcntl (2) | |
199 | byte-range locks on the entire file. | |
50bfd0b3 MK |
200 | This means that |
201 | .BR fcntl (2) | |
202 | and | |
203 | .BR flock () | |
204 | locks | |
205 | .I do | |
206 | interact with one another over NFS. | |
ff8bc614 MK |
207 | It also means that in order to place an exclusive lock, |
208 | the file must be opened for writing. | |
c6d039a3 | 209 | .P |
50bfd0b3 MK |
210 | Since Linux 2.6.37, |
211 | .\" commit 5eebde23223aeb0ad2d9e3be6590ff8bbfab0fc2 | |
212 | the kernel supports a compatibility mode that allows | |
213 | .BR flock () | |
214 | locks (and also | |
215 | .BR fcntl (2) | |
216 | byte region locks) to be treated as local; | |
217 | see the discussion of the | |
218 | .I "local_lock" | |
219 | option in | |
220 | .BR nfs (5). | |
4131356c | 221 | .SH NOTES |
334ed979 | 222 | .BR flock () |
4131356c AC |
223 | places advisory locks only; given suitable permissions on a file, |
224 | a process is free to ignore the use of | |
225 | .BR flock () | |
226 | and perform I/O on the file. | |
c6d039a3 | 227 | .P |
334ed979 | 228 | .BR flock () |
334ed979 | 229 | and |
4131356c AC |
230 | .BR fcntl (2) |
231 | locks have different semantics with respect to forked processes and | |
232 | .BR dup (2). | |
233 | On systems that implement | |
334ed979 | 234 | .BR flock () |
4131356c AC |
235 | using |
236 | .BR fcntl (2), | |
237 | the semantics of | |
238 | .BR flock () | |
239 | will be different from those described in this manual page. | |
c6d039a3 | 240 | .P |
4131356c AC |
241 | Converting a lock |
242 | (shared to exclusive, or vice versa) is not guaranteed to be atomic: | |
243 | the existing lock is first removed, and then a new lock is established. | |
244 | Between these two steps, | |
245 | a pending lock request by another process may be granted, | |
246 | with the result that the conversion either blocks, or fails if | |
247 | .B LOCK_NB | |
248 | was specified. | |
249 | (This is the original BSD behavior, | |
250 | and occurs on many other implementations.) | |
251 | .\" Kernel 2.5.21 changed things a little: during lock conversion | |
252 | .\" it is now the highest priority process that will get the lock -- mtk | |
47297adb | 253 | .SH SEE ALSO |
278742f7 | 254 | .BR flock (1), |
fea681da MK |
255 | .BR close (2), |
256 | .BR dup (2), | |
257 | .BR execve (2), | |
258 | .BR fcntl (2), | |
259 | .BR fork (2), | |
260 | .BR open (2), | |
0d7ae27d MK |
261 | .BR lockf (3), |
262 | .BR lslocks (8) | |
c6d039a3 | 263 | .P |
fa9efa86 | 264 | .I Documentation/filesystems/locks.txt |
173fe7e7 | 265 | in the Linux kernel source tree |
de544d7f | 266 | .RI ( Documentation/locks.txt |
173fe7e7 | 267 | in older kernels) |