summaryrefslogtreecommitdiffstats
path: root/man2/flock.2
blob: 5f2917f428b7ac4e8b427c8db1fd034051b5c3aa (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
.\" Copyright 1993 Rickard E. Faith (faith@cs.unc.edu) and
.\" and Copyright 2002 Michael Kerrisk
.\"
.\" SPDX-License-Identifier: Linux-man-pages-copyleft
.\"
.\" Modified Fri Jan 31 16:26:07 1997 by Eric S. Raymond <esr@thyrsus.com>
.\" Modified Fri Dec 11 17:57:27 1998 by Jamie Lokier <jamie@imbolc.ucc.ie>
.\" Modified 24 Apr 2002 by Michael Kerrisk <mtk.manpages@gmail.com>
.\"	Substantial rewrites and additions
.\" 2005-05-10 mtk, noted that lock conversions are not atomic.
.\"
.\" FIXME Maybe document LOCK_MAND, LOCK_RW, LOCK_READ, LOCK_WRITE
.\" which only have effect for SAMBA.
.\"
.TH flock 2 2023-03-30 "Linux man-pages 6.05.01"
.SH NAME
flock \- apply or remove an advisory lock on an open file
.SH LIBRARY
Standard C library
.RI ( libc ", " \-lc )
.SH SYNOPSIS
.nf
.B #include <sys/file.h>
.PP
.BI "int flock(int " fd ", int " operation );
.fi
.SH DESCRIPTION
Apply or remove an advisory lock on the open file specified by
.IR fd .
The argument
.I operation
is one of the following:
.RS 4
.TP 9
.B LOCK_SH
Place a shared lock.
More than one process may hold a shared lock for a given file
at a given time.
.TP
.B LOCK_EX
Place an exclusive lock.
Only one process may hold an exclusive lock for a given
file at a given time.
.TP
.B LOCK_UN
Remove an existing lock held by this process.
.RE
.PP
A call to
.BR flock ()
may block if an incompatible lock is held by another process.
To make a nonblocking request, include
.B LOCK_NB
(by ORing)
with any of the above operations.
.PP
A single file may not simultaneously have both shared and exclusive locks.
.PP
Locks created by
.BR flock ()
are associated with an open file description (see
.BR open (2)).
This means that duplicate file descriptors (created by, for example,
.BR fork (2)
or
.BR dup (2))
refer to the same lock, and this lock may be modified
or released using any of these file descriptors.
Furthermore, the lock is released either by an explicit
.B LOCK_UN
operation on any of these duplicate file descriptors, or when all
such file descriptors have been closed.
.PP
If a process uses
.BR open (2)
(or similar) to obtain more than one file descriptor for the same file,
these file descriptors are treated independently by
.BR flock ().
An attempt to lock the file using one of these file descriptors
may be denied by a lock that the calling process has
already placed via another file descriptor.
.PP
A process may hold only one type of lock (shared or exclusive)
on a file.
Subsequent
.BR flock ()
calls on an already locked file will convert an existing lock to the new
lock mode.
.PP
Locks created by
.BR flock ()
are preserved across an
.BR execve (2).
.PP
A shared or exclusive lock can be placed on a file regardless of the
mode in which the file was opened.
.SH RETURN VALUE
On success, zero is returned.
On error, \-1 is returned, and
.I errno
is set to indicate the error.
.SH ERRORS
.TP
.B EBADF
.I fd
is not an open file descriptor.
.TP
.B EINTR
While waiting to acquire a lock, the call was interrupted by
delivery of a signal caught by a handler; see
.BR signal (7).
.TP
.B EINVAL
.I operation
is invalid.
.TP
.B ENOLCK
The kernel ran out of memory for allocating lock records.
.TP
.B EWOULDBLOCK
The file is locked and the
.B LOCK_NB
flag was selected.
.SH VERSIONS
Since Linux 2.0,
.BR flock ()
is implemented as a system call in its own right rather
than being emulated in the GNU C library as a call to
.BR fcntl (2).
With this implementation,
there is no interaction between the types of lock
placed by
.BR flock ()
and
.BR fcntl (2),
and
.BR flock ()
does not detect deadlock.
(Note, however, that on some systems, such as the modern BSDs,
.\" E.g., according to the flock(2) man page, FreeBSD since at least 5.3
.BR flock ()
and
.BR fcntl (2)
locks
.I do
interact with one another.)
.SS CIFS details
Up to Linux 5.4,
.BR flock ()
is not propagated over SMB.
A file with such locks will not appear locked for remote clients.
.PP
Since Linux 5.5,
.BR flock ()
locks are emulated with SMB byte-range locks on the entire file.
Similarly to NFS, this means that
.BR fcntl (2)
and
.BR flock ()
locks interact with one another.
Another important side-effect is that the locks are not advisory anymore:
any IO on a locked file will always fail with
.B EACCES
when done from a separate file descriptor.
This difference originates from the design of locks in the SMB protocol,
which provides mandatory locking semantics.
.PP
Remote and mandatory locking semantics may vary with
SMB protocol, mount options and server type.
See
.BR mount.cifs (8)
for additional information.
.SH STANDARDS
BSD.
.SH HISTORY
4.4BSD (the
.BR flock ()
call first appeared in 4.2BSD).
A version of
.BR flock (),
possibly implemented in terms of
.BR fcntl (2),
appears on most UNIX systems.
.SS NFS details
Up to Linux 2.6.11,
.BR flock ()
does not lock files over NFS
(i.e., the scope of locks was limited to the local system).
Instead, one could use
.BR fcntl (2)
byte-range locking, which does work over NFS,
given a sufficiently recent version of
Linux and a server which supports locking.
.PP
Since Linux 2.6.12, NFS clients support
.BR flock ()
locks by emulating them as
.BR fcntl (2)
byte-range locks on the entire file.
This means that
.BR fcntl (2)
and
.BR flock ()
locks
.I do
interact with one another over NFS.
It also means that in order to place an exclusive lock,
the file must be opened for writing.
.PP
Since Linux 2.6.37,
.\" commit 5eebde23223aeb0ad2d9e3be6590ff8bbfab0fc2
the kernel supports a compatibility mode that allows
.BR flock ()
locks (and also
.BR fcntl (2)
byte region locks) to be treated as local;
see the discussion of the
.I "local_lock"
option in
.BR nfs (5).
.SH NOTES
.BR flock ()
places advisory locks only; given suitable permissions on a file,
a process is free to ignore the use of
.BR flock ()
and perform I/O on the file.
.PP
.BR flock ()
and
.BR fcntl (2)
locks have different semantics with respect to forked processes and
.BR dup (2).
On systems that implement
.BR flock ()
using
.BR fcntl (2),
the semantics of
.BR flock ()
will be different from those described in this manual page.
.PP
Converting a lock
(shared to exclusive, or vice versa) is not guaranteed to be atomic:
the existing lock is first removed, and then a new lock is established.
Between these two steps,
a pending lock request by another process may be granted,
with the result that the conversion either blocks, or fails if
.B LOCK_NB
was specified.
(This is the original BSD behavior,
and occurs on many other implementations.)
.\" Kernel 2.5.21 changed things a little: during lock conversion
.\" it is now the highest priority process that will get the lock -- mtk
.SH SEE ALSO
.BR flock (1),
.BR close (2),
.BR dup (2),
.BR execve (2),
.BR fcntl (2),
.BR fork (2),
.BR open (2),
.BR lockf (3),
.BR lslocks (8)
.PP
.I Documentation/filesystems/locks.txt
in the Linux kernel source tree
.RI ( Documentation/locks.txt
in older kernels)