1 .\" Copyright 1993 Rickard E. Faith (faith@cs.unc.edu) and
2 .\" and Copyright 2002 Michael Kerrisk
4 .\" %%%LICENSE_START(VERBATIM)
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.
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.
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15 .\" manual page may be incorrect or out-of-date.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
22 .\" Formatted or processed versions of this manual, if unaccompanied by
23 .\" the source, must acknowledge the copyright and authors of this work.
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>
28 .\" Modified 24 Apr 2002 by Michael Kerrisk <mtk.manpages@gmail.com>
29 .\" Substantial rewrites and additions
30 .\" 2005-05-10 mtk, noted that lock conversions are not atomic.
32 .\" FIXME: Maybe document LOCK_MAND, LOCK_RW, LOCK_READ, LOCK_WRITE
33 .\" which only have effect for SAMBA.
35 .TH FLOCK 2 2014-04-22 "Linux" "Linux Programmer's Manual"
37 flock \- apply or remove an advisory lock on an open file
39 .B #include <sys/file.h>
41 .BI "int flock(int " fd ", int " operation );
43 Apply or remove an advisory lock on the open file specified by
47 is one of the following:
52 More than one process may hold a shared lock for a given file
56 Place an exclusive lock.
57 Only one process may hold an exclusive lock for a given
61 Remove an existing lock held by this process.
66 may block if an incompatible lock is held by another process.
67 To make a nonblocking request, include
70 with any of the above operations.
72 A single file may not simultaneously have both shared and exclusive locks.
76 are associated with an open file description (see
78 This means that duplicate file descriptors (created by, for example,
82 refer to the same lock, and this lock may be modified
83 or released using any of these descriptors.
84 Furthermore, the lock is released either by an explicit
86 operation on any of these duplicate descriptors, or when all
87 such descriptors have been closed.
91 (or similar) to obtain more than one descriptor for the same file,
92 these descriptors are treated independently by
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
96 already placed via another descriptor.
98 A process may hold only one type of lock (shared or exclusive)
102 calls on an already locked file will convert an existing lock to the new
107 are preserved across an
110 A shared or exclusive lock can be placed on a file regardless of the
111 mode in which the file was opened.
113 On success, zero is returned.
114 On error, \-1 is returned, and
116 is set appropriately.
121 is not an open file descriptor.
124 While waiting to acquire a lock, the call was interrupted by
125 delivery of a signal caught by a handler; see
133 The kernel ran out of memory for allocating lock records.
136 The file is locked and the
142 call first appeared in 4.2BSD).
145 possibly implemented in terms of
147 appears on most UNIX systems.
150 does not lock files over NFS.
153 instead: that does work over NFS, given a sufficiently recent version of
154 Linux and a server which supports locking.
158 is implemented as a system call in its own right rather
159 than being emulated in the GNU C library as a call to
161 This yields true BSD semantics:
162 there is no interaction between the types of lock
169 does not detect deadlock.
172 places advisory locks only; given suitable permissions on a file,
173 a process is free to ignore the use of
175 and perform I/O on the file.
180 locks have different semantics with respect to forked processes and
182 On systems that implement
188 will be different from those described in this manual page.
191 (shared to exclusive, or vice versa) is not guaranteed to be atomic:
192 the existing lock is first removed, and then a new lock is established.
193 Between these two steps,
194 a pending lock request by another process may be granted,
195 with the result that the conversion either blocks, or fails if
198 (This is the original BSD behavior,
199 and occurs on many other implementations.)
200 .\" Kernel 2.5.21 changed things a little: during lock conversion
201 .\" it is now the highest priority process that will get the lock -- mtk
212 .I Documentation/filesystems/locks.txt
213 in the Linux kernel source tree
214 .RI ( Documentation/locks.txt
217 This page is part of release 3.67 of the Linux
220 A description of the project,
221 information about reporting bugs,
222 and the latest version of this page,
224 \%http://www.kernel.org/doc/man\-pages/.