flock — apply or remove an advisory lock on an open file
Apply or remove an advisory lock on the open file
one of the following:
Place a shared lock. More than one process may hold a shared lock for a given file at a given time.
Place an exclusive lock. Only one process may hold an exclusive lock for a given file at a given time.
Remove an existing lock held by this process.
A call to
flock() may block
if an incompatible lock is held by another process. To make a
nonblocking request, include
LOCK_NB (by ORing) with any of the above
A single file may not simultaneously have both shared and exclusive locks.
Locks created by
associated with an open file description (see open(2)). This means that
duplicate file descriptors (created by, for example,
fork(2) or 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
operation on any of these duplicate file descriptors, or when
all such file descriptors have been closed.
If a process uses open(2) (or similar) to
obtain more than one file descriptor for the same file, these
file descriptors are treated independently by
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
A process may hold only one type of lock (shared or
exclusive) on a file. Subsequent
flock() calls on an already locked file
will convert an existing lock to the new lock mode.
Locks created by
preserved across an execve(2).
A shared or exclusive lock can be placed on a file regardless of the mode in which the file was opened.
On success, zero is returned. On error, −1 is
errno is set
fd is not an
open file descriptor.
While waiting to acquire a lock, the call was interrupted by delivery of a signal caught by a handler; see signal(7).
The kernel ran out of memory for allocating lock records.
The file is locked and the
LOCK_NB flag was selected.
first appeared in 4.2BSD). A version of
flock(), possibly implemented in terms of
fcntl(2), appears on most
Since kernel 2.0,
implemented as a system call in its own right rather than
being emulated in the GNU C library as a call to fcntl(2). With this
implementation, there is no interaction between the types of
lock placed by
flock() does not detect deadlock. (Note,
however, that on some systems, such as the modern BSDs,
flock() and fcntl(2) locks
do interact with one
In Linux kernels up to 2.6.11,
flock() does not lock files over NFS (i.e.,
the scope of locks was limited to the local system). Instead,
one could use fcntl(2) byte-range
locking, which does work over NFS, given a sufficiently
recent version of Linux and a server which supports locking.
Since Linux 2.6.12, NFS clients support
flock() locks by emulating them as
byte-range locks on the entire file. This means that
do interact with one another
over NFS. Since Linux 2.6.37, the kernel supports a
compatibility mode that allows
flock() locks (and also fcntl(2) byte region locks)
to be treated as local; see the discussion of the
local_lock option in
flock() places advisory
locks only; given suitable permissions on a file, a process
is free to ignore the use of
flock() and perform I/O on the file.
flock() and fcntl(2) locks have
different semantics with respect to forked processes and
dup(2). On systems that
fcntl(2), the semantics of
flock() will be different from
those described in this manual page.
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
specified. (This is the original BSD behavior, and occurs on
many other implementations.)
Documentation/filesystems/locks.txt in the
Linux kernel source tree (
Documentation/locks.txt in older
This page is part of release 4.07 of the Linux
man-pages project. A
description of the project, information about reporting bugs,
and the latest version of this page, can be found at
Copyright 1993 Rickard E. Faith (faithcs.unc.edu) and
and Copyright 2002 Michael Kerrisk
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Since the Linux kernel and libraries are constantly changing, this
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Modified Fri Jan 31 16:26:07 1997 by Eric S. Raymond <esrthyrsus.com>
Modified Fri Dec 11 17:57:27 1998 by Jamie Lokier <jamieimbolc.ucc.ie>
Modified 24 Apr 2002 by Michael Kerrisk <mtk.manpagesgmail.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.