NAME
flock —
apply or remove an advisory
lock on an open file
LIBRARY
Standard C Library (libc, -lc)
SYNOPSIS
#include <fcntl.h>
#define LOCK_SH 1 /* shared lock */
#define LOCK_EX 2 /* exclusive lock */
#define LOCK_NB 4 /* don't block when locking */
#define LOCK_UN 8 /* unlock */
int
flock(
int
fd,
int operation);
DESCRIPTION
flock() applies or removes an
advisory lock
on the file associated with the file descriptor
fd. A
lock is applied by specifying an
operation parameter
that is one of
LOCK_SH
or
LOCK_EX
with the optional addition of
LOCK_NB
. To unlock an existing lock
operation
should be
LOCK_UN
.
Advisory locks allow cooperating processes to perform consistent operations on
files, but do not guarantee consistency (i.e., processes may still access
files without using advisory locks possibly resulting in inconsistencies).
The locking mechanism allows two types of locks:
shared locks
and
exclusive locks. At any time multiple shared locks may
be applied to a file, but at no time are multiple exclusive, or both shared
and exclusive, locks allowed simultaneously on a file.
A shared lock may be
upgraded to an exclusive lock, and vice
versa, simply by specifying the appropriate lock type; this results in the
previous lock being released and the new lock applied (possibly after other
processes have gained and released the lock).
Requesting a lock on an object that is already locked normally causes the caller
to be blocked until the lock may be acquired. If
LOCK_NB
is included in
operation, then this will not happen; instead the call
will fail and the error
EAGAIN
will be returned.
NOTES
Locks are on files, not file descriptors. That is, file descriptors duplicated
through
dup(2) or
fork(2) do not result in multiple
instances of a lock, but rather multiple references to a single lock. If a
process holding a lock on a file forks and the child explicitly unlocks the
file, the parent will lose its lock.
Processes blocked awaiting a lock may be awakened by signals.
RETURN VALUES
Zero is returned if the operation was successful; on an error a -1 is returned
and an error code is left in the global location
errno.
ERRORS
The
flock() call fails if:
-
-
- [
EAGAIN
]
- The file is locked and the
LOCK_NB
option was specified.
-
-
- [
EBADF
]
- The argument fd is an invalid
descriptor.
-
-
- [
EINVAL
]
- The argument operation does not
include exactly one of
LOCK_EX
,
LOCK_SH
, or LOCK_UN
.
-
-
- [
EOPNOTSUPP
]
- The argument fd refers to an object
other than a file.
SEE ALSO
close(2),
dup(2),
execve(2),
fork(2),
open(2),
flockfile(3),
lockf(3)
HISTORY
The
flock() function call appeared in
4.2BSD.