FLOCK(2) | System Calls Manual | FLOCK(2) |
flock
—
#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);
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.
Processes blocked awaiting a lock may be awakened by signals.
flock
() call fails if:
EAGAIN
]LOCK_NB
option was
specified.EBADF
]EINVAL
]LOCK_EX
, LOCK_SH
, or
LOCK_UN
.EOPNOTSUPP
]flock
() function call appeared in
4.2BSD.
October 15, 2011 | NetBSD 9.2 |