|FLOCK(2)||System Calls Manual||FLOCK(2)|
flock — apply or
remove an advisory lock on an open file
#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 */
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_EX with the
optional addition of
LOCK_NB. To unlock an existing
operation should be
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.
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.
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.
flock() call fails if:
flock() function call appeared in
|October 15, 2011||NetBSD-7.0.1|