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.