int
flock(
int fd
, int operation
)
)
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.
)
call fails if:
EAGAIN
]
LOCK_NB
option was specified.
EBADF
]
fd
is an invalid descriptor.
EOPNOTSUPP
]
fd
refers to an object other than a file.
EINVAL
]
operation
does not include one of
LOCK_EX
,
LOCK_SH
or
LOCK_UN
.
)
function call appeared in
4.2BSD.