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
EWOULDBLOCK
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:
[EWOULDBLOCK]
The file is locked and the
LOCK_NB
option was specified.
[EBADF]
The argument
fd
is an invalid descriptor.
[EINVAL]
The argument
operation
has an invalid value.
[EOPNOTSUPP]
The referenced descriptor is not of the correct type.