applies or removes an
lock on the file associated with the file descriptor
A lock is applied by specifying an
parameter that is one of
with the optional addition of
To unlock an existing lock,
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
The locking mechanism allows two types of 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
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.
is included in
then this will not happen; instead the call will fail and the error
will be returned.
Locks are on files, not file descriptors.
That is, file descriptors duplicated through
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
call fails if:
The file is locked and the
option was specified.
is an invalid descriptor.
has an invalid value.
The referenced descriptor is not of the correct type.