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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 or with
the optional addition of To unlock an existing lock 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: locks and 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. If 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 or
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
The call fails if: The file
is locked and the option was specified. The argument is an invalid descriptor.
The argument refers to an object other than a file.
The
function call appeared in
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