— apply or
remove an advisory lock on an open file
() applies or removes an
lock on the file associated with the
file descriptor fd
argument is one of:
may be combined with the optional
for nonblocking mode.
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. 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
is the bitwise OR of
, 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
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.
Upon successful completion, the value 0 is returned; otherwise the
value -1 is returned and the global variable
is set to indicate the error.
() call fails if:
- The file is locked and the
LOCK_NB option was specified.
- The argument fd is an
- The argument operation has
an invalid value.
- The referenced descriptor is not of the correct type.
() system call first appeared in