|FLOCK(2)||System Calls Manual||FLOCK(2)|
flock — apply or
remove an advisory lock on an open file
flock() applies or removes an
advisory lock on the file associated with the file
descriptor fd. The operation
argument is one of:
LOCK_EX may be combined with the optional
LOCK_NB 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: 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
operation is the bitwise OR of
LOCK_EX, then this will not happen; instead the call
will fail and the error
EWOULDBLOCK will be
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.
Upon successful completion, the value 0 is returned; otherwise the value -1 is returned and the global variable errno is set to indicate the error.
flock() call fails if:
flock() system call first appeared in
|December 10, 2014||OpenBSD-6.2|