flock - apply or remove an advisory lock on an open file
#define LOCK_SH 1 /* shared lock */
#define LOCK_EX 2 /* exclusive lock */
#define LOCK_NB 4 /* don't block when locking */
#define LOCK_UN 8 /* unlock */
flock(int fd, int operation);
flock() applies or removes an advisory lock on the file associated with
the file descriptor fd. A lock is applied by specifying an
that is one of LOCK_SH or LOCK_EX with the optional
LOCK_NB. To unlock an existing lock, operation 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: shared
exclusive locks. At any time multiple shared locks may be
applied to a
file, but at no time are multiple exclusive, or both shared
locks allowed simultaneously on a file.
A shared lock may be upgraded to an exclusive lock, and vice
by specifying the appropriate lock type; this results in
lock being released and the new lock applied (possibly after
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
fail and the error EWOULDBLOCK will be returned.
Locks are on files, not file descriptors. That is, file descriptors duplicated
through dup(2) or fork(2) do not result in multiple
a lock, but rather multiple references to a single lock. If
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 flock() call fails if:
The file is locked and the LOCK_NB option was
[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.
close(2), dup(2), execve(2), fcntl(2), fork(2), open(2)
The flock() function call appeared in 4.2BSD.
OpenBSD 3.6 December 11, 1993
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