FLOCK(2) BSD System Calls Manual FLOCK(2)
flock -- apply or remove an advisory lock on an open file
flock(int fd, int operation);
flock() applies or removes an advisory lock on the file associated with
the file descriptor fd. The operation argument is one of:
LOCK_SH Apply a shared lock.
LOCK_EX Apply an exclusive lock.
LOCK_UN Remove an existing lock.
LOCK_SH and LOCK_EX may be combined with the optional LOCK_NB for non-
Advisory locks allow cooperating processes to perform consistent opera-
tions 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: 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 exclu-
sive, locks allowed simultaneously on a file.
A shared lock may be upgraded to an exclusive lock, and vice versa, sim-
ply by specifying the appropriate lock type; this results in the previous
lock being released and the new lock applied (possibly after other pro-
cesses 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_NB and LOCK_SH or LOCK_EX, then this will not happen;
instead the call will 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 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
The flock() call fails if:
[EWOULDBLOCK] The file is locked and the LOCK_NB option was speci-
[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() system call first appeared in 4.1cBSD.
BSD December 10, 2014 BSD