FreeBSD man pages : flock (2)
FLOCK(2) FreeBSD System Calls Manual FLOCK(2)
flock - apply or remove an advisory lock on an open file
Standard C Library (libc, -lc)
#define LOCK_SH 0x01 /* shared file lock */
#define LOCK_EX 0x02 /* exclusive file lock */
#define LOCK_NB 0x04 /* don't block when locking */
#define LOCK_UN 0x08 /* unlock file */
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 operation
parameter that is one of LOCK_SH or LOCK_EX with the optional addition of
LOCK_NB. To unlock an existing lock operation should be LOCK_UN.
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 LOCK_NB is
included in operation, 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.
The flock() function returns the value 0 if successful; 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 fd refers to an object other than a file.
[EOPNOTSUPP] The argument fd refers to an object that does not sup-
port file locking.
close(2), dup(2), execve(2), fork(2), open(2)
The flock() function call appeared in 4.2BSD.
FreeBSD 4.8 December 11, 1993 FreeBSD 4.8