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FLOCK(2) NetBSD Programmer's Manual FLOCK(2)
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Maintained for NetBSD
by Kimmo Suominen.
Based on man-cgi by Panagiotis Christias.
flock - apply or remove an advisory lock on an open file
Standard C Library (libc, -lc)
#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 operation pa-
rameter 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 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 in-
cluded 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 du-
plicated 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.
Zero is returned if the operation was successful; on an error a -1 is re-
turned and an error code is left in the global location errno.
The flock() call fails if:
The file is locked and the LOCK_NB option was specified.
[EBADF] The argument fd is an invalid descriptor.
[EOPNOTSUPP] The argument fd refers to an object other than a file.
[EINVAL] The argument operation does not include one of LOCK_EX,
LOCK_SH or LOCK_UN.
open(2), close(2), dup(2), execve(2), fork(2)
The flock() function call appeared in 4.2BSD.
NetBSD 1.5 December 11, 1993 2