FLOCK(2) BSD System Calls Manual FLOCK(2)
NAME
flock -- apply or remove an advisory lock on an open file
SYNOPSIS
#include <sys/file.h>
#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 */
int
flock(int fd, int operation);
DESCRIPTION
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 operations on files, but do not guar-antee guarantee
antee consistency (i.e., processes may still access files without using advisory locks possibly result-ing resulting
ing in inconsistencies).
The locking mechanism allows two types of locks: shared locks and exclusive locks. At any time multi-ple multiple
ple 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 appropri-ate appropriate
ate 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 LOCK_NB is included in operation, then this will not happen; instead the
call will fail and the error EWOULDBLOCK will be returned.
NOTES
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.
RETURN VALUES
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 errno.
ERRORS
The flock() call fails if:
[EWOULDBLOCK] The file is locked and the LOCK_NB option was specified.
[EBADF] The argument fd is an invalid descriptor.
[EINVAL] The argument fd refers to an object other than a file.
[ENOTSUP] The referenced descriptor is not of the correct type.
SEE ALSO
close(2), dup(2), execve(2), fork(2), open(2)
HISTORY
The flock() function call appeared in 4.2BSD.
4.2 Berkeley Distribution December 11, 1993 4.2 Berkeley Distribution
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