NAME
flock
—
apply or remove an advisory lock on an
open file
LIBRARY
library “libc”
SYNOPSIS
#include
<fcntl.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 guarantee consistency (i.e., processes may still access files without using advisory locks possibly resulting in inconsistencies).
A shared lock may be upgraded to an exclusive lock, and vice versa, simply by specifying the appropriate 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 EAGAIN
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:
- [
EAGAIN
] - The file is locked and the
LOCK_NB
option was specified. - [
EBADF
] - The argument fd is an invalid descriptor.
- [
EINVAL
] - The argument operation does not include exactly one
of
LOCK_EX
,LOCK_SH
, orLOCK_UN
. - [
EOPNOTSUPP
] - The argument fd refers to an object other than a file.
SEE ALSO
close(2), dup(2), execve(2), fork(2), open(2), flockfile(3), lockf(3)
HISTORY
The flock
() function call appeared in
4.2BSD.