flock - apply or remove an advisory lock on an open file
#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 fd, 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 the inclusive or of
LOCK_SH or LOCK_EX and, possibly, LOCK_NB. To unlock an existing lock operation should be
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 advi-
sory locks possibly resulting in inconsistencies).
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 exclu-
sive, 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 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 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 ref-
erences 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 returned and an
error code is left in the global location errno.
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.
open(2), close(2), dup(2), execve(2), fork(2)
4.2 Berkeley Distribution May 22, 1986 FLOCK(2)