FLOCK(2) MachTen Programmer’s 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 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-
consistencies).

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.

NOTES
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.

RETURN VALUES
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.

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.

SEE ALSO
open(2), close(2), dup(2), execve(2), fork(2)

HISTORY
The flock function call appeared in 4.2BSD.

4.2 Berkeley Distribution December 11, 1993 2