FLOCK(2)

NAME
flock - apply or remove an advisory lock on an open file

SYNOPSIS
#include <fcntl.h>
int
flock(int fd, int operation);

DESCRIPTION
flock() applies or removes an advisory lock on the file associated with the file descriptor fd. The operation argument is one of:

LOCK_SH
    Apply a shared lock.
LOCK_EX
    Apply an exclusive lock.
LOCK_UN
    Remove an existing lock.

LOCK_SH and LOCK_EX may be combined with the optional LOCK_NB for nonblocking mode.

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).

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 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 operation is the bitwise OR of LOCK_NB and LOCK_SH or LOCK_EX, 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

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 operation has an invalid value.

[EOPNOTSUPP]
    The referenced descriptor is not of the correct type.

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

HISTORY
The flock() system call first appeared in 4.1cBSD.