FLOCK(2)

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

SYNOPSIS
#include <sys/file.h>
#define LOCK_SH 0x01 /* shared file lock */
#define LOCK_EX 0x02 /* exclusive file lock */
#define LOCK_NB 0x04 /* don't block when locking */
#define LOCK_UN 0x08 /* unlock file */
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).

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 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.

IMPLEMENTATION NOTES

In the non-threaded library flock() is implemented as the flock syscall.

In the threaded library, the flock syscall is assembled to _thread_sys_flock() and flock() is implemented as a function which locks fd for read and write, then calls _thread_sys_flock(). Before returning, flock() unlocks fd.

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.

[EOPNOTSUPP]
    The argument fd refers to an object that does not support file locking.

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

HISTORY
The flock() function call appeared in 4.2BSD.