flock(2) apply or remove an advisory lock on an open file


Lb libc


In sys/file.h Fd #define LOCK_SH 0x01 /* shared file lock */ Fd #define LOCK_EX 0x02 /* exclusive file lock */ Fd #define LOCK_NB 0x04 /* do not block when locking */ Fd #define LOCK_UN 0x08 /* unlock file */ Ft int Fn flock int fd int operation


The Fn flock system call applies or removes an advisory lock on the file associated with the file descriptor Fa fd . A lock is applied by specifying an Fa operation argument 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 Fa operation , then this will not happen; instead the call will fail and the error Er 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 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.

The Fn flock , fcntl(2), and lockf(3) locks are compatible. Processes using different locking interfaces can cooperate over the same file safely. However, only one of such interfaces should be used within the same process. If a file is locked by a process through Fn flock , any record within the file will be seen as locked from the viewpoint of another process using fcntl(2) or lockf(3), and vice versa.

Processes blocked awaiting a lock may be awakened by signals.


Rv -std flock


The Fn flock system call fails if:

The file is locked and the LOCK_NB option was specified.
The argument Fa fd is an invalid descriptor.
The argument Fa fd refers to an object other than a file.
The argument Fa fd refers to an object that does not support file locking.
A lock was requested, but no locks are available.


The Fn flock system call appeared in BSD 4.2