Manual Reference Pages - FLOCK (2)
- apply or remove an advisory lock on an open file
.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 */
flock int fd int operation
system call applies or removes an
lock on the file associated with the file descriptor
A lock is applied by specifying an
argument that is one of
with the optional addition of
an existing lock
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
The locking mechanism allows two types of 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
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
is included in
then this will not happen; instead the call will fail and
will be returned.
Locks are on files, not file descriptors.
That is, file descriptors
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.
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
any record within the file will be seen as locked
from the viewpoint of another process using
and vice versa.
Processes blocked awaiting a lock may be awakened by signals.
.Rv -std flock
system call fails if:
The file is locked and the
option was specified.
is an invalid descriptor.
refers to an object other than a file.
refers to an object that does not support file locking.
A lock was requested, but no locks are available.
system call appeared in
BSD 4.2 .
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