.\" Copyright (c) 1983, 1991, 1993
.\" The Regents of the University of California. All rights reserved.
.\" %sccs.include.redist.man%
.\" @(#)flock.2 8.2 (Berkeley) %G%
.Nd "apply or remove an advisory lock on an open file"
.Fd #include <sys/file.h>
.Fd #define LOCK_SH 1 /* shared lock */
.Fd #define LOCK_EX 2 /* exclusive lock */
.Fd #define LOCK_NB 4 /* don't block when locking */
.Fd #define LOCK_UN 8 /* unlock */
.Fn flock "int fd" "int operation"
lock on the file associated with the file descriptor
A lock is applied by specifying an
with the optional addition of
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.
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
then this will not happen; instead the call will fail and
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
Processes blocked awaiting a lock may be awakened by signals.
Zero is returned if the operation was successful;
on an error a -1 is returned and an error code is left in
.Bl -tag -width EWOULDBLOCKAA
The file is locked and the
is an invalid descriptor.
refers to an object other than a file.
function call appeared in
projects / unix-history / blob