.\"	$OpenBSD: flock.2,v 1.20 2014/12/10 19:19:00 schwarze Exp $
.\"	$NetBSD: flock.2,v 1.5 1995/02/27 12:32:32 cgd Exp $
.\"
.\" Copyright (c) 1983, 1991, 1993
.\"	The Regents of the University of California.  All rights reserved.
.\"
.\" Redistribution and use in source and binary forms, with or without
.\" modification, are permitted provided that the following conditions
.\" are met:
.\" 1. Redistributions of source code must retain the above copyright
.\"    notice, this list of conditions and the following disclaimer.
.\" 2. Redistributions in binary form must reproduce the above copyright
.\"    notice, this list of conditions and the following disclaimer in the
.\"    documentation and/or other materials provided with the distribution.
.\" 3. Neither the name of the University nor the names of its contributors
.\"    may be used to endorse or promote products derived from this software
.\"    without specific prior written permission.
.\"
.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
.\" ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
.\" SUCH DAMAGE.
.\"
.\"     @(#)flock.2	8.2 (Berkeley) 12/11/93
.\"
.Dd $Mdocdate: December 10 2014 $
.Dt FLOCK 2
.Os
.Sh NAME
.Nm flock
.Nd apply or remove an advisory lock on an open file
.Sh SYNOPSIS
.In fcntl.h
.Ft int
.Fn flock "int fd" "int operation"
.Sh DESCRIPTION
.Fn flock
applies or removes an
.Em advisory
lock on the file associated with the file descriptor
.Fa fd .
The
.Fa operation
argument is one of:
.Pp
.Bl -tag -width LOCK_SH -offset indent -compact
.It Dv LOCK_SH
Apply a shared lock.
.It Dv LOCK_EX
Apply an exclusive lock.
.It Dv LOCK_UN
Remove an existing lock.
.El
.Pp
.Dv LOCK_SH
and
.Dv LOCK_EX
may be combined with the optional
.Dv LOCK_NB
for nonblocking mode.
.Pp
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).
.Pp
The locking mechanism allows two types of locks:
.Em shared
locks and
.Em 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.
.Pp
A shared lock may be
.Em 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).
.Pp
Requesting a lock on an object that is already locked normally causes
the caller to be blocked until the lock may be acquired.
If
.Fa operation
is the bitwise OR of
.Dv LOCK_NB
and
.Dv LOCK_SH
or
.Dv LOCK_EX ,
then this will not happen; instead the call will fail and the error
.Er EWOULDBLOCK
will be returned.
.Sh NOTES
Locks are on files, not file descriptors.
That is, file descriptors duplicated through
.Xr dup 2
or
.Xr 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.
.Pp
Processes blocked awaiting a lock may be awakened by signals.
.Sh RETURN VALUES
.Rv -std
.Sh ERRORS
The
.Fn flock
call fails if:
.Bl -tag -width Er
.It Bq Er EWOULDBLOCK
The file is locked and the
.Dv LOCK_NB
option was specified.
.It Bq Er EBADF
The argument
.Fa fd
is an invalid descriptor.
.It Bq Er EINVAL
The argument
.Fa operation
has an invalid value.
.It Bq Er EOPNOTSUPP
The referenced descriptor is not of the correct type.
.El
.Sh SEE ALSO
.Xr close 2 ,
.Xr dup 2 ,
.Xr execve 2 ,
.Xr fcntl 2 ,
.Xr fork 2 ,
.Xr open 2
.Sh HISTORY
The
.Fn flock
system call first appeared in
.Bx 4.1c .