160 lines
4.7 KiB
Groff
160 lines
4.7 KiB
Groff
.\" $NetBSD: flock.2,v 1.15 2002/10/01 18:10:44 wiz 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. All advertising materials mentioning features or use of this software
|
|
.\" must display the following acknowledgement:
|
|
.\" This product includes software developed by the University of
|
|
.\" California, Berkeley and its contributors.
|
|
.\" 4. 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 December 11, 1993
|
|
.Dt FLOCK 2
|
|
.Os
|
|
.Sh NAME
|
|
.Nm flock
|
|
.Nd "apply or remove an advisory lock on an open file"
|
|
.Sh LIBRARY
|
|
.Lb libc
|
|
.Sh SYNOPSIS
|
|
.Fd #include \*[Lt]fcntl.h\*[Gt]
|
|
.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 */
|
|
.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 .
|
|
A lock is applied by specifying an
|
|
.Fa operation
|
|
parameter that is one of
|
|
.Dv LOCK_SH
|
|
or
|
|
.Dv LOCK_EX
|
|
with the optional addition of
|
|
.Dv LOCK_NB .
|
|
To unlock
|
|
an existing lock
|
|
.Dv operation
|
|
should be
|
|
.Dv LOCK_UN .
|
|
.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
|
|
.Dv LOCK_NB
|
|
is included in
|
|
.Fa operation ,
|
|
then this will not happen; instead the call will fail and
|
|
the error
|
|
.Er EAGAIN
|
|
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
|
|
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
|
|
.Va errno .
|
|
.Sh ERRORS
|
|
The
|
|
.Fn flock
|
|
call fails if:
|
|
.Bl -tag -width Er
|
|
.It Bq Er EAGAIN
|
|
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 EOPNOTSUPP
|
|
The argument
|
|
.Fa fd
|
|
refers to an object other than a file.
|
|
.It Bq Er EINVAL
|
|
The argument
|
|
.Fa operation
|
|
does not include one of
|
|
.Dv LOCK_EX ,
|
|
.Dv LOCK_SH
|
|
or
|
|
.Dv LOCK_UN .
|
|
.El
|
|
.Sh SEE ALSO
|
|
.Xr close 2 ,
|
|
.Xr dup 2 ,
|
|
.Xr execve 2 ,
|
|
.Xr fork 2 ,
|
|
.Xr open 2
|
|
.Sh HISTORY
|
|
The
|
|
.Fn flock
|
|
function call appeared in
|
|
.Bx 4.2 .
|