7e6cabd06e
Renumber cluase 4 to 3, per what everybody else did when BSD granted them permission to remove clause 3. My insistance on keeping the same numbering for legal reasons is too pedantic, so give up on that point. Submitted by: Jan Schaumann <jschauma@stevens.edu> Pull Request: https://github.com/freebsd/freebsd/pull/96
174 lines
4.9 KiB
Groff
174 lines
4.9 KiB
Groff
.\" Copyright (c) 1983, 1991, 1993
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.\" The Regents of the University of California. All rights reserved.
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.\"
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.\" Redistribution and use in source and binary forms, with or without
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.\" modification, are permitted provided that the following conditions
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.\" are met:
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.\" 1. Redistributions of source code must retain the above copyright
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.\" notice, this list of conditions and the following disclaimer.
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.\" 2. Redistributions in binary form must reproduce the above copyright
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.\" notice, this list of conditions and the following disclaimer in the
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.\" documentation and/or other materials provided with the distribution.
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.\" 3. Neither the name of the University nor the names of its contributors
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.\" may be used to endorse or promote products derived from this software
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.\" without specific prior written permission.
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.\"
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.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
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.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
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.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
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.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
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.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
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.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
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.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
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.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
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.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
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.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
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.\" SUCH DAMAGE.
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.\"
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.\" @(#)flock.2 8.2 (Berkeley) 12/11/93
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.\" $FreeBSD$
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.\"
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.Dd November 9, 2011
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.Dt FLOCK 2
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.Os
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.Sh NAME
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.Nm flock
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.Nd "apply or remove an advisory lock on an open file"
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.Sh LIBRARY
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.Lb libc
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.Sh SYNOPSIS
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.In sys/file.h
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.Fd "#define LOCK_SH 0x01 /* shared file lock */"
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.Fd "#define LOCK_EX 0x02 /* exclusive file lock */"
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.Fd "#define LOCK_NB 0x04 /* do not block when locking */"
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.Fd "#define LOCK_UN 0x08 /* unlock file */"
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.Ft int
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.Fn flock "int fd" "int operation"
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.Sh DESCRIPTION
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The
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.Fn flock
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system call applies or removes an
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.Em advisory
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lock on the file associated with the file descriptor
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.Fa fd .
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A lock is applied by specifying an
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.Fa operation
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argument that is one of
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.Dv LOCK_SH
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or
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.Dv LOCK_EX
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with the optional addition of
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.Dv LOCK_NB .
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To unlock
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an existing lock
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.Dv operation
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should be
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.Dv LOCK_UN .
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.Pp
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Advisory locks allow cooperating processes to perform
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consistent operations on files, but do not guarantee
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consistency (i.e., processes may still access files
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without using advisory locks possibly resulting in
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inconsistencies).
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.Pp
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The locking mechanism allows two types of locks:
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.Em shared
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locks and
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.Em exclusive
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locks.
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At any time multiple shared locks may be applied to a file,
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but at no time are multiple exclusive, or both shared and exclusive,
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locks allowed simultaneously on a file.
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.Pp
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A shared lock may be
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.Em upgraded
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to an exclusive lock, and vice versa, simply by specifying
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the appropriate lock type; this results in the previous
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lock being released and the new lock applied (possibly
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after other processes have gained and released the lock).
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.Pp
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Requesting a lock on an object that is already locked
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normally causes the caller to be blocked until the lock may be
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acquired.
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If
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.Dv LOCK_NB
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is included in
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.Fa operation ,
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then this will not happen; instead the call will fail and
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the error
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.Er EWOULDBLOCK
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will be returned.
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.Sh NOTES
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Locks are on files, not file descriptors.
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That is, file descriptors
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duplicated through
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.Xr dup 2
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or
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.Xr fork 2
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do not result in multiple instances of a lock, but rather multiple
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references to a single lock.
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If a process holding a lock on a file
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forks and the child explicitly unlocks the file, the parent will
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lose its lock.
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.Pp
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The
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.Fn flock ,
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.Xr fcntl 2 ,
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and
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.Xr lockf 3
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locks are compatible.
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Processes using different locking interfaces can cooperate
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over the same file safely.
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However, only one of such interfaces should be used within
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the same process.
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If a file is locked by a process through
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.Fn flock ,
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any record within the file will be seen as locked
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from the viewpoint of another process using
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.Xr fcntl 2
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or
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.Xr lockf 3 ,
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and vice versa.
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.Pp
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Processes blocked awaiting a lock may be awakened by signals.
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.Sh RETURN VALUES
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.Rv -std flock
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.Sh ERRORS
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The
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.Fn flock
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system call fails if:
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.Bl -tag -width Er
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.It Bq Er EWOULDBLOCK
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The file is locked and the
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.Dv LOCK_NB
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option was specified.
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.It Bq Er EBADF
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The argument
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.Fa fd
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is an invalid descriptor.
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.It Bq Er EINVAL
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The argument
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.Fa fd
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refers to an object other than a file.
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.It Bq Er EOPNOTSUPP
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The argument
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.Fa fd
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refers to an object that does not support file locking.
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.It Bq Er ENOLCK
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A lock was requested, but no locks are available.
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.El
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.Sh SEE ALSO
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.Xr close 2 ,
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.Xr dup 2 ,
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.Xr execve 2 ,
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.Xr fcntl 2 ,
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.Xr fork 2 ,
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.Xr open 2 ,
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.Xr flopen 3 ,
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.Xr lockf 3
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.Sh HISTORY
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The
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.Fn flock
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system call appeared in
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.Bx 4.2 .
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