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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.\" 4. 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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