freebsd-dev/lib/libc/sys/flock.2

.\" 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.
.\" 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
.\" $FreeBSD$
.\"
.Dd November 9, 2011
.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
.In sys/file.h
.Fd "#define   LOCK_SH        0x01      /* shared file lock */"
.Fd "#define   LOCK_EX        0x02      /* exclusive file lock */"
.Fd "#define   LOCK_NB        0x04      /* do not block when locking */"
.Fd "#define   LOCK_UN        0x08      /* unlock file */"
.Ft int
.Fn flock "int fd" "int operation"
.Sh DESCRIPTION
The
.Fn flock
system call 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
argument 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 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
The
.Fn flock ,
.Xr fcntl 2 ,
and
.Xr lockf 3
locks are compatible.
Processes using different locking interfaces can cooperate
over the same file safely.
However, only one of such interfaces should be used within
the same process.
If a file is locked by a process through
.Fn flock ,
any record within the file will be seen as locked
from the viewpoint of another process using
.Xr fcntl 2
or
.Xr lockf 3 ,
and vice versa.
.Pp
Processes blocked awaiting a lock may be awakened by signals.
.Sh RETURN VALUES
.Rv -std flock
.Sh ERRORS
The
.Fn flock
system 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 fd
refers to an object other than a file.
.It Bq Er EOPNOTSUPP
The argument
.Fa fd
refers to an object that does not support file locking.
.It Bq Er ENOLCK
A lock was requested, but no locks are available.
.El
.Sh SEE ALSO
.Xr close 2 ,
.Xr dup 2 ,
.Xr execve 2 ,
.Xr fcntl 2 ,
.Xr fork 2 ,
.Xr open 2 ,
.Xr flopen 3 ,
.Xr lockf 3
.Sh HISTORY
The
.Fn flock
system call appeared in
.Bx 4.2 .
BSD 4.4 Lite Lib Sources 1994-05-27 05:00:24 +00:00			`.\" 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.`
			`.\" 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`
$Id$ -> $FreeBSD$ 1999-08-28 00:22:10 +00:00			`.\" $FreeBSD$`
BSD 4.4 Lite Lib Sources 1994-05-27 05:00:24 +00:00			`.\"`
Document that flock can return ENOLCK 2011-11-10 06:20:18 +00:00			`.Dd November 9, 2011`
BSD 4.4 Lite Lib Sources 1994-05-27 05:00:24 +00:00			`.Dt FLOCK 2`
mdoc(7) police: removed HISTORY info from the .Os call. 2001-07-10 13:41:46 +00:00			`.Os`
BSD 4.4 Lite Lib Sources 1994-05-27 05:00:24 +00:00			`.Sh NAME`
			`.Nm flock`
			`.Nd "apply or remove an advisory lock on an open file"`
Introduce ".Lb" macro to libc manpages. More libraries manpages updates following. 2000-04-21 09:42:15 +00:00			`.Sh LIBRARY`
			`.Lb libc`
BSD 4.4 Lite Lib Sources 1994-05-27 05:00:24 +00:00			`.Sh SYNOPSIS`
mdoc(7) police: Use the new .In macro for #include statements. 2001-10-01 16:09:29 +00:00			`.In sys/file.h`
mdoc: remove literal tabs where they don't belong 2010-06-08 16:48:59 +00:00			`.Fd "#define LOCK_SH 0x01 /* shared file lock */"`
			`.Fd "#define LOCK_EX 0x02 /* exclusive file lock */"`
			`.Fd "#define LOCK_NB 0x04 /* do not block when locking */"`
			`.Fd "#define LOCK_UN 0x08 /* unlock file */"`
BSD 4.4 Lite Lib Sources 1994-05-27 05:00:24 +00:00			`.Ft int`
			`.Fn flock "int fd" "int operation"`
			`.Sh DESCRIPTION`
The .Fn function. 2002-07-15 20:59:12 +00:00			`The`
			`.Fn flock`
mdoc(7) police: Tidy up the syscall language. Stop calling system calls "function calls". Use "The .Fn system call" a-la "The .Nm utility". When referring to a non-BSD implementation in the HISTORY section, call syscall a function, to be safe. 2002-12-18 09:22:32 +00:00			`system call applies or removes an`
BSD 4.4 Lite Lib Sources 1994-05-27 05:00:24 +00:00			`.Em advisory`
			`lock on the file associated with the file descriptor`
			`.Fa fd .`
			`A lock is applied by specifying an`
			`.Fa operation`
mdoc(7) police: "The .Fa argument.". 2002-12-19 09:40:28 +00:00			`argument that is one of`
BSD 4.4 Lite Lib Sources 1994-05-27 05:00:24 +00:00			`.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,`
Remove whitespace at EOL. 2001-07-15 07:53:42 +00:00			`locks allowed simultaneously on a file.`
BSD 4.4 Lite Lib Sources 1994-05-27 05:00:24 +00:00			`.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`
Mechanically kill hard sentence breaks. 2004-07-02 23:52:20 +00:00			`acquired.`
			`If`
BSD 4.4 Lite Lib Sources 1994-05-27 05:00:24 +00:00			`.Dv LOCK_NB`
			`is included in`
			`.Fa operation ,`
			`then this will not happen; instead the call will fail and`
			`the error`
			`.Er EWOULDBLOCK`
			`will be returned.`
			`.Sh NOTES`
Mechanically kill hard sentence breaks. 2004-07-02 23:52:20 +00:00			`Locks are on files, not file descriptors.`
			`That is, file descriptors`
BSD 4.4 Lite Lib Sources 1994-05-27 05:00:24 +00:00			`duplicated through`
			`.Xr dup 2`
			`or`
			`.Xr fork 2`
			`do not result in multiple instances of a lock, but rather multiple`
Mechanically kill hard sentence breaks. 2004-07-02 23:52:20 +00:00			`references to a single lock.`
			`If a process holding a lock on a file`
BSD 4.4 Lite Lib Sources 1994-05-27 05:00:24 +00:00			`forks and the child explicitly unlocks the file, the parent will`
			`lose its lock.`
			`.Pp`
Clarify and extend paragraphs on interoperation of fcntl(2), flock(2), and lockf(3) advisory locks. Add such a paragraph to the flock(2) manpage for the sake of consistency. Reviewed by: Cyrille Lefevre and Kirk McKusick on -arch MFC after: 2 weeks 2004-05-17 23:09:10 +00:00			`The`
			`.Fn flock ,`
			`.Xr fcntl 2 ,`
			`and`
			`.Xr lockf 3`
			`locks are compatible.`
			`Processes using different locking interfaces can cooperate`
			`over the same file safely.`
			`However, only one of such interfaces should be used within`
			`the same process.`
			`If a file is locked by a process through`
			`.Fn flock ,`
			`any record within the file will be seen as locked`
			`from the viewpoint of another process using`
			`.Xr fcntl 2`
			`or`
			`.Xr lockf 3 ,`
			`and vice versa.`
			`.Pp`
BSD 4.4 Lite Lib Sources 1994-05-27 05:00:24 +00:00			`Processes blocked awaiting a lock may be awakened by signals.`
			`.Sh RETURN VALUES`
Use the ``.Rv -std'' mdoc(7) macro in appropriate cases. Reviewed by: ru 2001-08-09 13:32:13 +00:00			`.Rv -std flock`
BSD 4.4 Lite Lib Sources 1994-05-27 05:00:24 +00:00			`.Sh ERRORS`
			`The`
			`.Fn flock`
mdoc(7) police: Tidy up the syscall language. Stop calling system calls "function calls". Use "The .Fn system call" a-la "The .Nm utility". When referring to a non-BSD implementation in the HISTORY section, call syscall a function, to be safe. 2002-12-18 09:22:32 +00:00			`system call fails if:`
Use `Er' variable to define first column width in ERRORS section. It was initially suggested by mdoc(7) style, but was broken over the years 2000-05-04 13:09:25 +00:00			`.Bl -tag -width Er`
BSD 4.4 Lite Lib Sources 1994-05-27 05:00:24 +00:00			`.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 fd`
			`refers to an object other than a file.`
Correct some xrefs/mlinks. 1996-12-14 23:27:20 +00:00			`.It Bq Er EOPNOTSUPP`
			`The argument`
			`.Fa fd`
			`refers to an object that does not support file locking.`
Document that flock can return ENOLCK 2011-11-10 06:20:18 +00:00			`.It Bq Er ENOLCK`
			`A lock was requested, but no locks are available.`
BSD 4.4 Lite Lib Sources 1994-05-27 05:00:24 +00:00			`.El`
			`.Sh SEE ALSO`
			`.Xr close 2 ,`
			`.Xr dup 2 ,`
			`.Xr execve 2 ,`
Clarify and extend paragraphs on interoperation of fcntl(2), flock(2), and lockf(3) advisory locks. Add such a paragraph to the flock(2) manpage for the sake of consistency. Reviewed by: Cyrille Lefevre and Kirk McKusick on -arch MFC after: 2 weeks 2004-05-17 23:09:10 +00:00			`.Xr fcntl 2 ,`
Sort cross references. 1997-01-20 23:23:22 +00:00			`.Xr fork 2 ,`
Clarify and extend paragraphs on interoperation of fcntl(2), flock(2), and lockf(3) advisory locks. Add such a paragraph to the flock(2) manpage for the sake of consistency. Reviewed by: Cyrille Lefevre and Kirk McKusick on -arch MFC after: 2 weeks 2004-05-17 23:09:10 +00:00			`.Xr open 2 ,`
Xref flopen.3 which references this manual page. PR: 112650 2008-01-22 15:56:48 +00:00			`.Xr flopen 3 ,`
Clarify and extend paragraphs on interoperation of fcntl(2), flock(2), and lockf(3) advisory locks. Add such a paragraph to the flock(2) manpage for the sake of consistency. Reviewed by: Cyrille Lefevre and Kirk McKusick on -arch MFC after: 2 weeks 2004-05-17 23:09:10 +00:00			`.Xr lockf 3`
BSD 4.4 Lite Lib Sources 1994-05-27 05:00:24 +00:00			`.Sh HISTORY`
			`The`
Update a bunch of man pages to use .Fn instead of .Nm when referencing funciton names. 1996-08-22 22:05:59 +00:00			`.Fn flock`
mdoc(7) police: Tidy up the syscall language. Stop calling system calls "function calls". Use "The .Fn system call" a-la "The .Nm utility". When referring to a non-BSD implementation in the HISTORY section, call syscall a function, to be safe. 2002-12-18 09:22:32 +00:00			`system call appeared in`
BSD 4.4 Lite Lib Sources 1994-05-27 05:00:24 +00:00			`.Bx 4.2 .`