[apple/xnu.git] / bsd / man / man2 / flock.2

.\"	$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. 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 BSD 4.2
.Sh NAME
.Nm flock
.Nd "apply or remove an advisory lock on an open file"
.Sh SYNOPSIS
.Fd #include <sys/file.h>
.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 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
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 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 ENOTSUP
The referenced descriptor is not of the correct type.
.El
.Sh SEE ALSO
.Xr open 2 ,
.Xr close 2 ,
.Xr dup 2 ,
.Xr execve 2 ,
.Xr fork 2
.Sh HISTORY
The
.Fn flock
function call appeared in
.Bx 4.2 .
Commit	Line	Data
9bccf70c A	1	.\" $NetBSD: flock.2,v 1.5 1995/02/27 12:32:32 cgd Exp $
	2	.\"
	3	.\" Copyright (c) 1983, 1991, 1993
	4	.\" The Regents of the University of California. All rights reserved.
	5	.\"
	6	.\" Redistribution and use in source and binary forms, with or without
	7	.\" modification, are permitted provided that the following conditions
	8	.\" are met:
	9	.\" 1. Redistributions of source code must retain the above copyright
	10	.\" notice, this list of conditions and the following disclaimer.
	11	.\" 2. Redistributions in binary form must reproduce the above copyright
	12	.\" notice, this list of conditions and the following disclaimer in the
	13	.\" documentation and/or other materials provided with the distribution.
	14	.\" 3. All advertising materials mentioning features or use of this software
	15	.\" must display the following acknowledgement:
	16	.\" This product includes software developed by the University of
	17	.\" California, Berkeley and its contributors.
	18	.\" 4. Neither the name of the University nor the names of its contributors
	19	.\" may be used to endorse or promote products derived from this software
	20	.\" without specific prior written permission.
	21	.\"
	22	.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
	23	.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
	24	.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
	25	.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
	26	.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
	27	.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
	28	.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
	29	.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
	30	.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
	31	.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
	32	.\" SUCH DAMAGE.
	33	.\"
	34	.\" @(#)flock.2 8.2 (Berkeley) 12/11/93
	35	.\"
	36	.Dd December 11, 1993
	37	.Dt FLOCK 2
	38	.Os BSD 4.2
	39	.Sh NAME
	40	.Nm flock
	41	.Nd "apply or remove an advisory lock on an open file"
	42	.Sh SYNOPSIS
	43	.Fd #include <sys/file.h>
	44	.Fd #define LOCK_SH 1 /* shared lock */
	45	.Fd #define LOCK_EX 2 /* exclusive lock */
	46	.Fd #define LOCK_NB 4 /* don't block when locking */
	47	.Fd #define LOCK_UN 8 /* unlock */
	48	.Ft int
	49	.Fn flock "int fd" "int operation"
	50	.Sh DESCRIPTION
	51	.Fn Flock
	52	applies or removes an
	53	.Em advisory
	54	lock on the file associated with the file descriptor
	55	.Fa fd .
	56	A lock is applied by specifying an
	57	.Fa operation
	58	parameter that is one of
	59	.Dv LOCK_SH
	60	or
	61	.Dv LOCK_EX
	62	with the optional addition of
	63	.Dv LOCK_NB .
	64	To unlock
65	an existing lock
66	.Dv operation
67	should be
68	.Dv LOCK_UN .
69	.Pp
70	Advisory locks allow cooperating processes to perform
71	consistent operations on files, but do not guarantee
72	consistency (i.e., processes may still access files
73	without using advisory locks possibly resulting in
74	inconsistencies).
75	.Pp
76	The locking mechanism allows two types of locks:
77	.Em shared
78	locks and
79	.Em exclusive
80	locks.
81	At any time multiple shared locks may be applied to a file,
82	but at no time are multiple exclusive, or both shared and exclusive,
83	locks allowed simultaneously on a file.
84	.Pp
85	A shared lock may be
86	.Em upgraded
87	to an exclusive lock, and vice versa, simply by specifying
88	the appropriate lock type; this results in the previous
89	lock being released and the new lock applied (possibly
90	after other processes have gained and released the lock).
91	.Pp
92	Requesting a lock on an object that is already locked
93	normally causes the caller to be blocked until the lock may be
94	acquired. If
95	.Dv LOCK_NB
96	is included in
97	.Fa operation ,
98	then this will not happen; instead the call will fail and
99	the error
100	.Er EWOULDBLOCK
101	will be returned.
102	.Sh NOTES
103	Locks are on files, not file descriptors. That is, file descriptors
104	duplicated through
105	.Xr dup 2
106	or
107	.Xr fork 2
108	do not result in multiple instances of a lock, but rather multiple
109	references to a single lock. If a process holding a lock on a file
110	forks and the child explicitly unlocks the file, the parent will
111	lose its lock.
112	.Pp
113	Processes blocked awaiting a lock may be awakened by signals.
114	.Sh RETURN VALUES
115	Zero is returned if the operation was successful;
116	on an error a -1 is returned and an error code is left in
117	the global location
118	.Va errno .
119	.Sh ERRORS
120	The
121	.Fn flock
122	call fails if:
123	.Bl -tag -width Er
124	.It Bq Er EWOULDBLOCK
125	The file is locked and the
126	.Dv LOCK_NB
127	option was specified.
128	.It Bq Er EBADF
129	The argument
130	.Fa fd
131	is an invalid descriptor.
132	.It Bq Er EINVAL
133	The argument
134	.Fa fd
135	refers to an object other than a file.
91447636	136	.It Bq Er ENOTSUP
9bccf70c A	137	The referenced descriptor is not of the correct type.
	138	.El
	139	.Sh SEE ALSO
	140	.Xr open 2 ,
	141	.Xr close 2 ,
	142	.Xr dup 2 ,
	143	.Xr execve 2 ,
	144	.Xr fork 2
	145	.Sh HISTORY
	146	The
	147	.Fn flock
	148	function call appeared in
	149	.Bx 4.2 .