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	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.
	136	.It Bq Er ENOTSUP
	137	The referenced descriptor is not of the correct type.
	138	.El
	139	.Sh SEE ALSO
	140	.Xr close 2 ,
	141	.Xr dup 2 ,
	142	.Xr execve 2 ,
	143	.Xr fork 2 ,
	144	.Xr open 2
	145	.Sh HISTORY
	146	The
	147	.Fn flock
	148	function call appeared in
	149	.Bx 4.2 .