[apple/libc.git] / gen / FreeBSD / lockf.3

.\" $NetBSD: lockf.3,v 1.10 2008/04/30 13:10:50 martin Exp $
.\"
.\" Copyright (c) 1997 The NetBSD Foundation, Inc.
.\" All rights reserved.
.\"
.\" This code is derived from software contributed to The NetBSD Foundation
.\" by Klaus Klein and S.P. Zeidler.
.\"
.\" 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.
.\"
.\" THIS SOFTWARE IS PROVIDED BY THE NETBSD FOUNDATION, INC. 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 FOUNDATION 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.
.\"
.\" $FreeBSD: src/lib/libc/gen/lockf.3,v 1.14 2009/03/04 01:01:26 delphij Exp $
.\"
.Dd December 19, 1997
.Dt LOCKF 3
.Os
.Sh NAME
.Nm lockf
.Nd record locking on files
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In unistd.h
.Ft int
.Fn lockf "int filedes" "int function" "off_t size"
.Sh DESCRIPTION
The
.Fn lockf
function allows sections of a file to be locked with advisory-mode locks.
Calls to
.Fn lockf
from other processes which attempt to lock the locked file section will
either return an error value or block until the section becomes unlocked.
All the locks for a process are removed when the process terminates.
.Pp
The argument
.Fa filedes
is an open file descriptor.
The file descriptor must have been opened either for write-only
.Dv ( O_WRONLY )
or read/write
.Dv ( O_RDWR )
operation.
.Pp
The
.Fa function
argument is a control value which specifies the action to be taken.
The permissible values for
.Fa function
are as follows:
.Bl -tag -width F_ULOCKXX -compact -offset indent
.It Sy Function
.Sy Description
.It Dv F_ULOCK
unlock locked sections
.It Dv F_LOCK
lock a section for exclusive use
.It Dv F_TLOCK
test and lock a section for exclusive use
.It Dv F_TEST
test a section for locks by other processes
.El
.Pp
.Dv F_ULOCK
removes locks from a section of the file;
.Dv F_LOCK
and
.Dv F_TLOCK
both lock a section of a file if the section is available;
.Dv F_TEST
detects if a lock by another process is present on the specified section.
.Pp
The
.Fa size
argument is the number of contiguous bytes to be locked or
unlocked.
The section to be locked or unlocked starts at the current
offset in the file and extends forward for a positive size or backward
for a negative size (the preceding bytes up to but not including the
current offset).
However, it is not permitted to lock a section that
starts or extends before the beginning of the file.
If
.Fa size
is 0, the section from the current offset through the largest possible
file offset is locked (that is, from the current offset through the
present or any future end-of-file).
.Pp
The sections locked with
.Dv F_LOCK
or
.Dv F_TLOCK
may, in whole or in part, contain or be contained by a previously
locked section for the same process.
When this occurs, or if adjacent
locked sections would occur, the sections are combined into a single
locked section.
If the request would cause the number of locks to
exceed a system-imposed limit, the request will fail.
.Pp
.Dv F_LOCK
and
.Dv F_TLOCK
requests differ only by the action taken if the section is not
available.
.Dv F_LOCK
blocks the calling process until the section is available.
.Dv F_TLOCK
makes the function fail if the section is already locked by another
process.
.Pp
File locks are released on first close by the locking process of any
file descriptor for the file.
.Pp
.Dv F_ULOCK
requests release (wholly or in part) one or more locked sections
controlled by the process.
Locked sections will be unlocked starting
at the current file offset through
.Fa size
bytes or to the end of file if size is 0.
When all of a locked section
is not released (that is, when the beginning or end of the area to be
unlocked falls within a locked section), the remaining portions of
that section are still locked by the process.
Releasing the center
portion of a locked section will cause the remaining locked beginning
and end portions to become two separate locked sections.
If the
request would cause the number of locks in the system to exceed a
system-imposed limit, the request will fail.
.Pp
An
.Dv F_ULOCK
request in which size is non-zero and the offset of the last byte of
the requested section is the maximum value for an object of type
off_t, when the process has an existing lock in which size is 0 and
which includes the last byte of the requested section, will be treated
as a request to unlock from the start of the requested section with a
size equal to 0.
Otherwise an
.Dv F_ULOCK
request will attempt to unlock only the requested section.
.Pp
A potential for deadlock occurs if a process controlling a locked
region is put to sleep by attempting to lock the locked region of
another process.
This implementation detects that sleeping until a
locked region is unlocked would cause a deadlock and fails with an
.Er EDEADLK
error.
.Pp
The
.Fn lockf ,
.Xr fcntl 2 ,
and
.Xr flock 2
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
.Xr flock 2 ,
any record within the file will be seen as locked
from the viewpoint of another process using
.Xr fcntl 2
or
.Fn lockf ,
and vice versa.
.Pp
Blocking on a section is interrupted by any signal.
.Sh RETURN VALUES
.Rv -std lockf
In the case of a failure, existing locks are not changed.
.Sh ERRORS
The
.Fn lockf
function
will fail if:
.Bl -tag -width Er
.It Bq Er EAGAIN
The argument
.Fa function
is
.Dv F_TLOCK
or
.Dv F_TEST
and the section is already locked by another process.
.It Bq Er EBADF
The argument
.Fa filedes
is not a valid open file descriptor.
.Pp
The argument
.Fa function
is
.Dv F_LOCK
or
.Dv F_TLOCK ,
and
.Fa filedes
is not a valid file descriptor open for writing.
.It Bq Er EDEADLK
The argument
.Fa function
is
.Dv F_LOCK
and a deadlock is detected.
.It Bq Er EINTR
The argument
.Fa function
is F_LOCK
and
.Fn lockf
was interrupted by the delivery of a signal.
.It Bq Er EINVAL
The argument
.Fa function
is not one of
.Dv F_ULOCK ,
.Dv F_LOCK ,
.Dv F_TLOCK
or
.Dv F_TEST .
.Pp
The argument
.Fa filedes
refers to a file that does not support locking.
.It Bq Er ENOLCK
The argument
.Fa function
is
.Dv F_ULOCK ,
.Dv F_LOCK
or
.Dv F_TLOCK ,
and satisfying the lock or unlock request would result in the number
of locked regions in the system exceeding a system-imposed limit.
.El
.Sh SEE ALSO
.Xr fcntl 2 ,
.Xr flock 2
.Sh STANDARDS
The
.Fn lockf
function conforms to
.St -xpg4.2 .
Commit	Line	Data
1f2f436a	1	.\" $NetBSD: lockf.3,v 1.10 2008/04/30 13:10:50 martin Exp $
5b2abdfb A	2	.\"
	3	.\" Copyright (c) 1997 The NetBSD Foundation, Inc.
	4	.\" All rights reserved.
	5	.\"
	6	.\" This code is derived from software contributed to The NetBSD Foundation
	7	.\" by Klaus Klein and S.P. Zeidler.
	8	.\"
	9	.\" Redistribution and use in source and binary forms, with or without
	10	.\" modification, are permitted provided that the following conditions
	11	.\" are met:
	12	.\" 1. Redistributions of source code must retain the above copyright
	13	.\" notice, this list of conditions and the following disclaimer.
	14	.\" 2. Redistributions in binary form must reproduce the above copyright
	15	.\" notice, this list of conditions and the following disclaimer in the
	16	.\" documentation and/or other materials provided with the distribution.
5b2abdfb A	17	.\"
	18	.\" THIS SOFTWARE IS PROVIDED BY THE NETBSD FOUNDATION, INC. AND CONTRIBUTORS
	19	.\" ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED
	20	.\" TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
	21	.\" PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE FOUNDATION OR CONTRIBUTORS
	22	.\" BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
	23	.\" CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
	24	.\" SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
	25	.\" INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
	26	.\" CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
	27	.\" ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
	28	.\" POSSIBILITY OF SUCH DAMAGE.
	29	.\"
1f2f436a	30	.\" $FreeBSD: src/lib/libc/gen/lockf.3,v 1.14 2009/03/04 01:01:26 delphij Exp $
5b2abdfb A	31	.\"
	32	.Dd December 19, 1997
	33	.Dt LOCKF 3
	34	.Os
	35	.Sh NAME
	36	.Nm lockf
	37	.Nd record locking on files
	38	.Sh LIBRARY
	39	.Lb libc
	40	.Sh SYNOPSIS
	41	.In unistd.h
	42	.Ft int
	43	.Fn lockf "int filedes" "int function" "off_t size"
	44	.Sh DESCRIPTION
	45	The
	46	.Fn lockf
	47	function allows sections of a file to be locked with advisory-mode locks.
	48	Calls to
	49	.Fn lockf
	50	from other processes which attempt to lock the locked file section will
	51	either return an error value or block until the section becomes unlocked.
	52	All the locks for a process are removed when the process terminates.
	53	.Pp
	54	The argument
	55	.Fa filedes
	56	is an open file descriptor.
	57	The file descriptor must have been opened either for write-only
	58	.Dv ( O_WRONLY )
	59	or read/write
	60	.Dv ( O_RDWR )
	61	operation.
	62	.Pp
	63	The
	64	.Fa function
	65	argument is a control value which specifies the action to be taken.
	66	The permissible values for
	67	.Fa function
	68	are as follows:
	69	.Bl -tag -width F_ULOCKXX -compact -offset indent
	70	.It Sy Function
	71	.Sy Description
	72	.It Dv F_ULOCK
	73	unlock locked sections
	74	.It Dv F_LOCK
	75	lock a section for exclusive use
	76	.It Dv F_TLOCK
	77	test and lock a section for exclusive use
	78	.It Dv F_TEST
	79	test a section for locks by other processes
	80	.El
	81	.Pp
	82	.Dv F_ULOCK
	83	removes locks from a section of the file;
	84	.Dv F_LOCK
	85	and
	86	.Dv F_TLOCK
	87	both lock a section of a file if the section is available;
	88	.Dv F_TEST
	89	detects if a lock by another process is present on the specified section.
	90	.Pp
	91	The
	92	.Fa size
	93	argument is the number of contiguous bytes to be locked or
	94	unlocked.
95	The section to be locked or unlocked starts at the current
96	offset in the file and extends forward for a positive size or backward
97	for a negative size (the preceding bytes up to but not including the
3d9156a7 A	98	current offset).
3d9156a7 A	99	However, it is not permitted to lock a section that
5b2abdfb A	100	starts or extends before the beginning of the file.
	101	If
	102	.Fa size
	103	is 0, the section from the current offset through the largest possible
	104	file offset is locked (that is, from the current offset through the
	105	present or any future end-of-file).
	106	.Pp
	107	The sections locked with
	108	.Dv F_LOCK
	109	or
	110	.Dv F_TLOCK
	111	may, in whole or in part, contain or be contained by a previously
	112	locked section for the same process.
	113	When this occurs, or if adjacent
	114	locked sections would occur, the sections are combined into a single
	115	locked section.
	116	If the request would cause the number of locks to
	117	exceed a system-imposed limit, the request will fail.
	118	.Pp
	119	.Dv F_LOCK
	120	and
	121	.Dv F_TLOCK
	122	requests differ only by the action taken if the section is not
	123	available.
	124	.Dv F_LOCK
	125	blocks the calling process until the section is available.
	126	.Dv F_TLOCK
	127	makes the function fail if the section is already locked by another
	128	process.
	129	.Pp
	130	File locks are released on first close by the locking process of any
	131	file descriptor for the file.
	132	.Pp
	133	.Dv F_ULOCK
	134	requests release (wholly or in part) one or more locked sections
	135	controlled by the process.
	136	Locked sections will be unlocked starting
	137	at the current file offset through
	138	.Fa size
3d9156a7 A	139	bytes or to the end of file if size is 0.
3d9156a7 A	140	When all of a locked section
5b2abdfb A	141	is not released (that is, when the beginning or end of the area to be
	142	unlocked falls within a locked section), the remaining portions of
	143	that section are still locked by the process.
	144	Releasing the center
	145	portion of a locked section will cause the remaining locked beginning
	146	and end portions to become two separate locked sections.
	147	If the
	148	request would cause the number of locks in the system to exceed a
	149	system-imposed limit, the request will fail.
	150	.Pp
	151	An
	152	.Dv F_ULOCK
	153	request in which size is non-zero and the offset of the last byte of
	154	the requested section is the maximum value for an object of type
	155	off_t, when the process has an existing lock in which size is 0 and
	156	which includes the last byte of the requested section, will be treated
	157	as a request to unlock from the start of the requested section with a
3d9156a7 A	158	size equal to 0.
3d9156a7 A	159	Otherwise an
5b2abdfb A	160	.Dv F_ULOCK
	161	request will attempt to unlock only the requested section.
	162	.Pp
	163	A potential for deadlock occurs if a process controlling a locked
	164	region is put to sleep by attempting to lock the locked region of
3d9156a7 A	165	another process.
3d9156a7 A	166	This implementation detects that sleeping until a
5b2abdfb A	167	locked region is unlocked would cause a deadlock and fails with an
	168	.Er EDEADLK
	169	error.
	170	.Pp
9385eb3d	171	The
5b2abdfb	172	.Fn lockf ,
3d9156a7	173	.Xr fcntl 2 ,
5b2abdfb A	174	and
5b2abdfb A	175	.Xr flock 2
3d9156a7 A	176	locks are compatible.
	177	Processes using different locking interfaces can cooperate
	178	over the same file safely.
	179	However, only one of such interfaces should be used within
	180	the same process.
	181	If a file is locked by a process through
	182	.Xr flock 2 ,
	183	any record within the file will be seen as locked
	184	from the viewpoint of another process using
	185	.Xr fcntl 2
	186	or
	187	.Fn lockf ,
	188	and vice versa.
5b2abdfb A	189	.Pp
	190	Blocking on a section is interrupted by any signal.
	191	.Sh RETURN VALUES
	192	.Rv -std lockf
	193	In the case of a failure, existing locks are not changed.
	194	.Sh ERRORS
9385eb3d	195	The
5b2abdfb	196	.Fn lockf
9385eb3d	197	function
5b2abdfb A	198	will fail if:
	199	.Bl -tag -width Er
	200	.It Bq Er EAGAIN
	201	The argument
	202	.Fa function
	203	is
	204	.Dv F_TLOCK
	205	or
	206	.Dv F_TEST
	207	and the section is already locked by another process.
	208	.It Bq Er EBADF
	209	The argument
	210	.Fa filedes
	211	is not a valid open file descriptor.
	212	.Pp
	213	The argument
	214	.Fa function
	215	is
	216	.Dv F_LOCK
	217	or
	218	.Dv F_TLOCK ,
	219	and
	220	.Fa filedes
	221	is not a valid file descriptor open for writing.
	222	.It Bq Er EDEADLK
	223	The argument
	224	.Fa function
	225	is
	226	.Dv F_LOCK
	227	and a deadlock is detected.
	228	.It Bq Er EINTR
	229	The argument
	230	.Fa function
	231	is F_LOCK
	232	and
	233	.Fn lockf
	234	was interrupted by the delivery of a signal.
	235	.It Bq Er EINVAL
	236	The argument
	237	.Fa function
	238	is not one of
	239	.Dv F_ULOCK ,
	240	.Dv F_LOCK ,
	241	.Dv F_TLOCK
	242	or
	243	.Dv F_TEST .
	244	.Pp
	245	The argument
	246	.Fa filedes
	247	refers to a file that does not support locking.
	248	.It Bq Er ENOLCK
	249	The argument
	250	.Fa function
	251	is
	252	.Dv F_ULOCK ,
	253	.Dv F_LOCK
	254	or
	255	.Dv F_TLOCK ,
	256	and satisfying the lock or unlock request would result in the number
	257	of locked regions in the system exceeding a system-imposed limit.
	258	.El
	259	.Sh SEE ALSO
	260	.Xr fcntl 2 ,
	261	.Xr flock 2
262	.Sh STANDARDS
263	The
264	.Fn lockf
265	function conforms to
266	.St -xpg4.2 .