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

.\" $NetBSD: lockf.3,v 1.2 1998/02/05 18:47:28 perry 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.
.\" 3. All advertising materials mentioning features or use of this software
.\"    must display the following acknowledgement:
.\"        This product includes software developed by the NetBSD
.\"        Foundation, Inc. and its contributors.
.\" 4. Neither the name of The NetBSD Foundation 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 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 1.13 2004/07/02 23:52:10 ru 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
5b2abdfb A	1	.\" $NetBSD: lockf.3,v 1.2 1998/02/05 18:47:28 perry Exp $
	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.
	17	.\" 3. All advertising materials mentioning features or use of this software
	18	.\" must display the following acknowledgement:
	19	.\" This product includes software developed by the NetBSD
	20	.\" Foundation, Inc. and its contributors.
	21	.\" 4. Neither the name of The NetBSD Foundation nor the names of its
	22	.\" contributors may be used to endorse or promote products derived
	23	.\" from this software without specific prior written permission.
	24	.\"
	25	.\" THIS SOFTWARE IS PROVIDED BY THE NETBSD FOUNDATION, INC. AND CONTRIBUTORS
	26	.\" ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED
	27	.\" TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
	28	.\" PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE FOUNDATION OR CONTRIBUTORS
	29	.\" BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
	30	.\" CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
	31	.\" SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
	32	.\" INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
	33	.\" CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
	34	.\" ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
	35	.\" POSSIBILITY OF SUCH DAMAGE.
	36	.\"
3d9156a7	37	.\" $FreeBSD: src/lib/libc/gen/lockf.3,v 1.13 2004/07/02 23:52:10 ru Exp $
5b2abdfb A	38	.\"
	39	.Dd December 19, 1997
	40	.Dt LOCKF 3
	41	.Os
	42	.Sh NAME
	43	.Nm lockf
	44	.Nd record locking on files
	45	.Sh LIBRARY
	46	.Lb libc
	47	.Sh SYNOPSIS
	48	.In unistd.h
	49	.Ft int
	50	.Fn lockf "int filedes" "int function" "off_t size"
	51	.Sh DESCRIPTION
	52	The
	53	.Fn lockf
	54	function allows sections of a file to be locked with advisory-mode locks.
	55	Calls to
	56	.Fn lockf
	57	from other processes which attempt to lock the locked file section will
	58	either return an error value or block until the section becomes unlocked.
	59	All the locks for a process are removed when the process terminates.
	60	.Pp
	61	The argument
	62	.Fa filedes
	63	is an open file descriptor.
	64	The file descriptor must have been opened either for write-only
	65	.Dv ( O_WRONLY )
	66	or read/write
	67	.Dv ( O_RDWR )
	68	operation.
	69	.Pp
	70	The
	71	.Fa function
	72	argument is a control value which specifies the action to be taken.
	73	The permissible values for
	74	.Fa function
	75	are as follows:
	76	.Bl -tag -width F_ULOCKXX -compact -offset indent
	77	.It Sy Function
	78	.Sy Description
	79	.It Dv F_ULOCK
	80	unlock locked sections
	81	.It Dv F_LOCK
	82	lock a section for exclusive use
	83	.It Dv F_TLOCK
	84	test and lock a section for exclusive use
	85	.It Dv F_TEST
	86	test a section for locks by other processes
	87	.El
	88	.Pp
	89	.Dv F_ULOCK
	90	removes locks from a section of the file;
	91	.Dv F_LOCK
	92	and
	93	.Dv F_TLOCK
	94	both lock a section of a file if the section is available;
	95	.Dv F_TEST
	96	detects if a lock by another process is present on the specified section.
	97	.Pp
	98	The
	99	.Fa size
	100	argument is the number of contiguous bytes to be locked or
	101	unlocked.
102	The section to be locked or unlocked starts at the current
103	offset in the file and extends forward for a positive size or backward
104	for a negative size (the preceding bytes up to but not including the
3d9156a7 A	105	current offset).
3d9156a7 A	106	However, it is not permitted to lock a section that
5b2abdfb A	107	starts or extends before the beginning of the file.
	108	If
	109	.Fa size
	110	is 0, the section from the current offset through the largest possible
	111	file offset is locked (that is, from the current offset through the
	112	present or any future end-of-file).
	113	.Pp
	114	The sections locked with
	115	.Dv F_LOCK
	116	or
	117	.Dv F_TLOCK
	118	may, in whole or in part, contain or be contained by a previously
	119	locked section for the same process.
	120	When this occurs, or if adjacent
	121	locked sections would occur, the sections are combined into a single
	122	locked section.
	123	If the request would cause the number of locks to
	124	exceed a system-imposed limit, the request will fail.
	125	.Pp
	126	.Dv F_LOCK
	127	and
	128	.Dv F_TLOCK
	129	requests differ only by the action taken if the section is not
	130	available.
	131	.Dv F_LOCK
	132	blocks the calling process until the section is available.
	133	.Dv F_TLOCK
	134	makes the function fail if the section is already locked by another
	135	process.
	136	.Pp
	137	File locks are released on first close by the locking process of any
	138	file descriptor for the file.
	139	.Pp
	140	.Dv F_ULOCK
	141	requests release (wholly or in part) one or more locked sections
	142	controlled by the process.
	143	Locked sections will be unlocked starting
	144	at the current file offset through
	145	.Fa size
3d9156a7 A	146	bytes or to the end of file if size is 0.
3d9156a7 A	147	When all of a locked section
5b2abdfb A	148	is not released (that is, when the beginning or end of the area to be
	149	unlocked falls within a locked section), the remaining portions of
	150	that section are still locked by the process.
	151	Releasing the center
	152	portion of a locked section will cause the remaining locked beginning
	153	and end portions to become two separate locked sections.
	154	If the
	155	request would cause the number of locks in the system to exceed a
	156	system-imposed limit, the request will fail.
	157	.Pp
	158	An
	159	.Dv F_ULOCK
	160	request in which size is non-zero and the offset of the last byte of
	161	the requested section is the maximum value for an object of type
	162	off_t, when the process has an existing lock in which size is 0 and
	163	which includes the last byte of the requested section, will be treated
	164	as a request to unlock from the start of the requested section with a
3d9156a7 A	165	size equal to 0.
3d9156a7 A	166	Otherwise an
5b2abdfb A	167	.Dv F_ULOCK
	168	request will attempt to unlock only the requested section.
	169	.Pp
	170	A potential for deadlock occurs if a process controlling a locked
	171	region is put to sleep by attempting to lock the locked region of
3d9156a7 A	172	another process.
3d9156a7 A	173	This implementation detects that sleeping until a
5b2abdfb A	174	locked region is unlocked would cause a deadlock and fails with an
	175	.Er EDEADLK
	176	error.
	177	.Pp
9385eb3d	178	The
5b2abdfb	179	.Fn lockf ,
3d9156a7	180	.Xr fcntl 2 ,
5b2abdfb A	181	and
5b2abdfb A	182	.Xr flock 2
3d9156a7 A	183	locks are compatible.
	184	Processes using different locking interfaces can cooperate
	185	over the same file safely.
	186	However, only one of such interfaces should be used within
	187	the same process.
	188	If a file is locked by a process through
	189	.Xr flock 2 ,
	190	any record within the file will be seen as locked
	191	from the viewpoint of another process using
	192	.Xr fcntl 2
	193	or
	194	.Fn lockf ,
	195	and vice versa.
5b2abdfb A	196	.Pp
	197	Blocking on a section is interrupted by any signal.
	198	.Sh RETURN VALUES
	199	.Rv -std lockf
	200	In the case of a failure, existing locks are not changed.
	201	.Sh ERRORS
9385eb3d	202	The
5b2abdfb	203	.Fn lockf
9385eb3d	204	function
5b2abdfb A	205	will fail if:
	206	.Bl -tag -width Er
	207	.It Bq Er EAGAIN
	208	The argument
	209	.Fa function
	210	is
	211	.Dv F_TLOCK
	212	or
	213	.Dv F_TEST
	214	and the section is already locked by another process.
	215	.It Bq Er EBADF
	216	The argument
	217	.Fa filedes
	218	is not a valid open file descriptor.
	219	.Pp
	220	The argument
	221	.Fa function
	222	is
	223	.Dv F_LOCK
	224	or
	225	.Dv F_TLOCK ,
	226	and
	227	.Fa filedes
	228	is not a valid file descriptor open for writing.
	229	.It Bq Er EDEADLK
	230	The argument
	231	.Fa function
	232	is
	233	.Dv F_LOCK
	234	and a deadlock is detected.
	235	.It Bq Er EINTR
	236	The argument
	237	.Fa function
	238	is F_LOCK
	239	and
	240	.Fn lockf
	241	was interrupted by the delivery of a signal.
	242	.It Bq Er EINVAL
	243	The argument
	244	.Fa function
	245	is not one of
	246	.Dv F_ULOCK ,
	247	.Dv F_LOCK ,
	248	.Dv F_TLOCK
	249	or
	250	.Dv F_TEST .
	251	.Pp
	252	The argument
	253	.Fa filedes
	254	refers to a file that does not support locking.
	255	.It Bq Er ENOLCK
	256	The argument
	257	.Fa function
	258	is
	259	.Dv F_ULOCK ,
	260	.Dv F_LOCK
	261	or
	262	.Dv F_TLOCK ,
	263	and satisfying the lock or unlock request would result in the number
	264	of locked regions in the system exceeding a system-imposed limit.
	265	.El
	266	.Sh SEE ALSO
	267	.Xr fcntl 2 ,
	268	.Xr flock 2
269	.Sh STANDARDS
270	The
271	.Fn lockf
272	function conforms to
273	.St -xpg4.2 .