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	1	.\" $NetBSD: fsync.2,v 1.4 1995/02/27 12:32:38 cgd Exp $
	2	.\"
	3	.\" Copyright (c) 1983, 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	.\" @(#)fsync.2 8.1 (Berkeley) 6/4/93
	35	.\"
	36	.Dd June 4, 1993
	37	.Dt FSYNC 2
	38	.Os BSD 4.2
	39	.Sh NAME
	40	.Nm fsync
	41	.Nd "synchronize a file's in-core state with that on disk"
	42	.Sh SYNOPSIS
	43	.Fd #include <unistd.h>
	44	.Ft int
	45	.Fn fsync "int fd"
	46	.Sh DESCRIPTION
	47	.Fn Fsync
	48	causes all modified data and attributes of
	49	.Fa fd
	50	to be moved to a permanent storage device.
	51	This normally results in all in-core modified copies
	52	of buffers for the associated file to be written to a disk.
	53	.Pp
	54	Note that while
	55	.Fn fsync
	56	will flush all data from the host
	57	to the drive (i.e. the "permanent storage
	58	device"), the
	59	drive itself may not physically
	60	write the data to the
	61	platters for quite some time
	62	and it may be written in an
	63	out-of-order sequence.
	64	.Pp
	65	Specifically, if the drive loses power
	66	or the OS crashes,
	67	the application
	68	may find that only some or none of their data was
	69	written. The disk drive may also re-order
	70	the data so that later writes
	71	may be present while earlier writes are not.
	72	.Pp
	73	This is not a theoretical
	74	edge case. This scenario is easily reproduced
	75	with real world workloads and drive
	76	power failures.
	77	.Pp
	78	For applications that require tighter guarantess about
	79	the integrity of their data, MacOS X provides the
	80	F_FULLFSYNC fcntl. The F_FULLFSYNC fcntl asks the
	81	drive to flush all buffered data to permanent
	82	storage. Applications such as databases that require
	83	a strict ordering of writes should use F_FULLFSYNC to
	84	ensure their data is written in the order they expect.
	85	Please see
	86	.Xr fcntl 2
	87	for more detail.
	88	.Pp
	89	.Sh RETURN VALUES
	90	A 0 value is returned on success. A -1 value indicates
	91	an error.
	92	.Sh ERRORS
	93	The
	94	.Fn fsync
	95	fails if:
	96	.Bl -tag -width Er
	97	.It Bq Er EBADF
	98	.Fa fd
	99	is not a valid descriptor.
	100	.It Bq Er EINVAL
	101	.Fa fd
	102	refers to a socket, not to a file.
	103	.It Bq Er EIO
	104	An I/O error occurred while reading from or writing to the file system.
	105	.El
	106	.Sh SEE ALSO
	107	.Xr sync 2 ,
	108	.Xr sync 8 ,
	109	.Xr update 8 ,
	110	.Xr fcntl 2
	111	.Sh HISTORY
	112	The
	113	.Fn fsync
	114	function call appeared in
	115	.Bx 4.2 .