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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	.Fo fsync
	46	.Fa "int fildes"
	47	.Fc
	48	.Sh DESCRIPTION
	49	.Fn Fsync
	50	causes all modified data and attributes of
	51	.Fa fildes
	52	to be moved to a permanent storage device.
	53	This normally results in all in-core modified copies
	54	of buffers for the associated file to be written to a disk.
	55	.Pp
	56	Note that while
	57	.Fn fsync
	58	will flush all data from the host to the drive
	59	(i.e. the "permanent storage device"),
	60	the drive itself may not physically write the data
	61	to the platters for quite some time
	62	and it may be written in an out-of-order sequence.
	63	.Pp
	64	Specifically, if the drive loses power
	65	or the OS crashes,
	66	the application may find that only some or none of their data was written.
	67	The disk drive may also re-order the data
	68	so that later writes may be present, while earlier writes are not.
	69	.Pp
	70	This is not a theoretical edge case.
	71	This scenario is easily reproduced with real world workloads
	72	and drive power failures.
	73	.Pp
	74	For applications that require tighter guarantees
	75	about the integrity of their data,
	76	Mac OS X provides the F_FULLFSYNC fcntl.
	77	The F_FULLFSYNC fcntl asks the drive to flush all buffered data
	78	to permanent storage.
	79	Applications, such as databases,
	80	that require a strict ordering of writes
	81	should use F_FULLFSYNC to ensure that their data
	82	is written in the order they expect.
	83	Please see
	84	.Xr fcntl 2
	85	for more detail.
	86	.Pp
	87	.Sh RETURN VALUES
	88	.Rv -std fsync
	89	.Sh ERRORS
	90	The
	91	.Fn fsync
	92	system call will fail if:
	93	.Bl -tag -width Er
	94	.\" ==========
	95	.It Bq Er EBADF
	96	.Fa fildes
	97	is not a valid descriptor.
	98	.\" ==========
	99	.It Bq Er EINTR
	100	Its execution is interrupted by a signal.
	101	.\" ==========
	102	.It Bq Er EINVAL
	103	.Fa fildes
	104	refers to a file type (e.g., a socket)
	105	that does not support this operation.
	106	.\" ==========
	107	.It Bq Er EIO
	108	An I/O error occurred while reading from or writing to the file system.
	109	.El
	110	.Pp
	111	If a queued I/O operation fails,
	112	.Fn fsync
	113	may fail with any of the errors defined for
	114	.Xr read 2
	115	or
	116	.Xr write 2 .
	117	.Sh SEE ALSO
	118	.Xr fcntl 2 ,
	119	.Xr read 2 ,
	120	.Xr sync 2 ,
	121	.Xr write 2 ,
	122	.Xr sync 8 ,
	123	.Xr update 8
	124	.Sh HISTORY
	125	The
	126	.Fn fsync
	127	function call appeared in
	128	.Bx 4.2 .