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 50causes all modified data and attributes of 51.Fa fildes 52to be moved to a permanent storage device. 53This normally results in all in-core modified copies 54of buffers for the associated file to be written to a disk. 55.Pp 56Note that while 57.Fn fsync 58will flush all data from the host to the drive 59(i.e. the "permanent storage device"), 60the drive itself may not physically write the data 61to the platters for quite some time 62and it may be written in an out-of-order sequence. 63.Pp 64Specifically, if the drive loses power 65or the OS crashes, 66the application may find that only some or none of their data was written. 67The disk drive may also re-order the data 68so that later writes may be present, while earlier writes are not. 69.Pp 70This is not a theoretical edge case. 71This scenario is easily reproduced with real world workloads 72and drive power failures. 73.Pp 74For applications that require tighter guarantees 75about the integrity of their data, 76Mac OS X provides the F_FULLFSYNC fcntl. 77The F_FULLFSYNC fcntl asks the drive to flush all buffered data 78to permanent storage. 79Applications, such as databases, 80that require a strict ordering of writes 81should use F_FULLFSYNC to ensure that their data 82is written in the order they expect. 83Please see 84.Xr fcntl 2 85for more detail. 86.Pp 87.Sh RETURN VALUES 88.Rv -std fsync 89.Sh ERRORS 90The 91.Fn fsync 92system call will fail if: 93.Bl -tag -width Er 94.\" ========== 95.It Bq Er EBADF 96.Fa fildes 97is not a valid descriptor. 98.\" ========== 99.It Bq Er EINTR 100Its execution is interrupted by a signal. 101.\" ========== 102.It Bq Er EINVAL 103.Fa fildes 104refers to a file type (e.g., a socket) 105that does not support this operation. 106.\" ========== 107.It Bq Er EIO 108An I/O error occurred while reading from or writing to the file system. 109.El 110.Pp 111If a queued I/O operation fails, 112.Fn fsync 113may fail with any of the errors defined for 114.Xr read 2 115or 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 125The 126.Fn fsync 127function call appeared in 128.Bx 4.2 . 129