1.\" $OpenBSD: fsync.2,v 1.15 2019/04/18 23:51:13 tedu Exp $ 2.\" $NetBSD: fsync.2,v 1.4 1995/02/27 12:32:38 cgd Exp $ 3.\" 4.\" Copyright (c) 1983, 1993 5.\" The Regents of the University of California. All rights reserved. 6.\" 7.\" Redistribution and use in source and binary forms, with or without 8.\" modification, are permitted provided that the following conditions 9.\" are met: 10.\" 1. Redistributions of source code must retain the above copyright 11.\" notice, this list of conditions and the following disclaimer. 12.\" 2. Redistributions in binary form must reproduce the above copyright 13.\" notice, this list of conditions and the following disclaimer in the 14.\" documentation and/or other materials provided with the distribution. 15.\" 3. Neither the name of the University nor the names of its contributors 16.\" may be used to endorse or promote products derived from this software 17.\" without specific prior written permission. 18.\" 19.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND 20.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 21.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 22.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE 23.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 24.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 25.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 26.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 27.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 28.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 29.\" SUCH DAMAGE. 30.\" 31.\" @(#)fsync.2 8.1 (Berkeley) 6/4/93 32.\" 33.Dd $Mdocdate: April 18 2019 $ 34.Dt FSYNC 2 35.Os 36.Sh NAME 37.Nm fsync , 38.Nm fdatasync 39.Nd synchronize a file's in-core state with that on disk 40.Sh SYNOPSIS 41.In unistd.h 42.Ft int 43.Fn fsync "int fd" 44.Ft int 45.Fn fdatasync "int fd" 46.Sh DESCRIPTION 47The 48.Fn fsync 49function causes all modified data and attributes of 50.Fa fd 51to be moved to a permanent storage device. 52This normally results in all in-core modified copies 53of buffers for the associated file to be written to a disk. 54.Pp 55The 56.Fn fdatasync 57function is similar to 58.Fn fsync 59except that it only guarantees modified data 60.Pq and metadata necessary to read that data 61is committed to storage. 62Other file modifications may be left unsynchronized. 63.Pp 64.Fn fsync 65and 66.Fn fdatasync 67should be used by programs that require a file to be in a known state, 68for example, in building a simple transaction facility. 69.Pp 70If 71.Fn fsync 72or 73.Fn fdatasync 74fail with 75.Er EIO , 76the state of the on-disk data may have been only partially written. 77To guard against potential inconsistency, future calls will continue failing 78until all references to the file are closed. 79.Sh RETURN VALUES 80.Rv -std fsync fdatasync 81.Sh ERRORS 82The 83.Fn fsync 84and 85.Fn fdatasync 86functions fail if: 87.Bl -tag -width Er 88.It Bq Er EBADF 89.Fa fd 90is not a valid descriptor. 91.It Bq Er EINVAL 92.Fa fd 93does not refer to a file which can be synchronized. 94.It Bq Er EIO 95An I/O error occurred while reading from or writing to the file system. 96.El 97.Sh SEE ALSO 98.Xr sync 2 , 99.Xr sync 8 100.Sh STANDARDS 101The 102.Fn fsync 103and 104.Fn fdatasync 105functions conform to 106.St -p1003.1-2008 . 107.Sh HISTORY 108The 109.Fn fsync 110system call first appeared in 111.Bx 4.1c , 112and the 113.Fn fdatasync 114function has been available since 115.Ox 5.4 . 116.Sh BUGS 117The 118.Fn fdatasync 119function is currently a wrapper around 120.Fn fsync , 121so it synchronizes more state than necessary. 122