1.\" $NetBSD: sync.2,v 1.12 2002/10/01 18:10:46 wiz Exp $ 2.\" 3.\" Copyright (c) 1980, 1991, 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.\" @(#)sync.2 8.1 (Berkeley) 6/4/93 35.\" 36.Dd June 4, 1993 37.Dt SYNC 2 38.Os 39.Sh NAME 40.Nm sync 41.Nd "synchronize disk block in-core status with that on disk" 42.Sh LIBRARY 43.Lb libc 44.Sh SYNOPSIS 45.Fd #include \*[Lt]unistd.h\*[Gt] 46.Ft void 47.Fn sync void 48.Sh DESCRIPTION 49The 50.Fn sync 51function forces a write of dirty (modified) buffers 52in the block buffer cache out 53to disk. 54The kernel keeps this information in core to reduce 55the number of disk I/O transfers required by the system. 56As information in the cache is lost after a system crash, 57kernel thread 58.Nm ioflush 59ensures that dirty buffers are synced to disk 60eventually. 61By default, a dirty buffer is synced after 30 seconds, 62but some filesystems exploit 63.Nm ioflush 64features to sync directory data and metadata faster 65(after 15 and 10 seconds, respectively). 66.Pp 67The function 68.Xr fsync 2 69may be used to synchronize individual file descriptor 70attributes. 71.Sh SEE ALSO 72.Xr fsync 2 , 73.Xr sync 8 74.Sh HISTORY 75A 76.Fn sync 77function call appeared in 78.At v6 . 79.Sh BUGS 80.Fn sync 81may return before the buffers are completely flushed. 82