1.\" $NetBSD: sync.2,v 1.15 2003/08/07 16:44:10 agc 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. Neither the name of the University nor the names of its contributors 15.\" may be used to endorse or promote products derived from this software 16.\" without specific prior written permission. 17.\" 18.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND 19.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 20.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 21.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE 22.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 23.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 24.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 25.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 26.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 27.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 28.\" SUCH DAMAGE. 29.\" 30.\" @(#)sync.2 8.1 (Berkeley) 6/4/93 31.\" 32.Dd June 4, 1993 33.Dt SYNC 2 34.Os 35.Sh NAME 36.Nm sync 37.Nd "synchronize disk block in-core status with that on disk" 38.Sh LIBRARY 39.Lb libc 40.Sh SYNOPSIS 41.In unistd.h 42.Ft void 43.Fn sync void 44.Sh DESCRIPTION 45The 46.Fn sync 47function forces a write of dirty (modified) buffers 48in the block buffer cache out 49to disk. 50The kernel keeps this information in core to reduce 51the number of disk I/O transfers required by the system. 52As information in the cache is lost after a system crash, 53kernel thread 54.Nm ioflush 55ensures that dirty buffers are synced to disk 56eventually. 57By default, a dirty buffer is synced after 30 seconds, 58but some filesystems exploit 59.Nm ioflush 60features to sync directory data and metadata faster 61(after 15 and 10 seconds, respectively). 62.Pp 63The function 64.Xr fsync 2 65may be used to synchronize individual file descriptor 66attributes. 67.Sh SEE ALSO 68.Xr fsync 2 , 69.Xr sync 8 70.Sh HISTORY 71A 72.Fn sync 73function call appeared in 74.At v6 . 75.Sh BUGS 76.Fn sync 77may return before the buffers are completely flushed. 78