.\" $NetBSD: sync.2,v 1.17 2009/03/25 06:46:21 wiz Exp $ .\" .\" Copyright (c) 1980, 1991, 1993 .\" The Regents of the University of California. All rights reserved. .\" .\" Redistribution and use in source and binary forms, with or without .\" modification, are permitted provided that the following conditions .\" are met: .\" 1. Redistributions of source code must retain the above copyright .\" notice, this list of conditions and the following disclaimer. .\" 2. Redistributions in binary form must reproduce the above copyright .\" notice, this list of conditions and the following disclaimer in the .\" documentation and/or other materials provided with the distribution. .\" 3. Neither the name of the University nor the names of its contributors .\" may be used to endorse or promote products derived from this software .\" without specific prior written permission. .\" .\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND .\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE .\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE .\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE .\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL .\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS .\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) .\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT .\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF .\" SUCH DAMAGE. .\" .\" @(#)sync.2 8.1 (Berkeley) 6/4/93 .\" .Dd March 25, 2009 .Dt SYNC 2 .Os .Sh NAME .Nm sync .Nd "synchronize disk block in-core status with that on disk" .Sh LIBRARY .Lb libc .Sh SYNOPSIS .In unistd.h .Ft void .Fn sync void .Sh DESCRIPTION The .Fn sync function forces a write of dirty (modified) buffers in the block buffer cache out to disk. The kernel keeps this information in core to reduce the number of disk I/O transfers required by the system. As information in the cache is lost after a system crash, kernel thread .Nm ioflush ensures that dirty buffers are synced to disk eventually. By default, a dirty buffer is synced after 30 seconds, but some filesystems exploit .Nm ioflush features to sync directory data and metadata faster (after 15 and 10 seconds, respectively). .Pp The function .Xr fsync 2 may be used to synchronize individual file descriptor attributes. .Sh CAUTIONS Many modern disks contain write-back caches. In theory .Fn sync flushes these. In practice there are many possible ways for this mechanism to go astray. It is prudent (where possible) to allow a few seconds after syncing for everything to settle before e.g. turning off the power. .Pp It may also be desirable to use .Xr dkctl 8 or .Xr scsictl 8 to disable the write-back cache entirely. .Sh SEE ALSO .Xr fsync 2 , .Xr dkctl 8 , .Xr scsictl 8 , .Xr sync 8 .Sh HISTORY A .Fn sync function call appeared in .At v6 . .Pp Historically, .Fn sync would schedule buffers for writing but not actually wait for the writes to finish. It was necessary to issue a second or sometimes a third call to ensure that all buffers had in fact been written out. In .Nx , .Fn sync does not return until all buffers have been written. usr.bin/script/script.c?id=949a1d1b5526bceb958f84509f7fe860768be6cf'>use system to parse argschristos 2009-10-17add more features found in other uniceschristos 2009-04-13Fix WARNS=4 issues (-Wcast-qual -Wshadow)lukem 2008-07-21Remove the \n and tabs from the __COPYRIGHT() strings.lukem 2007-09-09PR/36955: Ighighi: Use fseeko instead of fseek.christos 2007-08-20Apparently seeking forward on a FIFO does not work (why?). Read instead.christos 2007-08-19deal with non-regular files.christos 2006-06-14Fix PR: 26363: script core dumps on incorrect input files.liamjfoy 2006-03-29ANSIfy. de-P.rpaulo 2004-07-13Add -d to usage; use getprogname(3). From Kouichirou Hiratsuka in PR 26227.wiz 2003-08-07Move UCB-licensed code from 4-clause to 3-clause licence.agc 2002-06-21Add an alternate "typescript mode" that contains timestamps and otheratatat 1998-12-19don't use union wait; an int is good enough.christos 1998-04-02Need <time.h> for time() and ctime() prototypes.kleink 1997-10-19WARNSify, fix .Nm usage, deprecate register, getopt returns -1 not EOFlukem