NetBSD/lib/libc/sys/sync.2

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.\" $NetBSD: sync.2,v 1.15 2003/08/07 16:44:10 agc Exp $
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.\" @(#)sync.2 8.1 (Berkeley) 6/4/93
.\"
.Dd June 4, 1993
.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 SEE ALSO
.Xr fsync 2 ,
.Xr sync 8
.Sh HISTORY
A
.Fn sync
function call appeared in
.At v6 .
.Sh BUGS
.Fn sync
may return before the buffers are completely flushed.