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.\"     @(#)sync.2	8.1 (Berkeley) 6/4/93
.\" $FreeBSD$
.\"
.Dd December 1, 2017
.Dt SYNC 2
.Os
.Sh NAME
.Nm sync
.Nd "schedule file system updates"
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In unistd.h
.Ft void
.Fn sync void
.Sh DESCRIPTION
The
.Fn sync
system call 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, a
.Fn sync
system call is issued
frequently
by the kernel process
.Xr syncer 4
(about every 30 seconds).
.Pp
The
.Xr fsync 2
system call
may be used to synchronize individual file descriptor
attributes.
.Sh SEE ALSO
.Xr fsync 2 ,
.Xr syncer 4 ,
.Xr sync 8
.Sh HISTORY
The
.Fn sync
function appeared in
.At v3 .
.Sh BUGS
The
.Fn sync
system call
may return before the buffers are completely flushed.