1.\" Copyright (c) 1980, 1991, 1993 2.\" The Regents of the University of California. All rights reserved. 3.\" 4.\" Redistribution and use in source and binary forms, with or without 5.\" modification, are permitted provided that the following conditions 6.\" are met: 7.\" 1. Redistributions of source code must retain the above copyright 8.\" notice, this list of conditions and the following disclaimer. 9.\" 2. Redistributions in binary form must reproduce the above copyright 10.\" notice, this list of conditions and the following disclaimer in the 11.\" documentation and/or other materials provided with the distribution. 12.\" 3. Neither the name of the University nor the names of its contributors 13.\" may be used to endorse or promote products derived from this software 14.\" without specific prior written permission. 15.\" 16.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND 17.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 18.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 19.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE 20.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 21.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 22.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 23.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 24.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 25.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 26.\" SUCH DAMAGE. 27.\" 28.\" @(#)sync.2 8.1 (Berkeley) 6/4/93 29.\" 30.Dd December 1, 2017 31.Dt SYNC 2 32.Os 33.Sh NAME 34.Nm sync 35.Nd "schedule file system updates" 36.Sh LIBRARY 37.Lb libc 38.Sh SYNOPSIS 39.In unistd.h 40.Ft void 41.Fn sync void 42.Sh DESCRIPTION 43The 44.Fn sync 45system call forces a write of dirty (modified) buffers 46in the block buffer cache out 47to disk. 48The kernel keeps this information in core to reduce 49the number of disk I/O transfers required by the system. 50As information in the cache is lost after a system crash, a 51.Fn sync 52system call is issued 53frequently 54by the kernel process 55.Xr syncer 4 56(about every 30 seconds). 57.Pp 58The 59.Xr fsync 2 60system call 61may be used to synchronize individual file descriptor 62attributes. 63.Sh SEE ALSO 64.Xr fsync 2 , 65.Xr syncer 4 , 66.Xr sync 8 67.Sh HISTORY 68The 69.Fn sync 70function appeared in 71.At v3 . 72.Sh BUGS 73The 74.Fn sync 75system call 76may return before the buffers are completely flushed. 77