- NetBSD Manual Pages
SYNC(2) NetBSD System Calls Manual SYNC(2)
Powered by man-cgi (2021-06-01).
Maintained for NetBSD
by Kimmo Suominen.
Based on man-cgi by Panagiotis Christias.
sync -- synchronize disk block in-core status with that on disk
Standard C Library (libc, -lc)
The 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
ioflush ensures that dirty buffers are synced to disk eventually. By
default, a dirty buffer is synced after 30 seconds, but some filesystems
exploit ioflush features to sync directory data and metadata faster
(after 15 and 10 seconds, respectively).
The function fsync(2) may be used to synchronize individual file descrip-
Many modern disks contain write-back caches. In theory 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.
It may also be desirable to use dkctl(8) or scsictl(8) to disable the
write-back cache entirely.
fsync(2), dkctl(8), scsictl(8), sync(8)
A sync() function call appeared in Version 6 AT&T UNIX.
Historically, 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 writ-
ten out. In NetBSD, sync() does not return until all buffers have been
NetBSD 7.0 March 25, 2009 NetBSD 7.0