SYNC(2) System Calls Manual SYNC(2)
sync - update super-block
Sync causes all information in core memory that should be on disk to be
written out. This includes modified super blocks, modified i-nodes,
and delayed block I/O.
It should be used by programs which examine a file system, for example
icheck, df, etc. It is mandatory before a boot.
The writing, although scheduled, is not necessarily complete upon
return from sync.
(sync = 36.)