       sync ?fileId?

              If fileId is not specified, or if it is and this system does not
              support the fsync system call, issues  a  sync  system  call  to
              flush  all  pending disk output.  If fileId is specified and the
              system does support the fsync system call, issues  an  fsync  on
              the  file corresponding to the specified Tcl fileId to force all
              pending output to that file out to the disk.

              If fileId is specified, the file must be writable.  A flush will
              be issued against the fileId before the sync.

              The  infox  have_fsync command can be used to determine if "sync
              fileId" will do a sync or a fsync.
