| SYNC(2) | System Calls Manual | SYNC(2) | 
sync —
#include <unistd.h>
void
  
  sync(void);
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 file systems 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 descriptor attributes.
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.
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 written out. In NetBSD,
    sync() does not return until all buffers have been
    written.
| March 25, 2009 | NetBSD 9.3 |