Updated: 2021/Apr/14

SYNC(2)                       System Calls Manual                      SYNC(2)

     sync - synchronize disk block in-core status with that on disk

     Standard C Library (libc, -lc)

     #include <unistd.h>


     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 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.

     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
     written out.  In NetBSD, sync() does not return until all buffers have
     been written.

NetBSD 9.99                     March 25, 2009                     NetBSD 9.99