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SYNC(2) System Calls Manual SYNC(2)

synchronize disk block in-core status with that on disk

#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 a sync() call is issued frequently by the in-kernel process update (about every 30 seconds).
The function fsync(2) may be used to synchronize individual file descriptor attributes.

fsync(2), sync(8)

The sync() function conforms to IEEE Std 1003.1-2008 (“POSIX.1”).

A sync() function appeared in Version 2 AT&T UNIX.

sync() may return before the buffers are completely flushed.
January 9, 2016 OpenBSD-6.1