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FreeBSD 11.0 - man page for sync (freebsd section 2)

SYNC(2) 						      BSD System Calls Manual							   SYNC(2)

NAME
sync -- schedule file system updates
LIBRARY
Standard C Library (libc, -lc)
SYNOPSIS
#include <unistd.h> void sync(void);
DESCRIPTION
The sync() system call 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() system call is issued frequently by the kernel process syncer(4) (about every 30 seconds). The fsync(2) system call may be used to synchronize individual file descriptor attributes.
SEE ALSO
fsync(2), syncer(4), sync(8)
HISTORY
The sync() function appeared in Version 6 AT&T UNIX.
BUGS
The sync() system call may return before the buffers are completely flushed.
BSD
June 4, 1993 BSD