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FSYNC(2) System Calls Manual FSYNC(2)

fsync, fdatasync
synchronize a file's in-core state with that on disk

#include <unistd.h>
fsync(int fd);
fdatasync(int fd);

The fsync() function causes all modified data and attributes of fd to be moved to a permanent storage device. This normally results in all in-core modified copies of buffers for the associated file to be written to a disk.
The fdatasync() function is similar to fsync() except that it only guarantees modified data (and metadata necessary to read that data) is committed to storage. Other file modifications may be left unsynchronized.
fsync() and fdatasync() should be used by programs that require a file to be in a known state, for example, in building a simple transaction facility.

The fsync() and fdatasync() functions return the value 0 if successful; otherwise the value -1 is returned and the global variable errno is set to indicate the error.

The fsync() and fdatasync() functions fail if:
fd is not a valid descriptor.
fd does not refer to a file which can be synchronized.
An I/O error occurred while reading from or writing to the file system.

sync(2), sync(8)

The fsync() and fdatasync() functions conform to IEEE Std 1003.1-2008 (“POSIX.1”).

The fsync() system call first appeared in 4.1cBSD, and the fdatasync() function has been available since OpenBSD 5.4.

The fdatasync() function is currently a wrapper around fsync(), so it synchronizes more state than necessary.
August 14, 2013 OpenBSD-5.8