fsync - synchronize a file's in-core state with that on disk
Fsync causes all modified data and attributes of fd to be moved to a permanent storage device. This normally results in all in-core modi-
fied copies of buffers for the associated file to be written to a disk.
Fsync should be used by programs that require a file to be in a known state, for example, in building a simple transaction facility.
A 0 value is returned on success. A -1 value indicates an error.
The fsync fails if:
[EBADF] Fd is not a valid descriptor.
[EINVAL] Fd refers to a socket, not to a file.
[EIO] An I/O error occurred while reading from or writing to the file system.