The output files are normally buffered only if they are not
    directed to a terminal, except for stderr, which is not buffered
    by default.

    The fflush function flushes the Compaq C RTL buffers. However,
    RMS has its own buffers. The fflush function does not guarantee
    that the file will be written to disk. (See the description of
    fsync for a way to flush buffers to disk.)

    If the file pointed to by file_ptr was opened in record mode
    and if there is unwritten data in the buffer, then fflush always
    generates a record.