A function which will return -1 (TRUE) if the data file whose file handle is the argument is at, or beyond, its end. In other words, when PTR# points beyond the current end of the file. When reading a serial file, EOF# would go true when the last byte of the file had been read.
Writing to a byte beyond the current end of file updates the file length immediately, whether the record is physically written to storage at that time or not. However, the file must be closed in order to ensure that all the data written to it is physically written to storage.