FIO/RIO uses internal file descriptors to maintain information about the files that it processes. The descriptors contain the FORTRAN unit number of the file, the name of the file, the access mode and the record size. Knowledge of the access mode allows FIO/RIO to check for invalid operations, such as writing to a read-only file. Checking for invalid I/O operations before they are actually performed makes programs more robust, since the corresponding I/O error is never generated.
The FIO/RIO file descriptors do not contain any more information about a file than could be obtained by using the FORTRAN INQUIRE statement, but they store the information in such a way that it is more efficient to use descriptors than the INQUIRE statement.