Direct access files

Direct access files are structured binary files, in which is it possible to go directly to a desired record.

To enable this, a file index is used.

• index = position for next I/O operation

When it can be manipulated:

• set index, then do I/O
• access records in any order
• read or write
• direct access file

The DIRECT_IO package is used

• not text files
• direct and sequential I/O both possible

An example program shows the use of a direct access file in a program dealing with bank accounts. A direct access file is appropriate here because only one of the many records in the file is updated.

Using direct access files

General file management

I/O library instantiation

• similar to SEQUENTIAL_IO
• record type must precede library instantiation

variable definition

• as for text files, sequential files

open/close files

• INOUT_FILE allows reading and  writing
• file mode can also be IN_FILE or OUT_FILE

procedures and functions that use the file index:

• SIZE - how many records in the file?
• INDEX - what is the current value of the file index?
• SET_INDEX - set the value of the file index

Input/Output

relative to index:

        SET_INDEX (accfile, recordnum);
        WRITE (accfile, rec);

specific location:

        WRITE (accfile, rec, recordnum);

file index is automatically incremented after I/O

• sequential access possible

        while INDEX(accfile) <= 100 loop
            READ (accfile, rec);
        end loop;