Relative Files

A relative file is a file in which each record is identified by its ordinal position in the file (record 1, record 2 and so on). This means that records can be accessed randomly as well as sequentially:

• For sequential access, simply execute a READ or WRITE statement to access the next record in the file.
• For random access, define a data item as the relative key. Then specify in the data item the ordinal number of the record that you need to READ or WRITE.

Because records can be accessed randomly, access to relative files is fast.

Although you can declare variable length records for a relative file, this can be wasteful of disk space because the system assumes the maximum record length for all WRITE statements to the file and pads the unused character positions. This is done so that the COBOL file handling routines can quickly calculate the physical location of any record, given the record's record number in the file.

As relative files always contain fixed length records, no space is saved by specifying data compression. In fact, if data compression is specified for a relative file, it is ignored by the File Handler.

Each record in a relative file is followed by a two-byte record marker which indicates the current status of the record. The status of a record can be:

x"0D0A" (Windows) or x"0A" (UNIX) - record present

x"0D00" (Windows) or x"00" (UNIX) - record deleted or never written

When you delete a record from a relative file, the record's contents are not removed immediately. The record's record marker is updated to show that it has been deleted, but the contents of the deleted record remain physically in the file until a new record is written. If you need to remove the data from the file for security reasons, follow the procedure below:

1. Use REWRITE to overwrite the record, for example with space characters.
2. Delete the record.

To define a relative file, specify ORGANIZATION IS RELATIVE in the SELECT clause for the file in your COBOL program.

To access records randomly, you must also:

• Specify ACCESS MODE IS RANDOM or ACCESS MODE IS DYNAMIC in the SELECT clause for the file
• Define a relative key in the Working-Storage Section of your program

For example:

 select relfil assign to "relfil.dat"
     organization is relative
     access mode is random
     relative key is relfil-key.
 ...
 working-storage section.
 01 relfil-key   pic 9(8) comp-x.

The example code above defines a relative file. The access mode is random and so a relative key relfil-key is defined. For random access, you must always supply a record number in the relative key, before trying to read a record from the file.

If you specify ACCESS MODE IS DYNAMIC, you can access the file both sequentially and randomly.