DDM-related differences between iSeries and System/36 files

Because of differences between the types of files supported by an iSeries™ server and a System/36™, several items need to be considered when DDM is used between these two servers.

Generally, when a System/36 file is created locally (by the BLDFILE utility, for example), the System/36 user specifies such things as the type of file (S = sequential, D = direct, or I = indexed), whether records or blocks are to be allocated, how many of them are to be allocated, and how many additional times this amount can be added to the file to extend it.

Also, you can specify whether the file is to be delete-capable (DFILE) or not (NDFILE). In files specified as not delete-capable, records can be added or changed in the file, but not deleted.

Once these attributes have been specified, System/36 then creates the file and fills it with the appropriate hexadecimal characters. If a System/36 user specifies the file as:

A sequential file, the entire file space is filled with hex 00 characters and the end-of-file (EOF) pointer is set to the beginning of the initial extent. If you attempt to read an empty sequential file, an EOF condition is received.
A direct file that is delete-capable, the entire file space is filled with hex FF characters (deleted records) and the EOF pointer is set to the end of the initial extent. If you attempt to read an empty direct file that is delete-capable, a record-not-found condition is received.
A direct file that is not delete-capable, the entire file space is filled with hex 40 characters (blank or null records) and the EOF pointer is set to the end of the initial extent. If you attempt to read an empty direct file that is not delete-capable, a blank record is returned for every record in the file until the end of the file is reached.
An indexed file, it is prepared in the same manner as sequential files.

Typically, once a delete-capable file has been in use, it contains a relatively continuous set of active records with only a few deleted records, possibly an end of data marker, and then a continuous set of deleted records to the end of the file (EOF) space. This means that, unless the file is reorganized, a user can undelete (recover) a deleted record.

Of the three types of System/36 files, System/36 indexed files differ little from iSeries-supported logical files. If an iSeries source program is to use DDM to access the other types of files on a System/36, the iSeries application programmer should first consider the items remaining in this topic selection that relate to System/36.