Re: DS fields

[dhandy@xxxxxxxxxxx (Douglas Handy)]

Dave,

Doug said:

>>  Subfields are not pointers per se, they just have addresses within the
>> address space for the DS as a whole.

Dave said:

>If that is the case, then how do you account for a field from a file that
>is "sub divided" in a DS?

(snip) 

>In this case, a portion of the DS is altered, and the updated Acct# is
>written to MasterFile.  I thought that the Acct# DS had a pointer to
>the I/O buffer where MasterFile is read from & written from.
>If pointers aren't used, how does this the change in the DS show
>up in the change in the file?

All fields are stored in memory separate from the I/O buffer.  The
fields are not pointers to within the buffer space.  After a record is
read, each field is moved from the buffer to the field's memory space.

The opposite happens when a record is written.  First the fields are
moved to the buffer, then the buffer is written.

If you have ever run a program in debug and tried to step into a disk
I/O operation, that is why you need to step over each field's movement
to/from the buffer.  This can by suppressed in RPG IV by using the
H-spec keyword Option( *NoDebugIO ), but is the default behavior.

So if you have a DB field as part of a DS, then overlay a portion of
the field, the DB field is copied to the buffer prior to the write and
the disk record reflects the overlay.

Doug
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