Intro to pointers was: Re: data structure overlay question.

[rob@xxxxxxxxx]

Comments below:
<snip>
>DDSINPUT        E DS                  EXTNAME(DSUPDPAG) INZ

>D @QzAmt          S               *   INZ(%addr(qzamt1))
>D @QzChk          S               *   INZ(%addr(qzchk1))

>D $QzAmt          S                   like(QZAMT1 ) based(@qzamt)
>D                                      dim(12)
>D $QzChk          S                   like(QZCHK1 ) based(@qzchk)
>D                                      dim(12)

I have a few questions on this technique before I change my program.

1. the '*' in the field length implies "variable length"?

2. is it the "based" keyword on the $qzxxx that eventually defines the
length of the @qzxxx fields?

3. or is it the fact that the next field in the DS (@qzchk) starts at the
address of QZCHK1 that defines the length of @qzamt?   if so, what defines
the length of @qzchk?
<endsnip>

My comments:

1.  The '*' is not in the field length.  It is in the field type.  'a' is
alphanumeric, 'p' is packed, '*' is a pointer.
2.  "based" means that the address of this field in memory starts at the
pointer location.  Thus $qzamt is stored in memory at the position set by
the pointer @qzamt.  The value of the pointer @qzamt is equal to the
address in memory of the field qzamt1, as set by the INZ(%addr... function.
An eval on one field will change the value of the other.

I used to be pointer phobic.  But once you get used to them they are quite
handy.  I especially like them in using list api's versus the QUSRTVUS api.

Rob Berendt
--
"They that can give up essential liberty to obtain a little temporary
safety deserve neither liberty nor safety."
Benjamin Franklin