RE: Help with parameter passing

["Bob cozzi" <cozzi@xxxxxxxxx>]

Scott,

OF course passing by value on the first parameter would help, but not solve
the issue at hand. We still need to use %ADDR as you pointed out. The other
changes would have no impact on what I'm looking. I don't think there's a
clear answer to this question.... something of an "anything" parameter is
want I'd like to be able to specify, but without using %ADDR(). 


Bob Cozzi
Cozzi Consulting
www.rpgiv.com


-----Original Message-----
From: rpg400-l-bounces@xxxxxxxxxxxx [mailto:rpg400-l-bounces@xxxxxxxxxxxx]
On Behalf Of Scott Klement
Sent: Thursday, October 09, 2003 6:22 PM
To: RPG programming on the AS400 / iSeries
Subject: Re: Help with parameter passing



> .....D myFunct        PI
>      D  anytype                     *
>      D   nLen                     10I 0
>      D   nDec                     10I 0
>      D   type                     10I 0
>
> Could I call myFunct and pass a characte field, or a numeric field
> or a data field?  If so, could I avoid using %ADDR to do it, or do I
> need to wrap the variable in %ADDR?

Actually the way you have this coded, you couldn't do either.  Because
your parms are passed by reference, you MUST pass a variable for each
parameter.  Like this:

    D myFunct        PR
    D  anytype                     *
    D   nLen                     10I 0
    D   nDec                     10I 0
    D   type                     10I 0

    D ALPHAVAR       C                  CONST(1)
    D ptr            S             *
    D myvar          S          100A
    D len            s           10I 0
    D dec            s           10I 0
    D type           s           10I 0

     /free
           ptr = %addr(myvar);
           len = %len(myvar);
           dec = 0;
           type = ALPHAVAR;
           myFunct(ptr: len: dec: type);
     /end-free

Personally, I think that's really ugly code.  I don't see why you'd want
to be able to change the address, length, dec or type parms...  so why
require them to be stored in variables?   Instead, pass the parms by
value:

    D myFunct        PR
    D  anytype                     *    value
    D   nLen                     10I 0  value
    D   nDec                     10I 0  value
    D   type                     10I 0  value

    D ALPHAVAR       C                  CONST(1)
    D myvar          S          100A

     /free
         myFunct(%addr(myvar): %len(myvar): 0: ALPHAVAR);
     /end-free

I think that code like this is much easier to read than the first example.
Of course, that's only a matter of opinion.

At any rate, you cannot avoid using %addr() unless you want to have a
separate prototype for each data type.

> My guess is that I would need to do pass %ADDR(CustNo), but is that
> allowed when I may want to change the data in the called proc?
> (i.e, I don't want to use CONST).

Yes, in fact passing the %addr(MYVAR) by value results in the same
underlying machine code as passing MYVAR by reference.   In either case
what's actually being passed from one procedure to another is an address.
The procedure that you're calling uses this address to access an area of
memory.  Since it's the same area of memory as the caller uses, the
changes are visible to the caller.

Since I passed the "anytype" parameter by VALUE, the newly called
procedure's changes to "anytype" won't be visible to the caller -- but
remember, "anytype" is not the data we want to change!   It's the ADDRESS
of the data we want to change.   We don't need to change the address.
Just the data that's at that address.

Maybe it helps to think of an address the same way you think of an RRN
(relative record number) of a file.   For example, if you call a
subprocedure and pass an RRN of 76, and you protect that 76 so the
procedure cannot change it...   That doesn't prevent the procedure from
updating the record #76!   It just prevents it from being able to change
the 76 to a 94.  Memory addresses are the same basic idea...


Anyway, the hard part about allowing "any" type of parameter to a
procedure is not going to be the prototype :)   It's going to be figuring
out how to write RPG code that works with any possible variable type, with
any possible length and any possible number of decimal places.   That'll
be a lot of work.

It's doable though... I've done it..
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