RE: [EXT] Re: Advice passing a large parameter -- RPG400-L

So my understanding is this...

Option 1:
dcl-pi *n;
inPlist1 likeds(plist1);
end-pi;

VERSUS

Option 2:
dcl-pi *n char(20);
inPlist1 likeds(plist1) CONST;
end-pi;

In #1 I can simply change ANY of the fields
In #2 I simply have to return the value (changed or not in the logic of my program)

Which is "best"?
Greg

-----Original Message-----
From: RPG400-L [mailto:rpg400-l-bounces@xxxxxxxxxxxxxxxxxx] On Behalf Of Vernon Hamberg
Sent: Friday, April 12, 2019 11:47 AM
To: RPG programming on IBM i <rpg400-l@xxxxxxxxxxxxxxxxxx>
Subject: Re: [EXT] Re: Advice passing a large parameter

Greg

If you want to change only one element of the DS, it might be better to pass that subfield to a stand-alone parameter with the same declaration.
The rest of the DS is then untouched.

If you use CONST on the parameter and pass the DS, you can't change the subfield - you can't change anything in the DS - there are probably ways, see an article by Ted Holt that I will not cite, to see how to shoot yourself in both feet!

You can also pass the DS and be sure nothing else in it is changed - this allows for future change.

Hope that makes sense - quick response a snowy (i.e., flaky!) morning.

Vern

On 4/12/2019 10:13 AM, Greg Wilburn wrote:

So I'm confused...
The value that may or may not change (or return) is CHAR(20). Everything else would remain unchanged.

Should I use CONST or not?
Do I need to define the DS as TEMPLATE?

dcl-pi *n;
inPlist1 likeds(plist1);
end-pi;

dcl-ds plist1 extname('PLIST1DS') template;
end-ds;

-----Original Message-----
From: RPG400-L [mailto:rpg400-l-bounces@xxxxxxxxxxxxxxxxxx] On Behalf
Of DeLong, Eric
Sent: Friday, April 12, 2019 11:02 AM
To: RPG programming on IBM i <rpg400-l@xxxxxxxxxxxxxxxxxx>
Subject: Re: [EXT] Re: Advice passing a large parameter

I agree with David’s assessment. By default, RPG passes parameters by reference, meaning that a pointer is passed from the caller’s scope, so that the called program can access the callers storage directly.

The size and format of the address space is agreed upon by both parties. Since no data is actually being moved, the size of the parameter data is irrelevant to the performance question.

Passing by value is the expensive option you should try to avoid. This also includes large “return values” which are also passed by value. IBM gave us RTNPARM option to resolve that issue, but it’s not the default behavior, so just be aware.

Passing with CONST option performs well despite some small overhead with initialization of constant value storage in caller.

-Eric DeLong

On Apr 12, 2019, at 9:18 AM, David Gibbs via RPG400-L <rpg400-l@xxxxxxxxxxxxxxxxxx> wrote:

CAUTION: EXTERNAL EMAIL

*********************************************************************
*

On 4/12/2019 9:11 AM, Greg Wilburn wrote:
1. Just define the DS the same and pass it in (it could be
CONST)

I'm pretty sure this is the best way to do it. If you define the parm as const, I'm pretty sure it will be passed by reference, so there will be no additional storage allocated.

3. Define the PI as pointer and pass the address

No need to do this, as mentioned above, that's essentially what's already happening.

david

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