RE: IFS api openDir() const vs value

Thanks John and Charles for help me further on this.

Best regards,
Danny


-----Original Message-----
From: RPG400-L [mailto:rpg400-l-bounces@xxxxxxxxxxxx] On Behalf Of Jon Paris
Sent: maandag 10 juli 2017 15:36
To: Rpg400 Rpg400-L <rpg400-l@xxxxxxxxxxxx>
Subject: Re: IFS api openDir() const vs value

Value is the correct translation - in fact most C APIs require the parms
passed by value.

Value causes the _content_ of the pointer to be passed. Const (or simply
leaving the keyword off) causes the _address_ of the pointer to be passed.
i.e. a pointer to the pointer. Not the same thing.

I don't think I've ever seen Options(*String) used with Const before - I'm
actually a little surprised that it is valid.

Ignoring pointers for a moment and looking at it more generically. If you
use Const then a pointer is passed - period. No matter what size the data
is. So it results in 16 bytes being pushed onto the stack. If you use value
then an item the size of the parm is pushed on the stack. Pass a 10i and 4
bytes get pushed. Pass 1,000 char and 1,000 bytes get pushed. As you can see
very different.

Const can be made to work in this scenario. For example passing (say) a 100A
field that includes the X'00' null terminator. That would result in a
pointer to the surname being passed - and would work. But the proto is not
strictly accurate and would not work generically for all situations.

So don't lie to the compiler - if you should be passing a pointer by value
then do so.


Jon Paris

www.partner400.com
www.SystemiDeveloper.com

On Jul 10, 2017, at 6:17 AM, Danny Rodius <danny.rodius@xxxxxxxxx> wrote:

Hi,



Try to use the ifs function to open and read a directory.



Doc state the following:

DIR *opendir(const char *dirname)



However when we write the following PR it's not working



d openIFSDir Pr * ExtProc('opendir')

d * const options( *string )



Found the similar definition in Scott klements 'Working with the IFS in

RPG'

chapter.7

Instead of using the const he uses the value. But isn't telling why.



When running now I get a correct opendir result.



Is there someone who know the reason for this difference between the
doc and the real world?



Best regards,

Danny

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