RE: Passing pointers as parms

[Joel Fritz <JFritz@xxxxxxxxxxxxxxxx>]

I looked back at your original message, and I think we're misunderstanding
each other.  I do understand that in dynamic calls all parameter passing is
by reference.  In fact, you can't use the VALUE keyword on a parameter for a
MAIN procedure. I've also encountered the problems you can run into with
writing to storage beyond the bounds of a parameter received on a dynamic
call. 

My question was how are pointers passed by reference?  Seems like if you
pass a pointer by reference you are passing a pointer to a pointer and the
receiving program that bases a variable on that parameter is using the
storage location that holds the pointer rather than the storage location
pointed to by the original pointer.  E.g. pgma has a field, buffer, and a
pointer to buffer called ptrtobuffer.  Pgmb takes a pointer as a parm and
bases a data structure, bufferstruct, on the pointer.  Pgma passes
ptrtobuffer to pgmb in a dynamic call.  The value  contained in ptrtobuffer
is the "address" of buffer.  Since ptrtobuffer is passed by reference, what
pgmb receives ought to be a pointer to the "address" of ptrtobuffer, not the
"address" of buffer.  I could be wrong, but I see two levels of indirection
here and one level of dereferencing. 

I find it perplexing that in a subprocedure defined within a program,
passing a pointer by reference or by value yields the same results in the
calling routine.  Seems like there's something going on behind the scenes
that I'd love to find out.    

BTW, the advantage of passing the pointer, rather than the data structure
itself is purely one of less typing.  It's a way of "directly" overlaying a
data structure on a parameter instead of explicitly assigning, in one way or
another, the value of a parameter to a data structure and vice versa.  I've
never done it before, but I'm thinking of using it.  It'll make my code more
obscure. <g>    

> -----Original Message----- 
> From: jpcarr@TREDEGAR.COM [mailto:jpcarr@TREDEGAR.COM]
> Sent: Tuesday, November 28, 2000 6:34 PM
> To: RPG400-L@midrange.com
> Subject: Re: Passing pointers as parms
> 
> 
> 
> Joel
> 
> Jim said
> >If you pass a pointer to the data structure yourself, then 
> have a data
> >structure pointer point to this value, you are doing 
> manually the same
> thing
> >that RPG and the OS are already doing.  There should be no 
> reason to do
> this
> >I could think of, unless you are looking for some side 
> affect of this.
> >>Jim Langston
> 
> That is what I was trying to say.
> 
> In this example you don't have a VALUE keyword on the pointer 
> parm.   So it
> will pass the parm by Reference
> (just like all  dynamic calls do)
> 
> ------------
> D ptrtst2         pr                  extpgm('PTRTST2')
> D pointer                         *
> -----------
> 
> This example would pass the value itself
> 
> D ptrtst2         pr                  extpgm('PTRTST2')
> D pointer                         *   VALUE
> --------------
> 
> You may have a problem doing it this way.   I never tried it.
> 
> If you try using the CONST keyword it would be passed as a READ ONLY
> Reference.
> 
> 
> Just for grin's did you try my example of 3 dynamic called 
> programs (PGMA,
> PGMB, PGMC) ?
> Which demonstates this idea.
> 
> John Carr
> 
> 
> 
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