Re: pointers

["Simon Coulter" <shc@xxxxxxxxxxxxxxxxx>]

Hello James,

Answers in line below ....

Regards,
Simon Coulter.

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//--- forwarded letter -------------------------------------------------------
> Date: Fri, 23 Jun 2000 16:38:59 -0600
> From: "James David Rich" <james@dansfoods.com>
> To: rpg400-l@midrange.com
> Reply-To: RPG400-L@midrange.com
> Subject: pointers

> 
> Can someone explain the difference between a basing pointer in RPG and a
> pointer in C?  

An RPG basing pointer has no type.  It is like a void pointer in C.  It just 
points to 
'stuff'.  You determine how that 'stuff' appears by defining a variable, 
structure, or 
array and basing it on the pointer.  The basing pointer locates the storage, 
the based 
variable defines the view of the storage.

That means that you cannot increment a structure pointer and have it move up in 
storage by the size of the structure.  You have to increment the pointer by the 
actual 
number of bytes.  For example, you can't do this in RPG:

 typedef struct { char stuff[100];
                  int  stuffLen;
                } var_t;

 var_t * ptr;  /* Declare a pointer of type var_t */

 ptr = malloc( (sizeof(var_t) * 10) );  /* Allocate storage for 10 structures */

 ptr++;  /* point to next structure */

That's actually a good thing when dealing with the system APIs because you 
should 
never increment the pointer by the size of the structure, rather you should use 
the 
entry length returned by the API (usually in the header for the various user 
space 
APIs.  However you can do something like:

        ptr = ptr + sizeof(var_t);

if you need to.  The RPG equivalent is:

        EVAL    ptr = ptr + %size(data)
 
>Is there some other kind of pointer besides a basing
> pointer in RPG?

RPG also supports a procedure pointer which is similar to a function pointer in 
C.

> 
> Can someone explain how to do write the following C code in RPG:
> 
> void
> main (){
> char *buffer;
> 
> buffer = (char *) malloc (sizeof(char) * 1024);
> return;
> }
> 

Aside from the C-style issues .... what you have below will allocate 1K of 
storage but 
there is not much you can do with it like that because you can't dereference 
the 
pointer.  You must use one or more based variables -- see the next example.

> I know that I need to do something like:
> 
> D ptr         S       *
> C               alloc(e)      1024    ptr
> 
> but now how do I specify that ptr points to a buffer of type char?  Even
> better:  how do I use alloc to allocate memory that looks like this:
> 
> D data                DS
> D field                       100A

  D data                DS               BASED(@data)
  D field                       100A

  D size                S         5I 0

  C               EVAL          size = %size(data)
  C               ALLOC(E)      size    @data

Now you can do stuff with field or with data simply by referencing the names.  
Better 
still is to wrap the allocation op-codes in procedures (or simply use the C 
runtime 
versions) so you can write the code like:

  C             EVAL            @data = allocStg(%size(data))
  C             EVAL            @data = reallocStg(%size(data) * 10)
  C             CALLP           deallocStg(@data)

> 
> basically I want a 2-dimensional array that I can dynamically increase or
> descrease.

You will need to manage the array dimensions yourself but you can use based 
arrays in 
a similar fashion to the above examples.  REALLOC will increase allocated space 
and 
DEALLOC will release the storage.

> 
> James Rich
> james@dansfoods.com

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