RE: CONST vs. VALUE on a pointer parameter

CONST = pass by constant reference ( a pointer in other words)
VALUE = pass by value

Using the following:
D data         s       10a
D ptr          s         *    inz(%addr(data))

D proc1        pr
D  in                    *    const

D proc2        pr
D  in                    *    value

D proc3        pr 
D  in                    *

Then:
Proc1(data) --> passes the addr of data, doesn't allow data to be
changed
Proc1(ptr) --> passes the addr of ptr, doesn't allow ptr to be changed

Proc2(data) --> won't compile, since the value of data is 10A and we
said we're passes a pointer.
Proc2(ptr) --> passes the value of ptr, ie. the addr of data. Allows
data to be changed.

Proc3(data) --> passes the addr of data, allows data to be changed
Proc3(ptr) --> passes the addr of ptr, allows ptr to be changed

These two calls are equivalent:
Proc2(ptr)
Proc3(data)

Proc1(data) is almost the same, except data can't be changed.



HTH,

Charles Wilt
--
iSeries Systems Administrator / Developer
Mitsubishi Electric Automotive America
ph: 513-573-4343
fax: 513-398-1121
  

> -----Original Message-----
> From: rpg400-l-bounces@xxxxxxxxxxxx 
> [mailto:rpg400-l-bounces@xxxxxxxxxxxx] On Behalf Of darren@xxxxxxxxx
> Sent: Friday, January 27, 2006 10:09 AM
> To: rpg400-l@xxxxxxxxxxxx
> Subject: CONST vs. VALUE on a pointer parameter
> 
> 
> I attempted to call the Qp0lGetAttr API to get the creation 
> date of a file
> in the IFS.  I used the following prototype.
> 
>      D Qp0lGetAttr     PR            10I 0 ExtProc('Qp0lGetAttr')
>      D  path                           *   Const
>      D  AttArrPtr                      *   Const
>      D  BuffPtr                        *   Const
>      D  BuffLenSent                  10U 0 Const
>      D  BuffLenNeed                  10U 0
>      D  BuffLenRet                   10U 0
>      D  FollowLink                   10U 0 Value
> 
> 
> The above did not work, apparently because I had used Const.  
> The program
> compiles fine both ways, but the API returns a -1 indicating 
> that some kind
> of error occurred.  I can toggle the 'path' parameter from 
> Value to Const
> and get it to fail, with everything else as shown below.  My 
> reason for
> using Const was to document that the variable being pointed 
> to was an Input
> only variable per the API's specs.  Can someone in the know, 
> educate me as
> to the different between Const and Value in this case?
> 
>      D Qp0lGetAttr     PR            10I 0 ExtProc('Qp0lGetAttr')
>      D  path                           *   Value
>      D  AttArrPtr                      *   Value
>      D  BuffPtr                        *   Value
>      D  BuffLenSent                  10U 0 Value
>      D  BuffLenNeed                  10U 0
>      D  BuffLenRet                   10U 0
>      D  FollowLink                   10U 0 Value
> 
> -- 
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