Re: Another ILE question : CONST vs VALUE

Hi Alan,

> Scott does this neat little trick with his EXPAT test programs (Scott
> I hope I am remembering this correctly) where he pass a pointer to a
> data structure defined in local storage to an intermediate program by
> value along with a address of a procedure to do a callback.

Yes, I do that... but I don't use CONST when I do that, since my goal is usually to use the parameter as *output*, not input. So it's just using a pointer by value in one place, and a straight reference field in another.

Callbacks are an area where the RPG compiler developers don't let us use a matching prototype. You can't simply define it in one place and re-use it, because one place has to have EXTPROC(PointerName) and the other has to be a straight PR/PI.

But, you're right... I've tricked the compiler into dereferencing a CONST parameter in the past. There's one situation I've found where there's no other way (other than making a copy of the data). The iconv() API...

The input to the iconv() API requires a pointer to a pointer. It will change the pointer as it translates the data from the input buffer to the output buffer.

For example, let's say you're writing a program that sends a string to a TCP socket. You want to wrap the process in a subprocedure, so you can simply say sendTcp('my string here') and it'll automatically add CRLF, and translate the whole thing to an arbitrary CCSID (presumably, ASCII or Unicode) then send it over the socket.

The iconv() API would be used in that subprocedure to translate the data. No problem... iconv() doesn't change the input buffer when it translates it... it puts the output into the output buffer.

Unfortunately, iconv() needs a pointer to a pointer as input. It actually increments the input pointer as it goes along, thus keeping track of where it left off during the translation. If it encounters an untranslatable character, you can use this mechanism to skip it.

Unfortunately, since the input to iconv() is a pointer to a pointer, there's no way to use it on a CONST field! You can't get a pointer to a CONST field in RPG. It's a limitation of the language syntax.

So you have no choice... you have to trick the compiler into dereferencing the pointer to the CONST field, even though you have no intention of changing the data it points to. It drives me nuts, but that's the way RPG works.