Re: [Rd] capturing value of C or Fortran function

From: Thomas Lumley <tlumley_at_u.washington.edu>
Date: Mon 18 Dec 2006 - 17:13:34 GMT

On Wed, 13 Dec 2006, Harris A. Jaffee wrote:

> Re: src/main/dotcode.c:do_dotCode()
>
> The value, if there is one, of a function called by .C or .Fortran is not
> captured, so one needs a wrapper. To avoid that, the user would have to
> declare that there is a value, specify its type, and supply an R variable
> to hold the value. Presumably, all of these could be combined in a new
> "VALUE=var" parameter. Then do_dotCode could arrange the correct type of
> C variable to receive the value (for every one of the 66 possible instances
> of the call), and then transfer that value to 'var'.

Ick. There is no way (I hope) that .C() is going to stop being a pure function and go and write in other variables

> It seems doable with
> a strong enough macro, if a bit complicated. Or one could say VALUE=<type>
> and get the value as a new, (n+1)_th element of the current list value.

AFAICS this is possible only for a predefined list of return types. Even worse "vector of 4 ints" and "vector of 5 ints" count as different types, since it is not possible for do_dotCode to work out how much memory is pointed to by a pointer.

It seems a lot of extra code for very little extra generality. If someone submitted a patch that also carefully documented that there was no slowdown for .C/.Fortran and no loss of ANSI C conformance it might be considered, but I can't predict any great enthusiasm for it.

         -thomas



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