[Rd] Calling FORTRAN function from R issue?

[Prof Brian Ripley]

On 06/03/2012 13:37, Berwin A Turlach wrote:
> G'day Berend,
>
> On Tue, 6 Mar 2012 13:06:34 +0100
> Berend Hasselman<bhh at xs4all.nl>  wrote:
>
> [... big snip ...]
>
>> But I would really like to hear from an Rexpert why you
>> shouldn't/can't use external here in the Fortran.
>
> Probably less a question for an Rexpert but for a Fortran expert....

Exactly ....

> If you insert "implicit none" (one of my favourite extensions that I
> always use) as the first statement after
> 	subroutine callzdotc(retval,n, zx, incx, zy, incy)
> you will see what is going on.  The compiler should refuse compilation
> and complain that the type of zdotc was not declared (at least my
> compiler does).  For FORTRAN to know that zdotc returns a double
> complex you need the
> 	double complex zdotc
> declaration in callzdotc.
>
> An
> 	external double complex zdotc
> would be necessary if you were to call another subroutine/function, say
> foo, that accepts functions as formal arguments and you want to pass
> zdotc via such an argument.  Something like
>
> 	subroutine callzdotc(....)
>          ....
>          external double complex zdotc
> 	....
>          call foo(a, b, zdotc)
>          ....
> 	return
> 	end
>
> 	subroutine(a, b, h)
> 	double complex h, t
> 	....
> 	t = h(..,..,..,..)
> 	....
> 	return
> 	end
>
> In C, the qualifier (or whatever the technical term is) "external" is

'extern' in C?

> used to indicate that the function/variable/symbol is defined in
> another compilation unit.  In FORTRAN77, "external" is used to tell the
> compiler that you are passing a function to another
> function/subroutine.  At least that is my understanding from what I
> re-read in my FORTRAN documentation.

Not quite.  It also means that you really want to externally link and 
not allow the compiler to find any routine of that name it knows about 
(e.g. an intrinsic).  See para 8.7 of 
http://www.fortran.com/F77_std/rjcnf-8.html (although I got this from my 
Fortran reference on my bookshelf).

> Thus, perhaps strangely, if there is only a
> 	external double complex zdotc
> declaration in your subroutine, the compiler doesn't know that a call

The only 'strangely' thing is that is accepted: AFAICS is it not valid 
according to the link above.

> to zdotc will return a double complex but will assume that the result
> has the implicitly defined type, a real*8 IIRC.

The Fortran default type for z* is real.

 > So zdotc is called, and
> puts a double complex as result on the stack (heap?), but within
> callzdotc a real as return is expected and that is taken from the
> stack (heap?), that real is than coerced to a double complex when
> assigned to retval.  Note that while I am pretty sure about the above,
> this last paragraph is more speculative. :)  But it would explain why
> the erroneous code returns 0 on little-endian machines.

We haven't been told which machines, and this actually matters down to 
the level of optimization used by the compilers.  But these days 
typically double complex gets passed in sse registers, and double is 
passed in fpu registers.

Note that passing double complex/Rcomplex as return values, from C or 
Fortran, is rather fragile in that it does depend on a pretty precise 
match of compilers (and R's configure does test the constructions it 
uses, and from time to time finds combinations which fail -- R 2.12.2 
was released to workaround one of them).

-- 
Brian D. Ripley,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
1 South Parks Road,                     +44 1865 272866 (PA)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595