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Date: Wed, 03 Dec 2008 14:21:35 +0100
From: Reinhold Bader <Reinhold.Bader@lrz.de>
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Subject: Re: (j3.2006) (SC22WG5.3699)  Comment on N1761
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Hello,

Aleksandar Donev schrieb:
> Hi,
> 
>> Note especially that MPI
>> calls on scalar buffers can not be done in a backward compatible manner at
>> all.
> I am aware of this but it has nothing to do with MPI. It exists with all of C 
> Interop, and I have complained about it before, and J3/WG5 has explicitly 
> decided to keep it as it is. That is, a C routine for which one makes a 
> BIND(C) interface can either operate on scalars or on arrays, not on both. 
> 
> But, you are convincing me that maybe we should work on fixing this for the 
> TR. We specifically wanted to "fix" it for assumed-rank dummies, that is, we 
> wanted to allow the actual to be either a scalar or an array. Let's discuss 
> and think about it, perhaps off-line on an Interop e-mail list?

before taking this offline, I'd like to understand something about BIND(C)
which other people might know ...

Take the following example program:

module mod_label_interf
  use, intrinsic :: iso_c_binding
  implicit none
  interface foo
    subroutine foo_1(x, n, i) bind(c, name='Foo')
      import :: c_float, c_int
      real(c_float) :: x
      integer(c_int), value :: n, i
    end subroutine
    subroutine foo_2(x, n, i) bind(c, name='Foo')
      import :: c_float, c_int
      real(c_float) :: x(*)
      integer(c_int), value :: n, i
    end subroutine
  end interface
end module
program label_interf
  use mod_label_interf
  implicit none
  integer(c_int) :: i
  real(c_float) :: xx, xxx(3)
  i = 2
  call foo(xx, 1, i)
  call foo(xxx, 3, i)
  write(*, *) xx
  write(*, *) xxx
end program

together with the C implementation

#include <math.h>

void Foo(float *x, int n, int i) {
  int j;
  for (j=0; j<n; j++) {
    *(x+j)  = (float) (i + j) + 0.1;
  }
}

This code is accepted by various processors and executes in accordance with my
personal expectations (for what that is worth :-)).
It is not accepted by NAG's compiler, and also not by IBM xlf. NAG's error message is

Error: label_interf.f90: Duplicate binding label 'Foo' for external procedure FOO_2 and external procedure FOO_1

which references the rules given in clause 16.1. The point I do not understand is that the
interfaces define an external procedure. This seems to contradict the separation between
the definition of a procedure and its interface. More specifically, 2.2.3.1 says that

"an external procedure is a procedure that is defined by an external subprogram or by means other
than Fortran. An external procedure may be invoked by the main program or by any procedure of a
program."

and 12.3 says that

"The interface of a procedure determines the forms of reference through which it may be invoked."

My interpretation is that  in the above interface block using BIND(C), foo_1 is not an external procedure,
but only an alias for the external procedure 'Foo', i.e. foo_1 (and, via the generic interface, foo) is only the
form of reference to 'Foo' as cited above. Through the identical binding label, foo_1 and foo_2 refer to the
same global entity.
In particular, it then should be possible to reference a C pointer argument with a scalar as well
as an array.

If this is wrong, can anyone point out to me what clause in the standard contradicts this?


Regards
Reinhold