C struct member : pointer to double (Fortran-C interop.)

1

Hello,

Considered the severe lacking of a mature and complete Fortran NURBS library, I’m trying to create an interface to the SISL NURBS (C language ) for Fortran. I’ve several doubts, but maybe it’s better to tackle one of them at a time.
For now, this is a struct I’m trying to interoperate with :

typedef struct SISLdir
{
  int igtpi;			/* 0 - The direction of the surface or curve
			               is not greater than pi in any
			               parameter direction.
			           1 - The direction of the surface or curve
			               is greater than pi in the first
			               parameter direction.
			           2 - The direction of the surface is greater
			               than pi in the second parameter
			               direction. 			     */
  double *ecoef;		/* The coordinates to the center of the cone.*/
  double aang;			/* The angle from the center whice describe the
			           cone.				     */
  double *esmooth;		/* Coordinates of object after smoothing.    */
} SISLdir;

This is the Fortran module with the struct interface:

module mod_sislfor

use iso_c_binding
implicit none

private 
public ::  sisldir 

!-------------------------------------

! interoperability with struct SISLdir

!------------------------------------

type, bind(c) :: sisldir

  integer (c_int) :: igtpi

  real(c_double):: ecoef(*)

  real(c_double):: aang

  real(c_double):: esmooth(*)

end type sisldir


end module mod_sislfor

I’m in doubt about how to treat the struct member double *esmooth and double *ecoef . Do you have some suggestions ?
Thanks

2

I believe they need to be type(c_ptr). I have a sneaking suspicion that they are pointers because the consuming code is going to use them kind of like arrays. You’ll need to be careful about allocating/deallocating the memory, and storing/retrieving the values from them. Probably you’ll need some extra C code to do that properly.

1 Like
3

Yes, as @everythingfunctional said, you will need to define everything with a star (or several stars) like this:

type(c_ptr) :: ecoef
type(c_ptr) :: esmooth

Here they seem to point toward arrays containing coordinates (but we can’t see if it’s 2D or 3D). You will need to read the C source code to be sure.
Note that in Fortran, there is some kind of loss of information since when you write type(c_ptr) :: ecoef you don’t even see that it’s pointers toward double. Just pointers… Therefore, adding comments seems a good practice…
And yes, if the library does not offer some functions to free those arrays, it’s another problem.

1 Like
4

If you try to compile your module, you will find it is not conforming syntax - the compiler will issue erros.

As mentioned by @everythingfunctional , if you attempt a direct Fortran interface, you will require type(c_ptr), say type(c_ptr) :: p_ecoef. And your Fortran code will need to use c_f_pointer to generate Fortran entities of double type for ecoef.

That is, unless you first write “wrappers” using Fortran 2018 capabilities on the C side and offer up modern Fortran interfaces to them for the Fortran users including yourself.

5

Thanks to all for the replies. The problem is now clearer. SISL library will not handle any pointer automatically. I’ve underestimated the difficulties involved, however it was a good occasion for reviewing some Fortran-C interop concept.
Thanks

6

The declaration real(c_double):: ecoef(*) declares ecoef to be an assumed-size array. This is appropriate for the declaration of a dummy argument, but not allowed for a type component.