Fortran interface to call a C function that returns a pointer to an array

GeoffH — Fri, 14 Aug 2020 11:25:58 GMT

Back again after being away for quite a while...

After much searching, I found what I believe to be the closest answer to my problem is on stackoverflow (SO) at https://stackoverflow.com/questions/3852790/fortran-interface-to-call-a-c-function-that-return-a-pointer, (posted nearly 10 years ago!)

I quote this because using that example keeps the code simple and still illustrates my problem.

I want to return an array that has been created/memory allocated in C++ and be able to analyse the answer in Fortran, because that is where the bulk of the code for this application lies. My application goes off into C++ to produce the integer array answer and returns it to the Fortran program via the C interface. The original SO example used a single double precision variable as the return. I’ve changed it to integer because that is what I will be dealing with in my application. The example code (as changed) works.

I have highlighted with comments the changes that I have tried to make to return an array pointer, but I’ve run out of ideas. (I could say, “Oh for the bad old days when I could just equivalence an integer to an iarray(1) and go beyond the size of the array”, but I won’t. It’s good to have coding protections, but sometimes it gets frustrating.)

I am using Visual Studio 2017 and Intel Fortran parallel_studio_xe_2019_update5_composer.

My modified example of the original SO code:

[code here

! ps_test_pointers.f90

program foo

use, intrinsic :: iso_c_binding, only : c_ptr, &

c_f_pointer, &

c_int

implicit none

type(c_ptr) :: c_p!(:) ! <-------

integer(c_int), pointer :: f_p!(:) ! <-------

interface

function foofunc() bind(c)

import :: c_ptr

implicit none

type(c_ptr) :: foofunc!(:) ! <-------

end function foofunc

end interface

c_p = foofunc()

call c_f_pointer(c_p, f_p)

print *, f_p

end program foo

##########################################################

// ps_test_pointersC.cpp : 'Subroutine' only.

extern "C" {

int bar[3] = { 2, 3, 4 };

int *foofunc() {

return bar;

}

]

As I said above, the code works, in the sense that it prints out the first element of the array (‘2’).

If I add the ‘(:)’ to the definition of f_p, the code compiles without error, but when I run it, the program fails with the run-time error: “forrtl: severe (408): fort: (7): Attempt to use pointer F_P when it is not associated with a target” at the line “call c_f_pointer(c_p, f_p)”.

I have tried declaring c_p as an array (“c_p(:)”), but I get the same error in the same place.

I have also tried calling c_p as an argument to a subroutine – still only using integers

[code here

! ps_test_pointers.f90

program foo

use, intrinsic :: iso_c_binding, only : c_ptr, &

c_f_pointer, &

c_int

implicit none

type(c_ptr) :: c_p!(:) ! <-------

integer(c_int), pointer :: f_p!(:) ! <-------

interface

subroutine foofunc(c_p) bind(c)

import :: c_ptr

implicit none

type(c_ptr) :: c_p!(:) ! <-------

end subroutine foofunc

end interface

call foofunc(c_p)

call c_f_pointer(c_p, f_p)

print *, f_p

end program foo

##########################################################

// ps_test_pointersC.cpp : 'Subroutine' only.

extern "C" {

int bar[3] = { 2, 3, 4 };

void foofunc(int *rtn) {

rtn = bar;

}

]

but the created pointer in the C function never gets assigned to c_p on return (hence f_p is never defined).

Reading around the problem, I hope I’m not at the bleeding edge of compiler implementation and have exposed a problem between restrictions tightening but not coping with all the use cases!

Is there a solution to this? Any?

Contributions will be gratefully received!

Re: Fortran interface to call a C function that returns a pointer to an array

Arjen_Markus — Fri, 14 Aug 2020 12:16:02 GMT

I think you are working a trifle too hard here:

type(c_ptr) :: c_p(:) would be an array of C pointers, not a C pointer to a C array.

type(cptr) :: c_p is the one you need, because C returns an array and whatever is behind that address is of no interest to the declaration on the Fortran side, because C does not pass on this information.

To turn c_p into an array on the Fortran side use:

call c_f_pointer( c_p, f_p, [3] ) ! [3] is the shape of the resulting Fortran array - fill in whatever is needed

I may be overlooking something - I have not tried it, but this is the principle :).

Re: Fortran interface to call a C function that returns a pointer to an array

GeoffH — Sat, 15 Aug 2020 01:35:43 GMT

Thank you Arjen_Markus. That's part of the solution. I hinted that this example is a very simplified form of my problem that illustrates the issue. That said, I do have the size of the array in the returned pointer array and I can extract that and return it in the foofunc (or more accurately for a Fortran user, foosub).

I still have some more work to do to get all the info back into the Fortran program, but you have broken my deadlock. Thanks.

Re: Fortran interface to call a C function that returns a pointer to an array

GeoffH — Fri, 21 Aug 2020 11:25:19 GMT

Yep. All working now.

topic Fortran interface to call a C function that returns a pointer to an array in Intel® Fortran Compiler

Fortran interface to call a C function that returns a pointer to an array

Re: Fortran interface to call a C function that returns a pointer to an array

Re: Fortran interface to call a C function that returns a pointer to an array

Re: Fortran interface to call a C function that returns a pointer to an array