Passing Fortran array via ISO_C_BINDING

Question

I'm trying to get a Fortran code base to call a C library call using ISO_C_BINDING, but I am having trouble with passing the Fortran array.

I have created a simple example that illustrates my problem.

main.f90

program main
    use iso_c_binding
    implicit none

    interface
            subroutine stuff(s,d) bind(c,name='doStuff')
                    import :: c_char
                    import :: c_ptr
                    import :: c_double

                    character(c_char), value :: s
                    type(c_ptr), value :: d
            end subroutine stuff
    end interface

    character(len=1) ::c1
    real, allocatable :: d1(:)

    c1 = 'a'
    allocate ( d1(0:10) )

    d1(0) = 2.0
    d1(1) = 2.5
    d1(2) = 3.0
    d1(3) = 4.0

    write (*,*) d1

    call stuff(c1,c_loc(d1))

end program main

func.c

//a function
int doStuff (char c, double* w)
{
    printf("%c\n",c);
    printf("%d %d %d %d\n",w[0], w[1], w[2], w[3]);
    return 1;
}

compile line

icc -c func.c
ifort -o magic.go main.f90 func.o

output

2.000000       2.500000       3.000000       4.000000      0.0000000E+00    0.0000000E+00  0.0000000E+00  0.0000000E+00  0.0000000E+00  0.0000000E+00  0.0000000E+00
a
0 -1938231792 1 1

The char is being passed correctly but the array isn't and I am at a loss how to correct it. The examples I've seen suggest changing

double* w

to

void* w

but I can't do that as I can't change the library code to make this work. I just tried creating a new function to cast a void* to double* to no effect :(. Likewise changing the array so that it is not allocatable would be infeasible.

Thoughts?

Timmy, in func.c, did you really mean: printf("%f %f %f %f\n",w[0], w[1], w[2], w[3]); ? (%d prints an 'int' value) — Mahonri Moriancumer
– Mahonri Moriancumer, Commented May 21, 2014 at 21:41
Informative SO post: stackoverflow.com/tags/fortran-iso-c-binding/info — Mahonri Moriancumer
– Mahonri Moriancumer, Commented May 21, 2014 at 21:45

francescalus · Accepted Answer · 2014-05-21 22:04:37Z

3

You have a problem with the declaration of d1. Better would be

real(c_double), allocatable, target :: d1(:)

That is: it needs to be the correct kind to be interoperable with C's double; to be an argument to c_loc it must have either the target or pointer attribute.

Also, as noted by Mahonri Moriancumer your printf format isn't appropriate for the double. And, possibly, having stuff as a function rather than a subroutine may be worth considering.

answered May 21, 2014 at 22:04

francescalus

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3 Comments

francescalus Over a year ago

The important part is real(c_double): I just remain surprised by how uncaring ifort is about attributes for c_loc.

Sangjun Lee Over a year ago

Why target attribute is needed here? I tested with the target attribute, and the code still works fine

francescalus Over a year ago

@SangjunLee, the target or pointer attribute is required by the Fortran standard (see F2018 18.2.3.6) for arguments to c_loc. Although some compilers (notably ifort) don't care and have things seem to work, for maximum correctness and portability it is desirable to have one of those attributes.

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