Passing Multidimensional C Array to FORTRAN Function for Modification

Question

I'm attempting to pass a multidimensional array, initialized in C, to a FORTRAN function and modify the array elements. I found a similar question here, but I've been unable to get it working with the multidimensional array. Attempting to write to the array results in a core dump. What am I missing?

Code example:

#include <stdio.h>
#include <stdlib.h>

double f_function(double ****);

double ****alloc_4D_double(int wlen, int xlen, int ylen, int zlen)
{
    int i,j,k;

    double ****ary = (double****)malloc(wlen*sizeof(double***));

    for (i = 0; i < wlen; i++) 
    {
        ary[i] = (double***)malloc(xlen*sizeof(double**));

        for (j = 0; j < xlen; j++) 
        {
            ary[i][j] = (double**)malloc(ylen*sizeof(double*));

            for (k = 0; k < ylen; k++) 
            {
                ary[i][j][k] = (double*)malloc(zlen*sizeof(double));
            }
        }
    }

    return ary;
}

int main ( void ) {

    double ****cary = alloc_4D_double(2, 2, 2, 2);

    // intialize values
    for (j=0; j < 2; j++)
    {       
        for (k=0; k < 2; k++)
        {
            for (l=0; l < 2; l++)
            {
                for (m=0; m < 2; m++)
                {
                    cary[j][k][l][m] = 0;
                }
            }
        }
    }   

    f_function (cary);

    return 0;
}

and

    real(4) function f_function(cary)
        use, intrinsic :: iso_c_binding    
        !DEC$ ATTRIBUTES C :: f_function

        implicit none    

        real(c_double)        , intent(out), dimension(2,2,2,2) :: cary

        ! attempt to overwrite value (core dump!)
        cary=1.1_c_double

        ...

    end function f_function

user1220978 · Accepted Answer · 2013-09-18 20:04:33Z

2

In Fortran, multidimensional arrays are stored as contiguous data in memory.

However, you declare your multidimensional array in C by pointer to pointers to... Obviously Fortran don't know what to do with this.

You simply have to malloc(wlen*xlen*ylen*zlen*sizeof(double)), and pass this single pointer to your Fortran program.

If you badly need your multiple pointer representation, you can first allocate your data array as contiguous data with a single malloc, and then allocate your pointers to pointers, making them ultimately point inside the data array. And like previously, you pass to Fortran only the pointer to data array. Thus Fortran won't see the multiple pointers, but you will still be able to use them in C.

answered Sep 18, 2013 at 20:04

user1220978

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2 Comments

javacavaj Over a year ago

i think this will work. do you have an example of "and then allocate your pointers to pointers, making them ultimately point inside the data array"?

user1220978 Over a year ago

Simply replace ary[i][j][k] = (double*)malloc(zlen*sizeof(double)); with ary[i][j][k] = &data[zlen*(k + ylen*(j + xlen*i))]; in your code, where data is your "main" pointer.

M. S. B. · Accepted Answer · 2013-09-18 22:16:37Z

0

I suggest using the ISO_C_BINDING in Fortran and the intrinsic from the module c_f_pointer, which will associate a C pointer with a Fortran array. There is an example in the gfortran manual under "Mixed Language Programming".

answered Sep 18, 2013 at 22:16

M. S. B.

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2 Comments

user1220978 Over a year ago

But there is still the problem that, as is, data is scattered among many double***, not contiguous as in a single double*.

M. S. B. Over a year ago

By C pointer, I meant a pointer, and not a pointer to a pointer to ..., as you already explained.

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