calling a fortran dll from python using cffi with multidimensional arrays - dll

Question

I use a dll that contains differential equation solvers among other useful mathematical tools. Unfortunately, this dll is written in Fortran. My program is written in python 3.7 and I use spyder as an IDE.
I successfully called easy functions from the dll. However, I can't seem to get functions to work that require multidimensional arrays.
This is the online documentation to the function I am trying to call:
https://www.nag.co.uk/numeric/fl/nagdoc_fl26/html/f01/f01adf.html
The kernel dies without an error message if I execute the following code:
import numpy as np
import cffi as cf
ffi=cf.FFI()
lib=ffi.dlopen("C:\Windows\SysWOW64\DLL20DDS")
ffi.cdef("""void F01ADF (const int *n, double** a, const int *lda, int *ifail);""")
#Integer
nx = 4
n = ffi.new('const int*', nx)
lda = nx + 1
lda = ffi.new('const int*', lda)
ifail = 0
ifail = ffi.new('int*', ifail)
#matrix to be inversed
ax1 = np.array([5,7,6,5],dtype = float, order = 'F')
ax2 = np.array([7,10,8,7],dtype = float, order = 'F')
ax3 = np.array([6,8,10,9],dtype = float, order = 'F')
ax4 = np.array([5,7,9,10], dtype = float, order = 'F')
ax5 = np.array([0,0,0,0], dtype = float, order = 'F')
ax = (ax1,ax2,ax3,ax4,ax5)
#Array
zx = np.zeros(nx, dtype = float, order = 'F')
a = ffi.cast("double** ", zx.__array_interface__['data'][0])
for i in range(lda[0]):
a[i] = ffi.cast("double* ", ax[i].__array_interface__['data'][0])
lib.F01ADF(n, a, lda, ifail)
Since function with 1D arrays work I assume that the multidimensional array is the issues.
Any kind of help is greatly appreciated,
Thilo

Answer 1

Not having access to the dll you refer to complicates giving a definitive answer, however, the documentation of the dll and the provided Python script may be enough to diagnose the problem. There are at least two issues in your example:
The C header interface:
Your documentation link clearly states what the function's C header interface should look like. I'm not very well versed in C, Python's cffi or cdef, but the parameter declaration for a in your function interface seems wrong. The double** a (pointer to pointer to double) in your function interface should most likely be double a[] or double* a (pointer to double) as stated in the documentation.
Defining a 2d Numpy array with Fortran ordering:
Note that your Numpy arrays ax1..5 are one dimensional arrays, since the arrays only have one dimension order='F' and order='C' are equivalent in terms of memory layout and access. Thus, specifying order='F' here, probably does not have the intended effect (Fortran using column-major ordering for multi-dimensional arrays).
The variable ax is a tuple of Numpy arrays, not a 2d Numpy array, and will therefore have a very different representation in memory (which is of utmost importance when passing data to the Fortran dll) than a 2d array.
Towards a solution
My first step would be to correct the C header interface. Next, I would declare ax as a proper Numpy array with two dimensions, using Fortran ordering, and then cast it to the appropriate data type, as in this example:
#file: test.py
import numpy as np
import cffi as cf
ffi=cf.FFI()
lib=ffi.dlopen("./f01adf.dll")
ffi.cdef("""void f01adf_ (const int *n, double a[], const int *lda, int *ifail);""")
# integers
nx = 4
n = ffi.new('const int*', nx)
lda = nx + 1
lda = ffi.new('const int*', lda)
ifail = 0
ifail = ffi.new('int*', ifail)
# matrix to be inversed
ax = np.array([[5, 7, 6, 5],
[7, 10, 8, 7],
[6, 8, 10, 9],
[5, 7, 9, 10],
[0, 0, 0, 0]], dtype=float, order='F')
# operation on matrix using dll
print("BEFORE:")
print(ax.astype(int))
a = ffi.cast("double* ", ax.__array_interface__['data'][0])
lib.f01adf_(n, a, lda, ifail)
print("\nAFTER:")
print(ax.astype(int))
For testing purposes, consider the following Fortran subroutine that has the same interface as your actual dll as a substitute for your dll. It will simply add 10**(i-1) to the i'th column of input array a. This will allow checking that the interface between Python and Fortran works as intended, and that the intended elements of array a are operated on:
!file: f01adf.f90
Subroutine f01adf(n, a, lda, ifail)
Integer, Intent (In) :: n, lda
Integer, Intent (Inout) :: ifail
Real(Kind(1.d0)), Intent (Inout) :: a(lda,*)
Integer :: i
print *, "Fortran DLL says: Hello world!"
If ((n < 1) .or. (lda < n+1)) Then
! Input variables not conforming to requirements
ifail = 2
Else
! Input variables acceptable
ifail = 0
! add 10**(i-1) to the i'th column of 2d array 'a'
Do i = 1, n
a(:, i) = a(:, i) + 10**(i-1)
End Do
End If
End Subroutine
Compiling the Fortran code, and then running the suggested Python script, gives me the following output:
> gfortran -O3 -shared -fPIC -fcheck=all -Wall -Wextra -std=f2008 -o f01adf.dll f01adf.f90
> python test.py
BEFORE:
[[ 5 7 6 5]
[ 7 10 8 7]
[ 6 8 10 9]
[ 5 7 9 10]
[ 0 0 0 0]]
Fortran DLL says: Hello world!
AFTER:
[[ 6 17 106 1005]
[ 8 20 108 1007]
[ 7 18 110 1009]
[ 6 17 109 1010]
[ 1 10 100 1000]]