Return an array of doubles via FORTRAN (DLL) for further processing in python

Yunzhi

I've been struggling with this issue for a while now, and searching for the query/applicable documentation hasn't yielded any workable results either. Hence posting here.

What do I want to accomplish:

  • I have some program written in FORTRAN77 that takes some parameters and returns a fixed length array of doubles.
  • I wish to use another programming language to call this function (currently, I use python for testing; but this will change at any time - so f2py is not an option here)

A FORTRAN routine can be summarized as follows:

      FUNCTION TST (IIN)
      IMPLICIT LOGICAL (A-Z)
cGCC$ ATTRIBUTES DLLEXPORT, STDCALL :: TST
      INTEGER           II,IIN
      DOUBLE PRECISION, DIMENSION(3) :: TST
C
C     Test function:
      DO 1001 II = 1,3
         TST(II) = II * 1.11D0 + IIN
 1001 CONTINUE
      RETURN
      END

Compile with gcc-fortran as follows:

gfortran -static -c -fdollar-ok -fno-align-commons TEST.for
gfortran -shared -mrtd -static -o TEST.dll TEST.def TEST.o

Where TEST.def maps TST to tst_

No problem so far, but the "how do I call this function and handle the return value?" question arises in python.

Using the "depwalker" tool; the TST function apparently expects 2 arguments ( tst_@8 ). In addition to integers, I think this should be a pointer to the output array or its length.

My python code is as follows:

import ctypes as cs
import numpy as np

#import dll library hooks:
tdll = cs.WinDLL(pathToDLL)
#test:
tdll.TST.restype = None
tdll.TST.argtypes = [cs.POINTER(cs.c_long), cs.POINTER(cs.c_double*3)]

#start testing:
Ar = [0.0, 0.0, 0.0]
_A = np.array(Ar).ctypes.data_as(cs.POINTER(cs.c_double*len(Ar)))
_L = cs.c_long(3)
tdll.TST(cs.byref(_L), _A)

The problem is that this code (and all the variations I've tried) will produce the error: OSError: exception: access violation writing 0x0000000F. If I try to pass the first parameter ByValue it results inOSError: exception: access violation reading 0x0000000F

Can someone point me in the right direction here?

Yunzhi

After some tools; also thanks to Vladimir F for his valuable advice, a solution to the above problem has been found.

In order to make it work, some minor changes are required on the python side:

import ctypes as cs
import numpy as np

#import dll library handle:
tdll = cs.WinDLL(pathToDLL)
#specify result and argument types
tdll.TST.restype = None
tdll.TST.argtypes = [cs.POINTER(cs.POINTER(cs.c_double*3)), cs.POINTER(cs.c_long)]

#call the dll function 'TST':
Ar = (cs.c_double*3)()
_A = cs.pointer(Ar)
tdll.TST(cs.byref(_A), cs.byref(cs.c_long(3)))
result = Ar[:]

Hope this article is helpful to others.

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