Type marshalling to call a fortran subroutine from C#

Question

I\'m trying to call a FORTRAN77 subroutine from C# code using P/invoke - in case you\'re interested, I\'m trying to wrap some of the functionality offered by the ARPACK library

Answer 1

1) Quoting Wikipedia:

Single precision, called "float" in the C language family, and "real" or "real*4" in Fortran. This is a binary format that occupies 32 bits (4 bytes) and its significand has a precision of 24 bits (about 7 decimal digits).

So marshal it as a float. You could have tested that one, it's either a float or a double.

2) Quoting this Fortran 77 Tutorial:

Fortran 77 uses the so-called call-by-reference paradigm. This means that instead of just passing the values of the function/subroutine arguments (call-by-value), the memory address of the arguments (pointers) are passed instead.

Pass every parameter by reference.

Answer 2

I suggest you start with some small test code. Compile a FORTRAN .dll with some subroutines with simple parameters and play around with C# to get the calling to work. Also you may wish to wrap the Fortran with many arguments into a single structure (TYPE keyword), which makes the interop so much easier.

Here is a working example that you can use the get many ideas of how it works.

Original FORTRAN code:

  SUBROUTINE CALC2(BLKL,BLKW, N_LAMINA,N_SLICE, LOAD, SLOP,SKW,    &
                    DIA1,DIA2, Y1, Y2, N1, N2, DROP1, DROP2,        &
                    PARRAY, P_MAX, P_MAX_INDEX, ENDEFCT)
  !DEC$ ATTRIBUTES DLLEXPORT :: CALC2
  !DEC$ ATTRIBUTES ALIAS:'CALC2' :: CALC2
  !DEC$ ATTRIBUTES VALUE :: BLKL, BLKW, N_LAMINA, N_SLICE, LOAD, SLOP, SKW
  !DEC$ ATTRIBUTES VALUE :: DIA1, DIA2, Y1, Y2, N1, N2
  IMPLICIT NONE
  INTEGER*4, INTENT(IN) ::N_LAMINA, N_SLICE
  REAL*4, INTENT(IN) :: BLKL, BLKW, LOAD, SLOP, SKW,     &
                        DIA1, DIA2, Y1, Y2, N1, N2,   &
                        DROP1(MAX_LAMINA), DROP2(MAX_LAMINA)
  REAL*4, INTENT(OUT):: PARRAY(MAX_PATCH), P_MAX
  INTEGER*4, INTENT(OUT) :: P_MAX_INDEX, ENDEFCT
  INTEGER*4 :: NDIAG, N_PATCH
  REAL*4 :: SLNG, SWID
  REAL*4 :: DROPS_1(MAX_LAMINA), DROPS_2(MAX_LAMINA)

...

  END SUBROUTINE CALC2

with various scalar and array values in real and integer form. For example DROP1 is input 1D array. PARRAY is outputing a 2D array as a 1D array. BLKL are input floats.

Notice the !DEC$ ATTRIBUTES VALUE decoration to avoid declaring everything as ref.

In C# the code is called by

    [DllImport("mathlib.dll")]
    static extern void CALC2(float major_dim, float minor_dim, 
        int N_lamina, int N_slices, float total_load, 
        float slope, float skew, float diameter_1, float diameter_2, 
        float youngs_1, float youngs_2, float nu_1, float nu_2, 
        float[] drops_1, float[] drops_2, float[] pressures, 
        ref float p_max, ref int p_max_index, ref EndEffect end_effect);

...
   {
        float max_pressure = 0;
        int max_pressure_index = 0;
        float[] pressures = new float[Definition.MAX_PATCH];
        EndEffect end_effect = EndEffect.NO;

        CALC2(length, width, lamina_count, slice_count, load, slope, skew, 
            dia_1, dia_2, y_1, y_2, n_1, n_2, drops_1, drops_2, pressures, 
            ref max_pressure, ref max_pressure_index, ref end_effect);
    }

note I do not pass any strings.