Question

I am working with both C++ and Fortran code. The C++ code needs to call a Fortran subroutine. One parameter of the Fortran subroutine has Fortran type complex.

The C++ code is in one file, and the Fortran code subroutine is in another file. I am using gcc and gfortran on a 64-bit GNU/Linux system.

Here is a snippet showing the Fortran subroutine declaration (and a few additional lines):

SUBROUTINE  minp (AMP,L,L2,FMINP,PHI)
   REAL*4 AMP( L ),FMINP( L )
   COMPLEX PHI( L )

In the C++ file, I understand that arguments passed to the Fortran code need to be passed by reference, and not by value. The Fortran subroutine is declared as a function in the C++ code (at the top of the source code file) using the extern keyword.

extern "C"
{
minp_ (float *amp, int &L, int &L2, float *fminp, complex *phi);

}

However, the last parameter of the function is a complex C array. How might it be possible to:

1. Allocate memory for a complex array in C/C++ to be passed in as the phi argument?
2. Pass the array as an argument to the Fortran subroutine in such a way that the memory can be used by the Fortran code?

Answer 1

For interfacing C (and C++ via extern C) and Fortran, I recommend using Fortran’s ISO C Binding. It provides complex types, C_FLOAT_COMPLEX, C_DOUBLE_COMPLEX, and C_LONG_DOUBLE_COMPLEX, to match C’s types. By using the ISO C Binding and writing a Fortran interface declaration to match the C code, you will instruct the Fortran compiler to match the calling conventions of the C compiler. If you use the complex type with the ISO C Binding, dealing with the array won’t be any different from a float/double/long double array. The gfortran manual has examples in the Chapter Mixed-Language Programing and describes the types and intrinsic procedures of the ISO C Binding in the Chapter Intrinsic Modules.