C-Fortran Mixed Programming

Question

I am working on a project where I have to use a Fortran library in C. In the Fortran library there is a common block containing a complex*16, 4x4 array. Now in C, a complex variable is simply a struct containing two elements and since it is complex*16, the elements should be long double, which is the corresponding C data type in Fortran. So I have a struct with two long doubles.

I am to access the elements of this array. Good thing is, I can already do that along with all other common variables of the library. The problem is that elements that I import from the array are,

1) Not in the order as the should be, "even after taking into account the difference in the array structure of C and Fotran".

2) While most elements are right, two are very different from what they should be.

3) I get the right elements (except for the two) only if I use double instead of long double. When I use long double (and the correct character conversions) I get something entirely different which clearly points to a problem with conversions.

I have exhausted every explaination I had but nothing works. My code for priting arrays in C is the following:

for (j=0;j<=3;j++){
    printf("%s", "\n");
    for(k=0;k<=3;k++){            
        printf("%s %d %s %d %s %s %LE %s %LE %s",
          "(", k+1, "," ,j+1, ")", "{",
          (long double)mssmmixing_.neunmx[k][j].dr,
          " ",
          (long double)mssmmixing_.neunmx[k][j].di,
          "}\n");           
    }
}

Additional Info: Since I have to mix Fortran Object files, I am using gfortran to compile the C files. If I use GNU C compiler instead, it throws errors about not recognizing gfortran routines. This might also be a source of problem, may be the gfortran does not recognize long doubles in C.

Any help will be useful.

Answer 1

For mixing Fortran and C, I recommend the use of the ISO_C_Binding. It even has a Fortran type C_LONG_DOUBLE_COMPLEX that matches the C type long double _Complex -- see http://gcc.gnu.org/onlinedocs/gfortran/ISO_005fC_005fBINDING.html. As part of the Fortran language standard these types are guaranteed to match (when you use compatible compilers). Very likely C_LONG_DOUBLE_COMPLEX is actually the same as complex*16 but you could try, in Fortran, copying between the two types in case the memory layout is different. You should compile the Fortran source files with gfortran and the C with gcc. It is easiest to link with gfortran. (Or use other "brand" compilers.)