Fortran and C interoperability: receive return value from c (calling from Fortran)

I have read a lot of posts on a similar topic but I have not yet succeeded resolving this. I should mention that I have simplified my code a lot for this post. My intention is to use a c function by calling it from fortran77 and receiving back values from c. The fact that I mention fortran77 is because I want to link my code to a much larger project that uses fortran77, but I am willing to consider solutions with other versions of fortran if they do the job and if you believe they will simplify my problem.

I have two files: Try_stack.f and client2.c. I am compiling my code as:

gcc -c client2.c
gfortran -g Try_stack.f client2.o -o combined

My Try_stack.f file:

      program circle

      call circle2
      stop
      end

      subroutine circle2 
      dimension rread(2)
      double precision r, area,rread
      external client

      area = 3.
      rread(1)=area
      rread(2)=area+10.
      write (*,*) 'Area = ',  rread(1)
      call client(rread)
      retNread = rread(1) * 2
      write(*,*) 'new nread is: ',retNread

      return
      end

And my client2.c file:

#include <stdio.h> 
#include <stdlib.h> 
#include <string.h> 
#include <stdint.h>

int client_(double rread[2]) 
{ 
    double result;
    result=1.;
    rread[1]=result;
    printf("%.2lf",rread);
    return 0; 
} 

After running the compiled version I am getting:

Area = 3.0000000000000000
0.00 new nread is: 6.00000000

But, I wanted the return value to the fortran program to have been equal to 8.000 instead of 6.0000 (because fortran sends the value 3., 1. is added to 3. and a 4.0 should return back to fortran for multiplying it with 2.). If I wanted to write this in a simple way to explain it, I would say:

• First, I want the fortran file to send number 3. to c (actually I want to exchange arrays).

• Second, I want the c file to take number 3. and add 1.

• Third, I want c to return back the result to the fortran file, i.e. number 4.

• Finally, I want fortran to continue computing, in this case multiply 4*2=8.

I read a lot about iso_c_binding but I have not obviously managed to utilise it, plus it requires recent versions of Fortran if my understanding is correct. Any help will be much appreciated.