Legacy GM Attempting to use Fortran dll with GMS1, having issues

[PhenomenalDev]

I have produced a dll with gfortran that performs a basic function (shown below) and have attempted to write code to interface with it but I get the classic 'error defining external function' when I attempt to define a function from it and I'm not sure why. I know the function is exposed as I've used a dll viewer to prove it is and I have tried to write all the usual necessary interfaces that allow C++ interop with Fortran so I'm left at a bit of a puzzle as to where the issue lies. The code for all my stuff is below and I've linked the compiled dll below.

The dll source:

module main

    use, intrinsic :: iso_c_binding

    implicit none

contains

    function area_of_circle(r) bind(c, name="area_of_circle")


        ! function result

        implicit none


        !DEC$ ATTRIBUTES DLLEXPORT :: area_of_circle


        double precision :: area_of_circle


        ! dummy arguments

        double precision :: area


        ! local variables

        double precision, intent(in) :: r

        double precision :: pi


        pi = 4 * atan (1.0)

        area = pi * r**2


        area_of_circle = area


        return

    end function area_of_circle


end module

The GameMaker call:

dll_area_of_circle = external_define("cspline.dll","area_of_circle",dll_cdecl,ty_real,1,ty_real)

DLL download link:

cspline

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