Step 1: Fortran 77 signatures

C file: signatures.f

      subroutine gridloop2(a, xcoor, ycoor, nx, ny, func1)
Cf2py intent(c) gridloop2
      integer nx, ny
Cf2py intent(c) nx,ny
      real*8 a(0:nx-1,0:ny-1), xcoor(0:nx-1), ycoor(0:ny-1), func1
      external func1
Cf2py intent(c, out) a
Cf2py intent(in) xcoor, ycoor
Cf2py depend(nx,ny) a

C sample call of callback function:
      real*8 x, y, r
      real*8 func1
Cf2py intent(c) x, y, r, func1
      x = 1
      y = 1.51981721222
      r = func1(x, y)
      end