program main_fortran
    use, intrinsic :: iso_c_binding
    
    integer(c_int) :: i
    real(c_float)  :: r
    common /com/ i, r

    INTERFACE
      subroutine sam
      end subroutine
      subroutine mc() bind(C,name='mc')
      end subroutine
    END INTERFACE
    
    call sam  ! load COMMON block in fortran
    call mc   ! Call C code that gets pointer to COMMON
    WRITE(*,'(A,I4,F6.2)')'Fortran has the block /com/ ', i, r
end program main_fortran

subroutine sam_c(comp) bind(C,name='sam_c')
    use, intrinsic :: iso_c_binding
! The C function mc calls sam_c() to get the
! location of the common block start.    
    type(c_ptr) :: comp
    
    integer(c_int), TARGET :: i
    real(c_float)          :: r
    common /com/ i, r
    
! Return a pointer to the first location of the common block.
    comp=c_loc(i)
    WRITE(*,'(A,I4,F6.2)')'Fortran has common block /com/ ', i, r
    return
end subroutine sam_c

subroutine sam

! This is the original Fortran function
! assigning values to the COMMON BLOCK
    
    integer :: i
    real    :: r
    common /com/ i, r
    
    i = 786
    r = 3.2

    return
end subroutine sam