Unused dummy argument and Return value of function not set

Question

This is my Fortran 90 program for Newton-Raphson method. However, it did not compile properly and gave this warning in geany

newraph.f90:18.17:
real function f(x)
                 1
Warning: Unused dummy argument 'x' at (1)
newraph.f90:18.15:
real function f(x)
               1
Warning: Return value of function 'f' at (1) not set
Compilation finished successfully.

        program newton_raphson
        implicit none
        real,parameter::error=1e-4
        integer::i
        real::xo,x1,f,fd
        read*,xo
        i=1
        10 x1=xo-(f(xo)/fd(xo))
        if(abs((x1-xo)/x1)<error) then
        print*,"root is", x1,"no. of iteration=",i
        else
        xo=x1
        i=i+1
        goto 10
        endif
        end

        real function f(x)
        real::x
        end

        real function fd(x)
        real::x
        fd=3*x**2-1
        end

Answer 1

You have just two warnings. Both just inform you that the function f does nothing:

    real function f(x)
    real::x
    end

Change it to return something useful and the warnings will vanish.

    real function f(x)
      real, intent(in)  :: x
      f = x**2
    end function

and then change fd to return the derivative of f, like fd = 2*x.

Other than that, try to use some decent programming style. Use indentation, use loops instead of go to where possible. Use vertical spacing (empty lines). Also it is better to use end function, end subroutine and end program for clarity.