Passing array length to function

I'm sorry if this is a trivial question. My fortran-fu is poor.

Is there a way in Fortran to pass the array length? Using common (which, from what I gather is equivalent to global) is an option as well. What I want is in the main program to call a function with an array. For example (This is typed in, not copy pasted from anywhere)

program prog
integer num
double precision x(num),v
double precision test

....
v=test(x,num)
....


function test(x,num)
double precision test
integer num
double precision x(num)
test=0.0d0
....
return
end

This won't compile since num is not a constant. The important thing is to know what the array size I'm passing is.

Edit: I'm using the GNU Fortran 95 compiler.

Edit2: I tried High Performance Mark's solution without luck:

  program prog
  integer v
  parameter (v=10)
  double precision x(v),test,k
  k=test(x)
  write (*,*) size(x)
  stop
  end

  function test(x)
  double precision, dimension(:),intent(in) :: x
  double precision test
  write (*,*) size(x)
  test = 0.0d0
  return
  end

The output should be two lines where 10 is written. Instead I got this:

/scpc:niels: #$ f95 err.f
/scpc:niels: #$ ./a.out 
           0
          10
/scpc:niels: #$

Solution

Fortran arrays 'know' how long they are, you shouldn't need to pass the array and its length as different arguments. (Unless, that is, you are interfacing with old Fortran codes.) Today you'd write something like this

function test(arr)
    real, dimension(:), intent(in) :: arr
    ...
    integer :: arrsize
    ...
    arrsize = size(arr)
    ...

If you have to interface to old code in which array sizes are passed you can make calls like this

  call old_fortran_subr(array, size(array, 1), other_arguments)

Oh, and while I'm writing, have nothing to do with common in any code you write from scratch, it's a (rightly) deprecated feature from the 70's and earlier. Instead use module variables.