c===========================================================
c     Demonstration main program and subprograms 
c     illustrating the 'EXTERNAL' statement and how 
c     subprograms may be passed as ARGUMENTS to other 
c     subprograms.  This technique is often used to 
c     pass "user-defined" functions to routines which 
c     can do generic things with such functions (such 
c     as integrating or differentiating them, for example).
c===========================================================
      program         texternal

c-----------------------------------------------------------
c     The 'external' statement tells the compiler that the 
c     specified names are names of externally-defined 
c     subprograms (i.e. subroutines or functions)
c-----------------------------------------------------------
      real*8          r8fcn
      external        r8fcn,        r8sub2

c-----------------------------------------------------------
c     Call 'r8fcncaller' which then invokes 'r8fcn'
c-----------------------------------------------------------
      call r8fcncaller(r8fcn)
c-----------------------------------------------------------
c     Call 'r8subcaller' which then invokes 'r8sub2'
c-----------------------------------------------------------
      call subcaller(r8sub2)

      stop
      end

c===========================================================
c     Input 'fcn' is the name of an externally defined 
c     real*8 function.  This routine invokes that function
c     with argument 10.0d0 and writes the result on 
c     standard error
c===========================================================
      subroutine  r8fcncaller(fcn)
         
         implicit     none

         real*8       fcn
         external     fcn

         real*8       fcnval

         fcnval = fcn(10.0d0)

         write(0,*) 'r8caller: ', fcnval

         return

      end

c===========================================================
c     Input 'sub' is the name of an externally defined 
c     subroutine.  This routine invokes that subroutine
c     with arguments 10.0d0 and 20.0d0.
c===========================================================
      subroutine  subcaller(sub)
         
         implicit     none

         external     sub

         call sub(10.0d0,20.0d0)

         return

      end

c-----------------------------------------------------------
c     Demonstration real*8 function
c-----------------------------------------------------------
      real*8 function r8fcn(x)

         implicit     none 

         real*8       x

         r8fcn = x**2

         return

      end

c===========================================================
c     Demonstration subroutine
c===========================================================
      subroutine r8sub2(x,y)

         implicit     none 

         real*8       x,       y

         write(0,*) 'r8sub: x = ', x, ' y = ', y

         return

      end