Consider an old FORTRAN program that’s organised like this:

      program main
      implicit none
      integer rr_sz
      parameter(rr_sz=10)
      real,dimension(:),allocatable :: rr
      allocate(rr(rr_sz))
      call sub1(rr_sz/2,rr)
      print *, rr_sz, rr
      deallocate(rr)
      end

      subroutine sub1(cc_sz,cc)
      implicit none
      integer cc_sz,min_cc_sz,i
      complex cc
      dimension cc(cc_sz)
c here we can determine what array size is really required
      min_cc_sz = 6
      if (cc_sz.lt.min_cc_sz)
     & error stop
        do i=1,min_cc_sz
      cc(i) = complex(i,i)
        end do
      end

I would like to put the rr array allocation into sub1, so that when rr needs more space than the hardcoded rr_sz, the program can continue if the system have enough RAM.

It seems like doing so will require an explicit interface, which makes the compiler fail with a Type mismatch in argument ‘cc’: REAL(4) to COMPLEX(4).

How can I get around this problem with the least possible modifications to the original code?