Could you help out with the following example?

Jane Sleightholme and I have been looking at simple
examples to illustrate dangling pointers and memory 
leaks.

PROGRAM C20_04
IMPLICIT NONE
REAL , POINTER :: P2=>null()

interface
  subroutine test(p2)
  implicit none
  REAL , POINTER :: P2
  end subroutine test
end interface 

  print *,' staring main program'
  
  if (associated(p2)) then
    print *,' p2 is associated '
  end if

  call test(p2)

  if (associated(p2)) then
    print *,' p2 is associated '
  end if

  print *,' p2  in main program = ',p2

  print *,' returning from test routine'

end program c20_04

subroutine test(p2)
implicit none
REAL , POINTER :: P1 
REAL , POINTER :: P2
integer :: istat
integer :: t
  print *,' entering test' 
  allocate(p1)
  P1=21.0

! The value 21.0 is now stored in the 
! memory location that P1 points to.

  P2=>P1

! verify that they are the same

  PRINT * , ' p1 = ',P1
  PRINT * , ' p2 = ',P2
  p2=p1+1

! verify that they are both the same

  PRINT * , ' p1 = ',P1
  PRINT * , ' p2 = ',P2

! Now release the memory.
! P1 becomes disassociated.

  deallocate(p1,stat=istat)

! check the status of the deallocate

  print *,' verify deallocate status ',istat

  if (associated(p1)) then
    print *,' p1 is associated '
  end if

  if (associated(p2)) then
    print *,' p2 is associated '
  end if

! what does p2 point to?

  PRINT * , ' p2 = ', P2
  p2=p2+1
  PRINT * , ' p2 = ', P2
  print *,' about to leave test'

! comment out the following statement and run again.

  read *,t
END subroutine test

we presume that you need to nullify p2 before 
deallocating p1.

we've tried the example as above but without the
input prompt and got different behaviour regarding p2.

we've also taken out the 

=>null()

and tried with some fortran 90 compilers.

thanks in advance

Ian Chivers
Jane Sleightholme

King's College London



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