问题
I have following Fortran code
type t_octree
integer :: max_num_point
class(t_octree_node), pointer :: root => null()
contains
final :: DESTROY_OCTREE
end type t_octree
type t_octree_node
real :: box(2,3)
integer :: depth
type(t_octree_node), pointer :: parent => null()
type(t_octree_node), pointer :: children(:) => null()
class(t_octree_leaf), pointer :: leaf => null()
contains
final :: CLEAN_NODE
end type t_octree_node
type, extends(t_octree_node) :: t_octree_leaf
integer, allocatable :: id(:)
integer :: num_point
contains
final :: DESTROY_LEAF
end type
After I have done my processing and now need to make sure that my stuff is deallocated properly.
In my `final routines I have
subroutine DESTROY_OCTREE( this )
implicit none
type(t_octree) :: this
type(t_octree_node), pointer :: node => null()
integer :: i
node => this% root
if( associated(node% leaf) )deallocate( node% leaf )
if(.not. associated(node% children) )RETURN
deallocate( node )
node => this% root
i = 0
nullify(node)
print*, associated(this% root) ! this is true!!
print*, associated(this% root% children) ! this is true!!
end subroutine DESTROY_OCTREE
recursive subroutine CLEAN_NODE ( this )
implicit none
type(t_octree_node) :: this
type(t_octree_node), pointer :: node => null(), next => null()
integer :: i
if( associated(this% leaf) )then
deallocate( this% leaf )
nullify( this% leaf )
endif
if(.not. associated(this% children) )RETURN
do i = 1, 8
node => this% children(i)
deallocate( node )
nullify(node)
! debug
! print*, i, "rec"
enddo
nullify(this% children)
nullify(this% parent)
end subroutine CLEAN_NODE
subroutine DESTROY_LEAF ( leaf )
implicit none
type(t_octree_leaf) :: leaf
deallocate( leaf% id )
end subroutine DESTROY_LEAF
In my main program I do following
program TEST_OCTREE
use MD_OCTREE
implicit none
type(t_octree), pointer :: octree
octree => t_octree( max_num_point, box )
(...) ! all processing to build the data structure
Now I deallocate by simply
deallocate( octree )
print*, associated(octree% root) ! this give a segmentation fault
The question
Can somebody explain why it seems that my print*, associated(this% root) commands still show TRUE while when printing in my main program it looks like it has been deallocated as it gives me a segmentation fault
回答1:
Deallocating a pointer causes the pointer association status of any other pointer that was associated with the same target to become undefined (F2018 9.7.3.3p2). This situation happens with node and this%root in the DESTROY_OCTREE procedure - both pointers reference the same object (via the pointer assignment node => this% root, the object is deallocated through the node pointer - which makes this%root have undefined association status.
The argument to the ASSOCIATED intrinsic must not be a pointer with undefined association status (F2018 16.9.16p3). The code is non-conforming, anything may happen - where "anything" quite reasonably includes the results you see.
When you deallocate the object through the node pointer, there is no simple way that the processor can also reliably update the status of the this%root pointer - it ends up pointing at something that no longer exists.
There are other suspicious constructs in the fragments of source shown, including use of a superfluous NULLIFY after a DEALLOCATE statement on the same pointer object (successful deallocation disassociates (nullifies) the pointer), use of nullify when perhaps DEALLOCATE is appropriate (hard to say without complete code), and use of what appears to be a structure constructor as a pointer target.
来源：https://stackoverflow.com/questions/53951210/correct-execution-of-final-routine-in-fortran