[patch, fortran] Fix pointers not escaping via C_PTR

[Thomas Koenig]

Hi Steve,

> On Thu, Feb 28, 2019 at 09:14:48PM +0100, Thomas Koenig wrote:
>>
>> the attached patch fixes a wrong-code bug for gfortran where pointers
>> were not marked as escaping.  A C_PTR can be stashed away and reused
>> later (at least as long as the variable it points to remains active).
>>
>> This is not a regression, but IMHO a bad wrong-code bug. The chances
>> of this patch introducing regressions are really, really low.
>>
>> Regression-tested.  OK for trunk?
>>
> 
> Is this a wrong code bug or a clever user wandering off into
> undefined behavior?
> 
>> subroutine init()
>>      use, intrinsic :: iso_c_binding
>>      implicit none
>>      integer(c_int), pointer :: a
>>      allocate(a)
>>      call save_cptr(c_loc(a))
>>      a = 100
>> end subroutine init
> 
> Of particular note, 'a' is an unsaved local variable.  When init()
> returns, 'a' goes out-of-scope.  Fortran normally deallocates
> an unsaved allocatable entity, but 'a' is an unsaved local variable
> with the pointer attribute.  For lack of a better term, 'allocate(a)'
> allocates an anonymous target and associates the anonymous target
> with the pointer 'a'.  save_ptr() caches the address of the anonymous
> target.  When init() returns does the anonymous target get deallocated
> or does the program leak memory?

Neither.  If there were no C_PTR pointing to the memory, it would
leak memory.

> In addition, when init() returns,
> what happens to the pointer association of 'a'? 

'a' becomes undefined, hence it is no longer associated with the
memory.

> I think it becomes
> undefined, which means the anonymous memory is inaccessible at least
> by Fortran means as there are no pointers associated with the
> anonymous target.

That is not correct.  Looking at F2018, 18.2.3.3, by using C_F_POINTER,
you can

" Associate a data pointer with the target of a C pointer and specify 
its shape. "

First, this talks about a C pointer having a target.  Second, you can
re-estabilsh the association to a different pointer to the Fortran
program.

Regards

	Thomas