A little pointer on POINTER in data would be helpful

[Steve Kargl]

Arjen,

Thanks for the feedback.

First two are definitely invalid and I think I know where
to try to check for the problem in gfortran.  The third one
with null() is fine.  It's the last one that has me 
scratch my head.  It seems that "data p /m/" with 
m having the target and save attribute is meant to be
interpreted as "p => m". I freely admit that I sometime
has trouble parsing the standard.

-- 
steve

On Fri, Mar 23, 2018 at 10:47:54AM +0100, Arjen Markus wrote:
> Hello Steve,
> 
> looking at the various versions of the program, I wonder if the data
> statement is supposed to work as an association. Normally one does:
> 
> p => target_variable
> p = 1
> 
> or the like, so first an explicit association via => (or an
> allocation) and then the assignment. A data statement simply assigns
> the values and can not refer to variables. Hence it seems unlikely
> that any of the above programs is standard conforming. With the
> possible exception of "data p / null() /".
> 
> Regards,
> 
> Arjen
> 
> 
> 2018-03-23 0:38 GMT+01:00 Steve Kargl <sgk@troutmask.apl.washington.edu>:
> > I don't use POINTERs in my codes, so have little
> > experience with them; especially when derived types
> > are involved.  In PR 70870, Gerhard added the code
> >
> > program p
> >    type t
> >       integer :: n
> >    end type
> >    type(t), pointer :: z
> >    data z%n / 3 /
> >    print *, z%n
> > end
> >
> > which results in an ICE.  I'm trying to parse
> >
> >   F2008:C567 (R536) A data-i-do-object or a variable
> >   that appears as a data-stmt-object shall not be an
> >   object designator in which a pointer appears other
> >   than as the entire rightmost part-ref.
> >
> > My understanding is that the POINTER attribute applied
> > to z, which is not the rightmost part-ref.  The rightmost
> > is n.  Thus, the above code is violates a constraint.  I
> > think I know where to fix this situation.
> >
> > So, I now, re-arrange the above code to
> >
> > program p
> >    type t
> >       integer, pointer :: n
> >    end type
> >    type(t) :: z
> >    data z%n / 3 /
> >    print *, z%n
> > end
> >
> > Here, the POINTER attribute applies to the component n,
> > and so it is the rightmost part.  This code actually
> > compiles, but segfaults on execution, which may be
> > expected as '3' is not a TARGET.  The code is invalid,
> > but gfortran is not required to issues an error message.
> > The standard goes on to say
> >
> >    A data-stmt-constant shall be null-init or initial-data-target
> >    if and only if the corresponding data-stmt-object has the
> >    POINTER attribute.  If data-stmt-constant is null-init, the initial
> >    association status of the corresponding data statement object is
> >    disassociated.
> >
> > OK, so change the code to
> >
> > program p
> >    type t
> >       integer, pointer :: n
> >    end type
> >    type(t) :: z
> >    data z%n / null() /
> >    print *, associated(z%n)
> > end
> >
> > % gfcx -o z b.f90 && ./z
> >  F
> >
> > Good, gfortran gets this correct.  The stand goes on to say
> >
> >    If data-stmt-constant is initial-data-target the corresponding
> >    data statement object shall be data-pointer-initialization
> >    compatible with the initial data target; the data statement
> >    object is initially associated with the target.
> >
> > This is where it gets interesting.
> >
> > R443 initial-data-target  is designator
> > R601 designator  is object-name
> >                  or array-element
> >                  or array-section
> >                  or coindexed-named-object
> >                  or complex-part-designator
> >                  or structure-component
> >                  or substring
> >
> > After bouncing around abit, one finds
> >
> >   C766  A designator that is an initial-data-target shall designate
> >   a nonallocatable, noncoindexed variable that has the TARGET and
> >   SAVE attributes and does not have a vector subscript.  Every
> >   subscript, section subscript, substring starting point, and
> >   substring ending point in designator shall be a constant expression.
> >
> > program p
> >    integer, target, save :: m = 3
> >    type t
> >       integer, pointer :: n
> >    end type
> >    type(t) :: z
> >    data z%n / m /
> >    m = 3
> >    print *, z%n
> > end
> >
> > % gfcx -o z b.f90
> > b.f90:7:15:
> >
> >     data z%n / m /
> >                1
> > Error: Symbol 'm' must be a PARAMETER in DATA statement at (1)
> >
> > Whoops.
> >
> > --
> > Steve

-- 
Steve
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