11-128
To: J3
From: Nick Maclaren
Subject: Interop TR: CFI_desc_t scoping issues
Date: 2011 January 27
Reference: N1838, 10-235



1. Summary
----------

This issue was raised in 10-235, especially in section 2.3.  I shall
not repeat that, but there are three critical points:

    1) The scoping and lifetime of a descriptor is clearly unrelated
to that of the data it refers to, because descriptors are closer to
being the instantiation of an association than to one of an object.
However, the TR says nothing about what that scoping and lifetime
might be.

     2) The rules on object scoping and lifetimes are wildly different
between Fortran and C, not just in detail but in their basic concepts.
There are important constraints that are obvious to Fortran experts, but
will be positively counter-intuitive to C experts.

    3) The C code can obtain the address of both a descriptor and an
object, (in summary) such pointers remain valid while that object does,
and caching them is normal programming practice in C.  However, Fortran
permits any object that does not have the TARGET, ASYNCHRONOUS or
VOLATILE attributes to be moved at the whim of the implementor.

Note that C has at least 5 very different classes of 'undefined', and an
object can become undefined while still leaving some uses of it as
defined behaviour.

That being so, some wording making it clear to C programmers what
Fortran's rules really are is essential.  Unlike most of 10-235, this
wording is easy to add to the current design.  I have changed it slightly,
in an attempt to clarify it.


2. Edits
--------

[16:18+] Add some new paragraphs to 5.2.7 Restrictions on lifetimes:

When a Fortran object or internal procedure is deallocated, execution of
its host instance is completed, or its allocation or association status
becomes undefined, all C descriptors and C pointers to any part of it
become undefined[*].

A C descriptor received as a dummy argument becomes undefined on return
from the procedure call.  If the dummy argument does not have any of the
TARGET, ASYNCHRONOUS or VOLATILE attributes, all C pointers to any part
of the object it describes become undefined on return from the procedure
call[*].

If a C descriptor is passed as an actual argument, its lifetime and that
of the object it describes shall not end before the return from the
procedure call.  A Fortran pointer variable that is associated with a C
descriptor shall not be accessed beyond the end of the lifetime of the C
descriptor or the object it describes.

    NOTE [*]
    In C terms, the pointers and values become indeterminate (ISO/IEC
    9899:1999 6.2.4 paragraph 2), because the lifetime of the descriptor
    or object pointed to has ended.