J3/01-395

Date:       December 5, 2001
To:         J3
From:       /interop
Subject:    Enhanced C interoperability
References: J3/01-381, WG5/N1442, J3/00-168

This paper is intended to document the thought process of /interop
when considering WG5/N1442 and J3/01-381. /interop is not prepared to
submit a proposal at this time, but sees merit in the requests in
N1442.

N1442 states:

It is impossible for a Fortran procedure,
while being called from C, to allocate an array and make that array
available to the C code. Conversely, it is impossible for a C function,
while being called from Fortran, to allocate an array and make that
array available to the Fortran code.

J3/00-168 attempts to resolve the former problem by introducing
a new intrinsic that creates a Fortran pointer out of a C
pointer.

The latter problem can be solved by allowing C_LOC to take allocatable
arrays as arguments. Richard Maine suggests that perhaps C_LOC should
be allowed to take pointers as arguments as well.

Subgroup is concerned about two issues: Memory leaks and aliasing.
Subgroup believes that memory leaks can be addressed by a note
admonishing the user to free any memory in C if the memory was
allocated in C. This requires that the user keep track of memory
allocated in C independent of its use through a Fortran pointer.

Aliasing is a larger concern. To comply with Fortran rules, two
fortran pointers that use the same C pointer (or overlapping pieces
of memory obtained from C) should have the same type and kind.
Further, if the address of a dummy argument is taken in a C routine
then that dummy argument must have the TARGET attribute.

Finally, subgroup B pointed out that section 16.8.6, items 17 and
18 reference association between variables of type C_PTR and targets;
however, this form of association does not appear to be defined
in the standard.

The following is an outline of a possible solution to these issues.
Edits are not included.

- Enlarge pointer association to include variables of type C_PTR
that are defined by the C_LOC intrinsic.

- Allow BIND(C) on allocatable arrays and pointers. Such variables
are interoperable with C. Thus C_LOC can be used on them.

- Require that procedures defined by means other than fortran obey
Fortran rules for aliasing. That is, the address of a dummy argument
can be taken only if the argument has the target attribute.

- Include the intrinsic CPTR_TO_FPTR (with a different name) as
specified in the references. Require that pointers returned by
that intrinsic obey Fortran aliasing rules. The concept of
partial association should be helpful here.