To: J3                                                     J3/22-118
From: Van Snyder
Subject: C_F_POINTER's SHAPE argument should be described as upper bounds
Date: 2022-February-03
Reference: 22-007

Before adding the LOWER argument to C_F_POINTER, the description of the
SHAPE argument argument as specifying the shape of the result was
harmless. If a LOWER argument is present, describing the SHAPE argument as
specifying the shape instead of the upper bounds is confusing, and
incompatible with pointer assignment.

Assume X is contiguous and has the POINTER or TARGET attribute. Assume F
is a pointer. Assume L and U are rank-1 arrays whose extents are equal to
the rank of F, and whose values are the desired lower and upper bounds of
F, respectively.

Then

    F(@L:U) => X

specifies those bounds.

Assume C is of type C_PTR. Then to achieve the effect of the above,
assuming some intermediate operation by C interoperability and not all
values of L are 1 (else why bother specifing it), one would confusingly
need to write

    C = C_LOC(X)
    ... (C is passed to a C function)
    call C_F_POINTER ( C, F, U-L+1, L )

to be equivalent to

    F(@L:U) => X

It would be less confusing if the SHAPE argument were described as the
upper bound of the result, allowing

    C => C_LOC(X)
    ... (C is passed to a C function)
    call C_F_POINTER ( C, F, U, L )

to be equivalent to

    F(@L:U) => X

This description is not incompatible with Fortran 2018.

When C interoperability was being developed in 1997-2003, after Michael
Hennecke's effort was rejected, it was urged that C_F_POINTER ought to
include provision to specify both lower and upper bounds, in that order,
but that proposal was rejected.

Edits
=====

[499:30 18.2.3.3p3 C_F_POINTER] Replace "shape is" with "upper bounds
are".

[499:35 18.2.3.3p3 C_F_POINTER] Replace "SHAPE shall specify a size of 1"
with "all elements of SHAPE, or SHAPE - LOWER + 1 if LOWER is present,
shall be equal to 1."

[499:37 18.2.3.3p3 C_F_POINTER] Replace "SHAPE" with "SHAPE, or SHAPE -
LOWER + 1 if LOWER is present,".

[499:37 18.2.3.3p3 C_F_POINTER] After "PRODUCT(SHAPE)" insert ", or
PRODUCT(SHAPE - LOWER + 1) if LOWER is present".

[500:1 18.2.3.3p3 C_F_POINTER] Replace "SHAPE shall specify a size of 1"
with "all elements of SHAPE, or SHAPE - LOWER + 1 if LOWER is present,
shall be equal to 1."

[500:3 18.2.3.3p3 C_F_POINTER] Replace "SHAPE" with "SHAPE, or SHAPE -
LOWER + 1 if LOWER is present,".

[500:5 18.2.3.3p3 C_F_POINTER] After "PRODUCT(SHAPE)" insert ", or
PRODUCT(SHAPE - LOWER + 1) if LOWER is present".