To: J3                                                     J3/22-101
From: Steve Lionel
Subject: C_SIZEOF and null pointers F18/026 revisited
Date: 2021-December-12

Reference: 18-007r1, 20-151, 21-134r2

Consider the following program:

program p
 use iso_c_binding
 implicit none
 integer(c_int), pointer :: int_s
 integer(c_int), pointer :: int_a(:)
 print *, c_sizeof (c_null_ptr) ! (A)
 print *, c_sizeof (null ())    ! (B)
 print *, c_sizeof (null (int_s)) ! (C)
 print *, c_sizeof (null (int_a)) ! (D)
end

By the current text in 18-007r1, (A) is valid because its argument
is "an interoperable data entity that is not an assumed-size array or an
assumed-rank array that is associated with an assumed-size array"
(18.2.3.7p3). (B),(C) and (D) are not valid because the arguments have the
POINTER attribute and are thus not interoperable (18.3.4p1).

Interp F18/026, passed by J3 at m224, made (C) valid because the
prohibition on pointers was removed and the argument does have
interoperable type and type parameters. Even though the pointer is
disassociated, C_SIZEOF returns the size of an object of that type.

(D) is now problematic because C_SIZEOF needs to know the number of
elements in its argument, and what it is given is a disassociated pointer
for which that is undefined.

(B) is similarly undefined. When supplied as an actual argument, NULL()
takes on the "characteristics of the corresponding dummy argument"
(16.9.144 Table 16.5), but the dummy argument of C_SIZEOF has no specific
characteristics.

21-134r2 discussed the POINTER issue, but did not consider the case of
a disassociated pointer nor the NULL() intrinsic as an argument to
C_SIZEOF.

Proposal
--------
Add to 18.2.3.7p3: "It shall not be an unallocated allocatable variable or
a pointer that is not associated." (see LBOUND, for example). This will
make cases (B), (C) and (D) all invalid, while preserving the usefulness
of C_SIZEOF for associated/allocated Fortran POINTER/ALLOCATABLE data
objects.