[J3] CFI_establish and elem_len for zero-length character strings and empty types/structs

[Malcolm Cohen]

Hi Tobias,

 

>Likewise,
>type, bind(C) :: t; end type

 

This is not valid Fortran.


>and
>struct t {};

 

And this is not valid C according to the syntax in 6.7.2.1 of C 2018.

 

Similarly, C forbids arrays of size zero (“If the expression is a constant expression, it shall have a value greater than zero.”), and Fortran character strings of length zero are not interoperable.

 

So it’s not immediately obvious that this is a defect that needs correction. Though admittedly zero-size arrays and empty structures are not uncommon extensions to C, so permitting elem_len to be zero might be a good idea on its own merits.

 

Cheers,

-- 

..............Malcolm Cohen, NAG Oxford/Tokyo.

 

From: J3 <j3-bounces at mailman.j3-fortran.org> On Behalf Of Tobias Burnus via J3
Sent: Thursday, July 22, 2021 4:41 PM
To: General J3 interest list <j3 at mailman.j3-fortran.org>
Cc: Tobias Burnus <burnus at net-b.de>
Subject: [J3] CFI_establish and elem_len for zero-length character strings and empty types/structs

 

Hi all,

"" or ["", ""] has len(...) == 0. For the C array descriptor,
that matches elem_len = 0.

Likewise,
type, bind(C) :: t; end type
and
struct t {};
have c_sizeof()/sizeof() == 0, which implies elem_len = 0.

However, CFI_establish requires:
"elem_len – If type is equal to CFI_type_struct, CFI_type_other, or a Fortran
character type code, elem_len shall be greater than zero and equal to the
storage size in bytes of an element of the object."

The other CFI_... functions do not have the > 0 requirement (or != 0, as
size_t is unsigned and, hence, implies >= 0).

Tobias



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