[MPI3 Fortran] [Mpi-comments] MPI 3.0: Fortran 2008 interface - issue with the LOGICAL kind

[Tobias Burnus]

Bill Long wrote:
>>
>> There isn't any difficulty about a Fortran wrapper using integers (as
>> C does), and converting them to LOGICAL, but it can't be BIND(C).
>
> There is also no difficulty in having the wrapper arguments being 
> LOGICAL.
>
> subroutine sub (x) bind(c, name="MPI_xxx_f")
> logical :: x
> integer(int) :: cx
>
> cx = merge(1, 0, x)

Well, except that compilers might simply reject a default-kind LOGICAL 
with BIND(C), in line with the Fortran standard.

Your code above assumes that a Fortran compiler allows passing a 
default-kind LOGICAL to a Bind(C) procedure and that it then can handle 
that LOGICAL. I think no one really doubts the second part. But the 
first part is not really covered by the Fortran standard and 
standard-conforming compilers do exist, which reject default-kind LOGICALs.

J3/WG5 could change the standard such that it supports other LOGICAL 
kinds besides C_Bool, including kind(.true.). However, if 
LOGICAL(kind=kind(.true.) / C_INT) exists, users will assume that it 
acts like C's "int", which will lead to problems. So far, I haven't seen 
a convincing proposal how to solve this.

Tobias