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

[Bill Long]

In C when any scalar value is converted to _Bool, the result is 0 if the 
value compares equal to 0; otherwise the result is 1.  (6.3.1.3 Boolean 
type).

The value of C_BOOL as a Fortran KIND type for LOGICAL is such that

1) The bitsize of LOGICAL(C_BOOL) matches the bitsize of _Bool in C for 
the companion processor.

2) The Fortran processor uses the internal bit pattern in LOGICAL 
(C_BOOL) of 0 to represent false, and 1 to represent true.

If both of these is true (which is the case for many implementations) 
then C_BOOL > 0.  Otherwise C_BOOL will be negative, and there is no 
interoperability with C for LOGICAL.


The fatal flaw in the MPI interface here is that the corresponding C 
formal parameter is not _Bool or anything that looks like LOGICAL. 
Instead it is 'int'.

There is no concept of a Fortran argument of type LOGICAL interoperating 
with a C formal parameter of type int.  Usage of that sort in an MPI 
interface is simply wrong.  One side or the other of the call needs to 
be fixed.  I would vote for changing the Fortran side to be type 
INTEGER(C_INT).

Cheers,
Bill



On 2/28/13 12:29 PM, Craig Rasmussen wrote:
>
> Ack....
>
> I just did a quick rereading of clause 15, Interoperability with C, and
> it appears ill defined to me.  Consider the following:
>
> 1. Suppose a compiler has a default logical size equivalent to a default
> integer.  Since the standard only requires one logical kind, this
> compiler may not have a type that is interoperable with a _Bool C type.
>   In this instance C_BOOL would be a negative value.
> 2. If a compiler does support LOGICAL(kind=C_BOOL), what values of a C
> type _Bool map to .TRUE.?   What values map to .FALSE.?  These mappings
> may be in clause 15 but I didn't see them at first glance.
> 3. Suppose a Fortran compiler has the following mapping:
>
>          _Bool flag = 0;      /* flag maps to .FALSE. */
>
>          if (flag != 0)  ...;     /* flag maps to .TRUE. */
>
> This seems to me to be the sane way to map values from C to Fortran.
>   The same mapping can be done for any C integer type to an equivalently
> sized Fortran LOGICAL.
>

-- 
Bill Long                                           longb at cray.com
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