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

[Malcolm Cohen]

Nick Maclaren writes:
>It would also allow (for example) the C library function
>isalnum() to be called from Fortran.

Already allowed, not a problem.  You just declare it with type 
INTEGER(C_int) and use isalnum(...)/=0.

The problem, such as it exists, arises if you allow LOGICAL instead of 
INTEGER on the Fortran side.  Fortran LOGICAL only has two values, .TRUE. 
and .FALSE., whereas C's "boolean" use of integers has many values but only 
2 equivalence classes (true and false).  This is very different: a Fortran 
compiler might, for example, test the sign bit for logical testing.  Or the 
bottom bit.  Or do .AND. with bitwise AND (this latter is quite common in 
particular circumstances!).  All these are perfectly valid for Fortran (I've 
seen 'em all), but do not interoperate well with C.

This means that if you have LOGICAL on the Fortran side, either
(a) the C routine always returns the same bitwise value for TRUE, and you 
got lucky and that bit-value is the one the Fortran compiler uses for 
LOGICAL,
or
(b) you need a wrapper that ensures the conditions in (a).

Otherwise cue all kinds of incredibly obscure "incorrect results", that only 
occur in some cases and only with some compilers and/or some compilation 
options.  Ugh.

FWIW the NAG compiler currently uses 0 and 1 for LOGICAL, which works nicely 
with C relationals, and does use bitwise-and for LOGICAL .AND. operations 
under some circumstances.  (On some ancient platforms we used 0 and -1 for 
compatibility with other Fortran compilers, but those machines aren't around 
any more.)

Cheers,
-- 
.......................Malcolm Cohen on holiday.