REAL(c_double)     matches "double" and is OK. However,
COMPLEX(c_double)  is invalid as the proper kind parameter
                   is c_double_complex matching "double _Complex"

Internally, both yield the same value and also sizeof(0.0_c_double) and sizeof(real(cmplx(0.0, c_double_cmpl)) is the same. Thus one could simply accept c_double as synonym for c_double_complex.

gfortran, however, has checks build in which ensure the right KIND is used; there is room for improvement however:

! ------------------------------
use iso_c_binding
implicit none
real(c_double) :: r
complex(c_double) :: c ! << Wrong
type(c_ptr) :: ptr
ptr = c_loc(c) ! <<< ERROR here
end
! ------------------------------

Error: C kind parameter is for type REAL but symbol 'c' at (1) is of type COMPLEX


Expected: The error is printed in the "COMPLEX(c_double)" line and not in the "c_loc" line. The other question is whether a WARNING is enough or whether one wants to have an ERROR.


PS: Found at
http://www.lrz-muenchen.de/services/software/mathematik/gsl/fortran/README
"    - gfortran 4.3.x does not support complex(c_double)."