In order to ensure that exactly the same variable type and kind is used
in C and Fortran, the named constants shall be used which are defined in the
ISO_C_BINDING intrinsic module. That module contains named constants
for kind parameters and character named constants for the escape sequences
in C. For a list of the constants, see ISO_C_BINDING.
For logical types, please note that the Fortran standard only guarantees
interoperability between C99’s _Bool and Fortran’s C_Bool-kind
logicals and C99 defines that true has the value 1 and false
the value 0. Using any other integer value with GNU Fortran’s LOGICAL
(with any kind parameter) gives an undefined result. (Passing other integer
values than 0 and 1 to GCC’s _Bool is also undefined, unless the
integer is explicitly or implicitly casted to _Bool.)