How should I import a fortran interface into C using extern?
Suppose I have the following fortran module:
!swapper module
module swapper
interface swap
module procedure swap_r, swap_i, swap_c
end interface
contains
!subroutine implementations here:
!swap_r takes two double precision as argument
!swap_i takes two integers as argument
!swap_c takes two characters as argument
end module swapper
Then can I just do in C, the following?
extern "C" void __swapper_MOD_swap(double*, double*)
extern "C" void __swapper_MOD_swap(int*, int*)
extern "C" void __swapper_MOD_swap(char*, char*)
Alternatively, if I promise to call swap with only double precision numbers, may I exclusively do this?
extern "C" void __swapper_MOD_swap(double*, double*)
It looks like you are actually using C++. But first lets answer for C or C-style C++:
No, you cannot do
extern "C" void __swapper_MOD_swap(double*, double*)
you cannot do it for three different types of arguments, you cannot do it even for a single type of arguments.
There actually should not be any __swapper_MOD_swap in the library in the first place.
What Fortran does is that it keeps an interface (which is just a description how to call something) for the three specific subroutines swap_r, swap_i, swap_c and lets you call it by name swap.
But there is NO actual subroutine swap in the module!!! Fortran will just let you call those three specifics under a different name, that's all.
There is no way how to call those functions from C like a generic. C does not have this type of generics! (It does have some functions which operate on any type using void*).
In C++ you can actually make generics which are specialized for different types and you can call the Fortran procedures as a generic, but you have to tell that to C++ yourself! You can make a template and manually specialize this template to call the appropriate extern "C" functions.
For an example see my header https://github.com/LadaF/PoisFFT/blob/master/src/poisfft.h where I make a C++ class which is linked to an extern "C" struct and some extern "C" functions which are however all implemented in Fortran.
Finally, DO NOT use the __swapper_MOD_ symbols. Create some Fortran binding using bind(C,name="some_name") and call some_name through extern "C", because different Fortran compilers DO use different name mangling schemes.
