how to call a Fortran90 function included in a module in c++ code?

Question

I m including a fortran90 program that is not mine in my C++ project .

In the first stept I try to call the function by their name_() and i get the error \"undefin

Answer 1

I am assuming you are using the g++, gfortran, mpif90 toolchain. If you have a module

module rocker
contains
subroutine bye_baby
...
end subroutine

The external C declaration for it in C++ is

extern "C"
{
    //          ,-- 2 Leading underscores to start
    //          | ,-- then the module name
    //          | |     ,-- then _MOD_
    //          | |     |    ,-- then the subroutine name
    //          V V     V    V
    extern void __rocker_MOD_bye_baby();
}

You may also need to add attribute((stdcall)) after the extern. By default, C assumes cdecl which stacks the parameters differently.

If you do not want the __rocker_MOD part, then the subroutine/function should not be declared in a module.