Fortran, ISO C BINDING and std::string

I have a c++ method with the signature

extern std::string os_dtoa_format(double  x);

that I would like to call from Fortran. I understand that I have to build an explicit interface, but I am not really sure how to go about that. I have a vague idea of how to handle the double, but I have not found anything about the std::string. char* seems difficult enough, which leads me to ask whether it is even possible to do what I want.