What is the best way for me to use boost::filesystem::wpath with a 3rd party C library that expects a const char* filename, on Windows?

On linux, its simple: my_file.to_external_string()  but on windows that gives me a wide-character string.

So far the only thing I can think of is to roll my own wstring --> UTF8 conversion function, but I'm not even sure that is correct (will fopen() work with UTF-8 encoded filenames on windows?).

thanks,

Paul