When I take a path in an `NSString` and turn around and use it in a C API, for example:

```
CGDataProviderRef CGDataProviderCreateWithFilename(const char *filename );

```

What is the correct encoding for the path when flattening to a `char*`? UTF8? (ASCII??)

I feel like this should be either obvious, or obviously documented, but I'm having trouble thinking and/or finding the answer.