Three things:

1) In Zorro, "string" is only a typedef of char*. Do you know how pointers work?
2) Rd is for retrieving a double, yet you're casting a double to a char*. How did you expect this to work?
3) Rv is for retrieving an array of doubles into a user-supplied array of doubles. That's not remotely what you seem to be wanting to do.

Read this manual page for details on how the functions work:
https://zorro-project.com/manual/en/rbridge.htm

For asset names, you might as well set up key-value pairs on the R side and retrieve assets names by index lookup on the Zorro side.

See also this post.