c-string and with-foreign-string questions

Dear uffi-devel,

I have a few more questions about c-strings and foreign strings. What 
exactly is a c-string? I assumed this was a string that one would pass 
to a C function that takes a string, but apparently, at least for SBCL, 
this is instead a foreign-string. Furthermore c-strings are basically 
no-ops. Is this intended?

Also, the with-foreign-string macro seems very useful but is undefined 
(at least it's not in the pdf file). Furthermore, there is a 
handy-dandy with-cstrings, but not a with-foreign-strings which would 
seem to also be useful.

Thanks,

Cyrus