Immutable CStrings

How would one implement an CStringLen-style type that (a) was efficient, in particular could be read and written to Handles efficiently; (b) would get automatically deallocated by the Haskell garbage collector, when Haskell no longer referred to it; (c) was immutable, so that once created the character data could not be changed; (d) consequently had the property that conversion to and from String would be a pure operation; (e) could be passed to and from C using the FFI. (Of course C would need you to split the length and character data up; the character data would presumably have type "const char *".) ? It would be rather nice to have such a type.