[Haskell-cafe] converting prefixes of CString <-> String

[Eric Stansifer]

I have been reading Foreign.C.String but it does not seem to provide
the functionality I was looking for.

Let 'c2h' convert CStrings to Haskell Strings, and 'h2c' convert
Haskell Strings to CStrings.  (If I understand correctly, c2h . h2c
=== id, but h2c . c2h is not the identity on all inputs;  or perhaps
c2h is not defined for all CStrings.  Probably this is all locale
dependent.)

I have an infinite Haskell String transferred byte-wise over a
network;  I would like to convert some prefix of the bytes received
into a prefix of the String I started with.  However, if I understand
correctly, if "s" is a Haskell String it is not necessarily true that
"c2h (take n (h2c s))" is a prefix of s for all n.  So I have two
questions:

Given a CString of the form "cs = take n (h2c s)", how do I know
whether "c2h cs" is a prefix of s or not?  Is there a way to recognize
whether a CString is "valid" as opposed to truncated in the middle of
a code point, or is this impossible?  Better yet, given a CString "cs
= take n (h2c s)", is there a way to find the maximal prefix cs' of cs
such that c2h cs' is a prefix of s?

If s == s1 ++ s2, is it necessarily true that s == (c2h (h2c s1)) ++
(c2h (h2c s2))?  If so, then I can perform my conversion a bit at a
time, otherwise I'd need to start from the beginning of the cstring
each time I receive additional data.

In practice, I think my solution will come down to restricting my
program to only using the lower 128 characters, but I'd like to know
how to handle this problem in full generality.

Thanks,
Eric