FPS: useAsCString variants
Donald Bruce Stewart
dons at cse.unsw.edu.au
Fri Jul 14 03:23:23 EDT 2006
> Hello Donald,
> Friday, July 14, 2006, 8:11:22 AM, you wrote:
> >> afaik, CStringLen don't require zero byte at the end of string. So the
> >> following implementation will be much faster and still correct (not
> >> tested, it's just sketch):
> > Well spotted! I've applied this patch, with a tiny tweak (you missed a
> > withForeignPtr in useAsCStringLen).
> thanks. after writing this letter i spotted unsafeUseAsCStringLen and
> used it. now i'm curious about having these 4 functions:
> and differences between their behavior. may be, it will be better to
> keep only first two functions, with semantics already implemented
> (with my patch) - `useAsCString` add zero byte while useAsCStringLen doesn't ?
Ok, the story is:
We'd like to be able to efficiently pass ByteStrings to C. However,
* CString has to be null terminated
* ByteStrings aren't null terminated.
So, we have:
useAsCString, which copies a ByteString, and adds a \0 byte, O(n)
unafeUseAsCString, zero-copy version, assumes there's already a null byte, O(1)
Now, on the other hand, we have the CStringLen type, which doesn't need
a null byte. The CString type is isomorphic to a ByteString, so:
useAsCStringLen, which is just a cast. O(1)
unsafeUseAsCStringLen is a synonym for useAsCStringLen.
I've clarified the documentation for the CString functions to make this
More information about the Libraries