Re: [Caml-list] String, Array, Bigarray.char

[Anil Madhavapeddy <anil@recoil.org>]

On 9 May 2013, at 09:32, Tom Ridge <tom.j.ridge+caml@googlemail.com> wrote:
> 
> Quick question: I am working a lot with arrays of byte, which at
> various points I want to view as strings, and at various points I want
> to view as arrays. The exact types involved should be discernible from
> the code below.
> 
> I have some conversion functions e.g.:
> 
>  type myfusebuffer = (char, Bigarray.int8_unsigned_elt,
> Bigarray.c_layout) Bigarray.Array1.t
> 
>  module A = Bigarray.Array1
> 
>  (* convenience only; don't use in production code *)
>  let array_of_string bs = (
>    let arr = (Array.init (String.length bs) (String.get bs)) in
>    let contents = A.of_array Bigarray.char Bigarray.c_layout arr in
>    contents)
>  let (_:string -> myfusebuffer) = array_of_string
> 
> This presumably takes O(n) time (where n is the length of the string
> bs). My question is: is there functionality to move values between
> these types at cost O(1)? Basically, I'm hoping that String is
> implemented as A.of_array Bigarray.char Bigarray.c_layout or
> similar...

Strings are represented as normal OCaml values within the OCaml heap,
whereas Bigarrays are simply pointers to externally allocated memory
(via malloc).  You do therefore need to copy across them in most cases.
One quick solution is to define a subset of the String module that uses
the Bigarray accessor functions, but this isn't ideal (especially when
external libraries that use strings are involved).

Your fusebuffer type probably means that you're working with filesystem
data.  Can you just use Bigarrays for everything, with copies to strings
only when you absolutely need to?  We haven't released this out of beta
yet, but the cstruct camlp4 extension helps map C structures to OCaml:
https://github.com/mirage/ocaml-cstruct

-anil