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

[Tom Ridge <tom.j.ridge+caml@googlemail.com>]

Thanks for this information.

I guess I will probably end up using arrays as much as possible. In
various places I have used strings as though they were immutable
arrays of byte. I guess the advantage of this approach is that strings
seem more familiar than arrays (especially Bigarrays). But it is
probably not much of a big deal to move to using arrays everywhere.

Thanks once again

Tom


On 9 May 2013 14:44, Anil Madhavapeddy <anil@recoil.org> wrote:
> 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
>
>
>