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

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

Although I see that this won't be so easy because various functions
such as Unix.write have the buffer argument being of type string :(

So at various points I seem to be forced to use strings. I suppose one
alternative is to reimplement the functions I use (such as Unix.write)
to work with arrays. Does anyone know if this has been done elsewhere?


On 9 May 2013 15:07, Tom Ridge <tom.j.ridge+caml@googlemail.com> wrote:
> 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
>>
>>
>>