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

[Markus Mottl <markus.mottl@gmail.com>]

The Core.Std.Bigstring-module offers a large number of Unix-I/O
functions for bigstrings, even for vectorized I/O (e.g. writev).  I am
sure you will find everything you need there.

On Thu, May 9, 2013 at 10:14 AM, Tom Ridge
<tom.j.ridge+caml@googlemail.com> wrote:
> 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
>>>
>>>
>>>
>
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--
Markus Mottl        http://www.ocaml.info        markus.mottl@gmail.com