bigarray_str.ml

open Core
open Bigarray

external c_cast
  :  ('a, 'b, 'c) Bigarray.Genarray.t
  -> (char, int8_unsigned_elt, 'c) Bigarray.Genarray.t
  = "ba_cast_to_char"

let elems ba = Array.reduce_exn ~f:Int.( * ) (Bigarray.Genarray.dims ba)

let cast ba =
  let ba = Bigarray.reshape ba [| elems ba |] in
  c_cast ba |> array1_of_genarray
;;

dune

(library
 (name bigarray_str)
 (public_name bigarray_str)
 (libraries core)
 (foreign_stubs
  (language c)
  (names stubs)
  (flags -std=c99 -pedantic)))

stubs.c

#define CAML_INTERNALS

#include <caml/custom.h>
#include <caml/fail.h>
#include <caml/memory.h>
#include <caml/bigarray.h>

// Source: https://github.com/ocaml/ocaml/blob/4.12/runtime/bigarray.c#L919-L941
static void caml_ba_update_proxy(struct caml_ba_array *b1,
                                 struct caml_ba_array *b2)
{
     struct caml_ba_proxy *proxy;
     /* Nothing to do for un-managed arrays */
     if ((b1->flags & CAML_BA_MANAGED_MASK) == CAML_BA_EXTERNAL)
          return;
     if (b1->proxy != NULL)
     {
          /* If b1 is already a proxy for a larger array, increment refcount of
       proxy */
          b2->proxy = b1->proxy;
          ++b1->proxy->refcount;
     }
     else
     {
          /* Otherwise, create proxy and attach it to both b1 and b2 */
          proxy = malloc(sizeof(struct caml_ba_proxy));
          if (proxy == NULL)
               caml_raise_out_of_memory();
          proxy->refcount = 2; /* original array + sub array */
          proxy->data = b1->data;
          proxy->size =
              b1->flags & CAML_BA_MAPPED_FILE ? caml_ba_byte_size(b1) : 0;
          b1->proxy = proxy;
          b2->proxy = proxy;
     }
}

CAMLprim value ba_cast_to_char(value vb)
{
     CAMLparam1(vb);
     CAMLlocal1(res);
#define b (Caml_ba_array_val(vb))
     intnat dim[CAML_BA_MAX_NUM_DIMS];
     for (int i = 0; i < b->num_dims; i++)
     {
          dim[i] = (i == b->num_dims - 1) ? b->dim[i] * caml_ba_element_size[b->flags & CAML_BA_KIND_MASK] : b->dim[i];
     }

     int flags = (b->flags & ~CAML_BA_KIND_MASK) | CAML_BA_CHAR;

     res = caml_ba_alloc(flags, b->num_dims, b->data, dim);
     /* Copy the finalization function from the original array (PR#8568) */
     Custom_ops_val(res) = Custom_ops_val(vb);
     /* Create or update proxy in case of managed bigarray */
     caml_ba_update_proxy(b, Caml_ba_array_val(res));
     /* Return result */
     CAMLreturn(res);

#undef b
}

As a mental model you can look at the caml_ba_slice function that takes a "slice" of a bigarray without copying the data. What you are doing is not very different; and you will see there that the flags of the "source" array are copied as-is to the destination array.

I was using caml_ba_reshape since it looks simpler, yet I didn't look close enough at what caml_ba_update_proxy is doing. I will get some sleep and try to make the kind an input, maybe it's possible to get that merged into ocaml trunk. It would be pretty awesome but I have no idea how the core team perceives adding new features to standard library.