Modifying OCaml Bigarray from C

main.ml

open Bigarray

external mutation_test : (float, float64_elt, c_layout) Array2.t -> unit = "mutation_test"

let a = Array2.create float64 c_layout 2 2

let () =
  Array2.set a 0 0 1.;
  Array2.set a 0 1 2.;
  Array2.set a 1 0 3.;
  Array2.set a 1 1 4.;
  mutation_test a; (* print elements and change a[0][0] to 5.0 *)
  Printf.printf "\nmutated value: %f\n" (Array2.get a 0 0)

Makefile

all: run opt
run: main.run
opt: main.opt

OCAMLC=ocamlfind ocamlc -annot
OCAMLOPT=ocamlfind ocamlopt

# C
%.o: %.c
	$(OCAMLOPT) -c $<

dllmatrix.so: matrix.o
	ocamlmklib -o matrix $<

#OCaml bytecode
%.cmo: %.ml
	$(OCAMLC) -c $<

main.cma: main.cmo dllmatrix.so
	$(OCAMLC) -a -o $@ $< -dllib -lmatrix -package bigarray -linkpkg

main.run: main.cma main.cmo
	$(OCAMLC) -custom -o $@ $^ libmatrix.a

#OCaml optcode
%.cmx: %.ml
	$(OCAMLOPT) -c $<

libmatrix.a: main.cmx dllmatrix.so
	$(OCAMLOPT) -a -o main.cmxa $< -cclib -lmatrix

main.opt: main.cmx libmatrix.a
	$(OCAMLOPT) -o $@ $^ -ccopt -L./ -package bigarray -linkpkg

.PHONY: clean all run opt
clean:
	rm -f *.[oa] *.so *.cm[ixoa] *.cmxa *.out *~ *.run *.opt *.annot

matrix.c

#include <stdio.h>
#include <caml/mlvalues.h>
#include <caml/bigarray.h>

value mutation_test(value bigarray){ // print elements of a OCaml bigarray and mutate (0,0) element.
     int i;
     int dimx = Bigarray_val(bigarray)->dim[0];
     int dimy = Bigarray_val(bigarray)->dim[1];
     printf("dim: %d, %d\n", dimx, dimy);
     double *array = Data_bigarray_val(bigarray);
     for (i = 0; i < dimx * dimy; ++i)
	  printf("val: %f\n", array[i]);
     array[0] = 5.0;
     printf("a[0][0] was changed to 5.0 in the C code\n");
     return Val_unit;
}

Result

dim: 2, 2
val: 1.000000
val: 2.000000
val: 3.000000
val: 4.000000
a[0][0] was changed to 5.0 in the C code

mutated value: 5.000000