Re: [Caml-list] Val_int vs caml_copy_nativeint

[Erik de Castro Lopo <mle+ocaml@mega-nerd.com>]

Xavier Leroy wrote:

> On 08/08/2011 10:03 AM, Guillaume Yziquel wrote:
> 
> > Then I do not see anything wrong if the code snippet you sent. However,
> > when you change Val_int to caml_copy_nativeint, the layout of the tuple
> > is different. [...] So if you keep the same OCaml code when reading
> > the result value, it's no surprise that the pointer is shown in
> > place of the integer you expected.
> 
> This is good advice indeed: make sure your Caml type declaration
> matches the data representation that your Caml/C stub implements...
> 
> >    /* Package up the result as a tuple. */
> >    v_response = caml_alloc_tuple (3) ;
> >    Store_field (v_response, 0, Val_int (width)) ;
> >    Store_field (v_response, 1, Val_int (height)) ;
> >    Store_field (v_response, 2, caml_copy_string (code)) ;
> >    CAMLreturn (v_response) ;
> 
> Additionally, do make sure that "v_response" is registered with the GC
> (declared with one of the CAMLlocal macros).

This is strange, it wasn't declared with a CAMLlocal macro and it
was working, but if I do declare it with one the program segfaults
during garbage collection (caml_oldify_local_roots).

The program is small, so I've included the working version of the
C stub code below. In the Ocaml code I have:

    external iec16022ecc200 :
	int -> (* size *)
	string -> (* code *)
	(int * int * string) = "caml_iec16022ecc200"

The C stub code below works. If I change "value v_response" to
"CAMLlocal1 (v_response)" it segfaults. If I use caml_copy_nativeint()
instead of Val_int when I'm preparing the v_response tuple I get what
looks like a pointer instead of the small integer (ie [16, 160] range)
I was expecting.

Cheers,
Erik

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

#include <stdio.h>
#include <string.h>
#include <assert.h>

#include <iec16022ecc200.h>


typedef union
{	unsigned char * u ;
    signed char * s ;
    char * c ;
} flexi_char_ptr_t ;


CAMLprim value
caml_iec16022ecc200 (value v_size, value v_code)
{	unsigned char buffer [2048] ;
	flexi_char_ptr_t code, barcode ;
	int width, height, k ;
	value v_response ;

	CAMLparam2 (v_size, v_code) ;

	width = height = Int_val (v_size) ;
	code.c = String_val (v_code) ;

	barcode.u = iec16022ecc200 (&width, &height, NULL, strlen (code.c), code.u, NULL, NULL, NULL) ;

	if (barcode.u == NULL)
	{	failwith ("iec16022ecc200 failed") ;
		fprintf (stderr, "%s %d: Should never get here.\n", __FILE__, __LINE__) ;
		exit (1) ;
		} ;

	/* Sanity check the buffer size. */
	assert (width * height < (int) sizeof (buffer) - 1) ;

	/* Need to convert the resulting 0x00 and 0x01 byte string to a text
	 * string that we can pass back to Ocaml. */
	for (k = 0 ; k < width * height ; k++)
		buffer [k] = barcode.c [k] ? '1' : '0' ;

	/* Make sure buffer is string terminated. */
	buffer [width * height] = 0 ;

	/* Free the memory allocated by iec16022ecc200(). */
	free (barcode.u) ;

	barcode.u = buffer ;

	/* Package up the result as a tuple. */
	v_response = caml_alloc_tuple (3) ;

	Store_field (v_response, 0, Val_int (width)) ;
	Store_field (v_response, 1, Val_int (height)) ;
	Store_field (v_response, 2, caml_copy_string (barcode.c)) ;

    CAMLreturn (v_response) ;
} /* caml_iec16022ecc200 */



-- 
----------------------------------------------------------------------
Erik de Castro Lopo
http://www.mega-nerd.com/