* Re: Need help: O'Caml C Interface
@ 1996-09-24 14:45 Juan Jose Quintela Carreira
1996-09-24 17:11 ` Jerome Vouillon
0 siblings, 1 reply; 4+ messages in thread
From: Juan Jose Quintela Carreira @ 1996-09-24 14:45 UTC (permalink / raw)
To: caml-list, christo
Hi,
If you changes the lines:
>
> value ocaml_getCharWidth(value code)
> {
> double x, y;
> Push_roots(r,1);
>
> r[0] = alloc_tuple(2);
> getCharWidth(Int_val(code),&x,&y);
- Store_double_val(Field(r[0],0),x);
+ modify(&Field(r[0],0),copy_double(x); /* Field(r[0],0)=copy_double(x)*/
- Store_double_val(Field(r[0],1),y);
+ modify(&Field(r[0],1),copy_double(y); /* Field(r[0],1)=copy_double(y)*/
> Pop_roots();
> return r[0];
> }
>
The coment sentences also work in this example, but the former work always,
For details see the manual section:
Chapter: Interfacing C with Objective Caml
Section: Living in harmony with the garbage collector
Regards, Juan.
^ permalink raw reply [flat|nested] 4+ messages in thread
* Re: Need help: O'Caml C Interface
1996-09-24 14:45 Need help: O'Caml C Interface Juan Jose Quintela Carreira
@ 1996-09-24 17:11 ` Jerome Vouillon
0 siblings, 0 replies; 4+ messages in thread
From: Jerome Vouillon @ 1996-09-24 17:11 UTC (permalink / raw)
To: Juan Jose Quintela Carreira; +Cc: caml-list, christo
> If you changes the lines:
> >
> > value ocaml_getCharWidth(value code)
> > {
> > double x, y;
> > Push_roots(r,1);
> >
> > r[0] = alloc_tuple(2);
> > getCharWidth(Int_val(code),&x,&y);
> - Store_double_val(Field(r[0],0),x);
> + modify(&Field(r[0],0),copy_double(x); /* Field(r[0],0)=copy_double(x)*/
> - Store_double_val(Field(r[0],1),y);
> + modify(&Field(r[0],1),copy_double(y); /* Field(r[0],1)=copy_double(y)*/
> > Pop_roots();
> > return r[0];
> > }
> >
>
> The coment sentences also work in this example, but the former work always,
Well, actually neither are correct, as copy_double allocate some memory.
Thus, the tuple should have been filled with dummy values prior to the
first call to copy_double, and one cannot use `Field(..) = ..'.
Jerome
^ permalink raw reply [flat|nested] 4+ messages in thread
* Need help: O'Caml C Interface
@ 1996-09-23 15:09 Frank Christoph
1996-09-24 13:36 ` Jerome Vouillon
0 siblings, 1 reply; 4+ messages in thread
From: Frank Christoph @ 1996-09-23 15:09 UTC (permalink / raw)
To: caml-list
I have a C function
void getCharWidth(int code, double *x, double *y);
which returns its values in the two pointer arguments. I tried to write a
stub function that creates a 2-tuple and passes it back:
external width : int -> float * float = "ocaml_getCharWidth"
via:
value ocaml_getCharWidth(value code)
{
double x, y;
Push_roots(r,1);
r[0] = alloc_tuple(2);
getCharWidth(Int_val(code),&x,&y);
Store_double_val(Field(r[0],0),x);
Store_double_val(Field(r[0],1),y);
Pop_roots();
return r[0];
}
where I used Store_double_val because, as far as I can see, there is no
"Val_double" macro. However, this gives me a segmentation fault. I'm not
really sure I understand how the *_roots functions work. Do the two local
variables x and y need to be declared as of type value? If so, how can I
pass them to getCharWidth (since Double_val doesn't return an lvalue)? What
am I doing wrong?
------------------------------------------------------------------------
Frank Christoph Next Solution Co. Tel: 0424-98-1811
christo@nextsolution.co.jp Fax: 0424-98-1500
^ permalink raw reply [flat|nested] 4+ messages in thread
* Re: Need help: O'Caml C Interface
1996-09-23 15:09 Frank Christoph
@ 1996-09-24 13:36 ` Jerome Vouillon
0 siblings, 0 replies; 4+ messages in thread
From: Jerome Vouillon @ 1996-09-24 13:36 UTC (permalink / raw)
To: Frank Christoph; +Cc: caml-list
> value ocaml_getCharWidth(value code)
> {
> double x, y;
> Push_roots(r,1);
>
> r[0] = alloc_tuple(2);
> getCharWidth(Int_val(code),&x,&y);
> Store_double_val(Field(r[0],0),x);
> Store_double_val(Field(r[0],1),y);
> Pop_roots();
> return r[0];
> }
I would write it this way:
value ocaml_getCharWidth(value code)
{
double x, y;
value res;
Push_roots(r,2);
getCharWidth(Int_val(code),&x,&y);
r[0] = copy_double(x);
r[1] = copy_double(y);
res = alloc_tuple(2);
Field(res, 0) = r[0];
Field(res, 1) = r[1];
Pop_roots();
return res;
}
Store_double_val is used to store a double in an array of double values
(which is a special kind of array). In other cases, you have to use
copy_double to allocate a block containing the double.
So, your code failed because Field(r[0],0) was not a pointer to an array,
but an uninitialized pointer.
*_roots are used to keep a pointer to both double values. Indeed, these
values can be moved by copy_double or alloc_tuple. The tuple is allocated
last, so that it can be directly filled (without having to use function
modify).
Jerome
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1996-09-24 13:36 ` Jerome Vouillon
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