about `smob' generated by swig

about `smob' generated by swig

[William Xu]

Hi, 

Swig could generate pointer types in C to smob. But users still have to
define smob related operations(like mark, free, etc) by hand?

-- 
William

It was a JOKE!!  Get it??  I was receiving messages from DAVID LETTERMAN!!
YOW!!


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Re: about `smob' generated by swig

[Marco Maggi]

"William Xu" wrote:
> Swig could generate pointer types  in C to smob. But users
> still  have to define  smob related  operations(like mark,
> free, etc) by hand?

Mh, no.   I have never used SWIG  but if I read  [1] and [2]
and   by   inspecting   "Examples/guile"   in   the   source
distribution  of SWIG  1.3.31,  it seems  to  me that  every
unknown data type used  as parameter to functions is treated
like a pointer.

[1] <http://www.swig.org/Doc1.1/HTML/SWIG.html#n3>
[2] "swig-1.3.31/Doc/Manual/Guile.html"

  That is: SWIG generates a SMOB holding two values:

1 - the pointer to the data instance;
2 - a string holding the name of the unknown type;

and uses  that as  reference to the  unknown type;  the data
instance  itself is  just a  block  of memory,  it is  *not*
wrapped in  an automatically  defined SMOB.  Let's  call the
SWIG generated SMOB: 'reference'  SMOB.  A reference SMOB is
of  a type  defined by  SWIG, so  we have  to do  nothing to
handle its  memory: by  looking in a  "*_wrap.c" file  I see
that  there  are SMOB  type  definitions  whose drivers  are
referenced     by    variables    like     'swig_tag'    and
'swig_collectable_tag',  and   SWIG  implements  the  driver
functions for them.

  When SWIG  processes a function  that has some  C language
type  as  argument  or   return  value,  it  just  uses  the
corresponding SMOB type as defined by Guile to handle it, so
we have to do nothing.

  When SWIG  processes a function that has  unknown types as
argument  or return value,  it wraps  the function  with one
that uses reference SMOBs.

  The reference  SMOB drivers have no  mark function. AFAICS
there is  no way to tell  SWIG to invoke a  mark function to
mark  fields in  a  custom  data structure;  that  is: if  a
function builds and returns a structure:

    struct my_type_t {
      int a;
      SCM b;
    };

there is  no way to  mark 'b'. We  have to register  it with
'scm_gc_protect_object()'.

  It is my  understanding that it is possible  to register a
destroy function to be  invoke when garbage collection runs,
to free the  data referenced by a reference  SMOB; but I was
not able to find how to do it.

  Anyway  a   destructor  for  custom   data  referenced  by
reference SMOBs has  to be written, and then  you can use it
in a guardian and with 'after-gc-hook'.

  By the way: to write a SMOB driver is not difficult if
you know some C language. Are you sure that you need
SWIG?

--
Marco Maggi

"They say jump!, you say how high?"
Rage Against the Machine - "Bullet in the Head"



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Re: about `smob' generated by swig

[William Xu]

"Marco Maggi" <marco.maggi-ipsu@poste.it> writes:

> "William Xu" wrote:
>> Swig could generate pointer types  in C to smob. But users
>> still  have to define  smob related  operations(like mark,
>> free, etc) by hand?
>
> Mh, no.   I have never used SWIG  but if I read  [1] and [2]
> and   by   inspecting   "Examples/guile"   in   the   source
> distribution  of SWIG  1.3.31,  it seems  to  me that  every
> unknown data type used  as parameter to functions is treated
> like a pointer.
>
> [1] <http://www.swig.org/Doc1.1/HTML/SWIG.html#n3>
> [2] "swig-1.3.31/Doc/Manual/Guile.html"
>
>   That is: SWIG generates a SMOB holding two values:
>
> 1 - the pointer to the data instance;
> 2 - a string holding the name of the unknown type;

Yes, it's more like a smob type pointer. Consider this, 

    void bar(unsigned char *s)
    {
         printf("s = %s\n", s);
    }
    
One problem is that how i create and pass a `unsigned char*' parameter
to `bar'. If SWIG requires me to write smob creation myself, that's a
pain...

>   By the way: to write a SMOB driver is not difficult if
> you know some C language. Are you sure that you need
> SWIG?

As we know, one of guile or scheme's big shortcomings is lack of
libraries. On the contrary, C libraries are pretty full. Hence, if SWIG
can do a good job on wrapping C codes for guile, that would really be
great..

-- 
William

I used to be disgusted, now I find I'm just amused.
		-- Elvis Costello


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Re: about `smob' generated by swig

[Marco Maggi]

"William Xu" wrote:
> Yes, it's more like a smob type pointer. Consider this,
>
>    void bar(unsigned char *s)
>    {
>         printf("s = %s\n", s);
>    }
>
> One problem is that how i create and pass a `unsigned
> char*' parameter to `bar'. If SWIG requires me to write
> smob creation myself, that's a pain...

AFAICT you have to write a C constructor:

  unsigned char *
  make_one_thing (int dim)
  {
     unsigned char * p = malloc(dim);

     put_something_in(p);
     return p;
  }

and SWIG will generate the Scheme/C wrapper, something
like:

  static SCM
  _wrap_make_one_thing (SCM s_0)
  {
    unsigned char * result = 0 ;
    int arg0;
    SCM gswig_result;

    arg0 = scm_to_int(s_0);
    result = (unsigned char *)new_matrix(arg0);
    gswig_result = SWIG_NewPointerObj (result,
                      SWIGTYPE_p_unsigned_char, 0);
    return gswig_result;
  }

so you do not have to write the SMOB driver.

--
Marco Maggi

"They say jump!, you say how high?"
Rage Against the Machine - "Bullet in the Head"



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Re: about `smob' generated by swig

[William Xu]

"Marco Maggi" <marco.maggi-ipsu@poste.it> writes:

> AFAICT you have to write a C constructor:

And as well as a C desctructor? 

Ooh, i thought SWIG can take care of all details for me. Apparently it
doesn't.

Thanks for your help.

-- 
William

We are MicroSoft.  You will be assimilated.  Resistance is futile.
		-- Attributed to B.G., Gill Bates


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