Re: C structures

[Mike Gran]

Yeah.  There's a bit of overhead in getting a SMOB set up, but, it has
the advantage that you aren't keeping duplicate copies (a C copy and a
Scheme copy) of your information in memory.

--- Neil Jerram <address@hidden> wrote:

> Leonardo,
> 
> Based on your replies to other people on the list, it sounds as
> though
> you probably want a SMOB.  A SMOB is a way of passing a C pointer
> (such as to an arbitrary struct) around opaquely in Scheme, and the
> way to use them in quite well documented in a couple of places in the
> Guile manual (1.7/CVS version): nodes "Defining New Types (Smobs)"
> and
> "Dia Smobs".
> 
> In case something simpler would do, however, I've also added a few
> comments below.
> 
> Leonardo Lopes Pereira <address@hidden> writes:
> 
> > Let me give a example. I can pass some simple data...
> >
> > --- scheme.scm ---
> > (define number 1)
> >
> > --- C program ---
> > #include <stdio.c>
> > #include <libguile.h>
> > #include <guile/gh.h>
> >
> > int main ()
> > {
> >   int number
> >   SCM s_symbol, s_value;
> >   scm_init_guile ();
> >
> >   scm_c_primitive_load ("scheme.scm");
> >   
> >   s_symbol = scm_c_lookup("number");
> >   s_value = scm_variable_ref(s_symbol);
> >   number = gh_scm2int(s_value, 0, "main");
> > }
> >
> > ---
> >
> > This convert the Scheme data in C data, but I do not know how to
> pass
> > a group of data together. ex.: How to convert a (cons x y) into a
> > struct?
> 
> Well, for example:
> 
> (define number (cons 3 4))
> 
> instead of (define number 1), and then
> 
>   struct point number
>   SCM s_symbol, s_value;
>   scm_init_guile ();
> 
>   scm_c_primitive_load ("scheme.scm");
>   
>   s_symbol = scm_c_lookup("number");
>   s_value = scm_variable_ref(s_symbol);
>   number.x = gh_scm2int(SCM_CAR(s_value), 0, "main");
>   number.y = gh_scm2int(SCM_CDR(s_value), 0, "main");
> 
> instead of the C code above.  Does that make sense?
> 
> A scheme list can be as long as you like, so can hold more than 2
> values, and each value can be a different type.  So you can equally
> well have 2 numbers and a string, for example.
> 
> >
> > I want to create a wrap to a function that has a sruct as arg, so,
> I
> > need to create that struct in scheme and convert it to C, how to do
> > that?
> 
> It depends what the C code expects as regards the lifetime of the
> struct.  If the struct only needs to be value for the duration of the
> call to your C func, you can allocate it on the stack, initialize it
> from a Scheme pair/list as shown above, and then call the C func.  If
> the struct needs to be longer lived than that, the SMOB approach is
> probably best.
> 
> Regards,
>         Neil
> 
> 
> 
> _______________________________________________
> Guile-user mailing list
> address@hidden
> http://lists.gnu.org/mailman/listinfo/guile-user
> 



        
                
__________________________________ 
Yahoo! for Good - Make a difference this year. 
http://brand.yahoo.com/cybergivingweek2005/