Re: C structures

[Neil Jerram]

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