Re: Strings from C?

[Matthias Koeppe]

"Robert A. Uhl" <address@hidden> writes:

> On Wed, Oct 17, 2001 at 11:21:11AM +0200, Matthias Koeppe wrote:
>> 
>> You might want to consider using an interface generator like SWIG 1.3
>> (http://www.swig.org) or g-wrap.  This may save some work, and both
>> tools know how to perform standard conversions between C and Scheme
>> values.
> 
> Well, part of my problem is that in several places my methods take
> GSList* (a singly-linked list from glib, for those who don't use that
> library) as either an argument or return value.  I was tempted to use
> SWIG, but everything I saw indicated that there was no way to convert
> between _my_ lists and guile's lists.  Given that my application is
> list-intensive, and that scheme is list-intensive, it would seem
> pointless to simply wrap up more functions to create C lists from
> scheme lists; something like:
> 
> (trav-mapobject-set-owners obj (gslist-new obj2 obj3 obj4))
> 
> or
> 
> (car (gslist-split (trav-mapobject-get-owners obj)))
> 
> is more than a little ugly, at least to my eyes.

Indeed.

> Perh. there is a way to do this from SWIG after all?  Obviously
> ideal would be able to write:
> 
> (trav-mapobject-set-owners obj '(obj2 obj3 obj4))
> 
> and
> 
> (car (trav-mapobject-get-owners obj))

In SWIG we have user-defined type conversions between arbitrary C and
Scheme types, so-called "typemaps".  The SWIG code for your specific
conversion problem would look like this:

       %typemap(in) GSList * {
         GSList *list = $source;
         SCM scheme_list = ... /* build the SCM list here */
         $target = scheme_list;
       }

       %typemap(out) GSList * {
         SCM scheme_list = $source;
         GSList *list = ... /* build GSList here */
         $target = list;
       }

If you make these typemap declarations, all functions taking or
returning GSList * values will automatically perform the conversion,
so you can write your Scheme code in the "ideal" way.

-- 
Matthias Köppe -- http://www.math.uni-magdeburg.de/~mkoeppe
SWIG makes Guile wrappers for C/C++ libs -- http://www.swig.org
ILISP does module-aware Emacs/Guile interaction -- http://sf.net/projects/ilisp