Re: [Swig-user] Chicken - underlying pointer for SWIG object

[John L. <jl...@ma...>]

On 05/22/2007 12:41 AM, Dan Muresan wrote:
> Hi,
> 
> 1. I have wrapped a C library using SWIG; soon I discovered that I 
> sometimes need access to the underlying C pointers from the target 
> language. I've also discovered that
> 
> (define swig-underlying-ptr
>    (foreign-lambda* c-pointer ((c-pointer swig_ptr))
>     "C_return (swig_ptr);"
>    ))
> 
> always does the trick. I find this puzzling, since after reading some 
> past messages I expected the correct accessor to be C_block_item 
> (swig_ptr, 0). More importantly, this means that I can't access the type 
> information which was supposed to be in C_block_item (swig_ptr, 1). Can 
> anyone shed some light?
> 

I would have to look closer into this, doesn't seem correct.

But I would actually try a typemap instead.  For example,

%typemap(in, numinputs=0) void **out (void *temp) {
   $1 = &temp;
}

%typemap(argout) void **out {
   $result = C_unsigned_long_to_num( (unsigned long) *($1));
   or
   $result = C_make_tagged_ptr(...)
   or whatever
}

%inline %{
void getSwigPtr(YourType *a, void **out) {
  *out = (void *) a;
}
%}

This way you do not depend on how SWIG represents the data underneath....

Hasn't been tested, but I hope you get the idea...

If you want the type information, I suggest you look up "External Access
to runtime" at http://www.swig.org/Doc1.3/Modules.html#Modules.  Then
write some C functions that lookup the type and for example return the
name as a string or something.

> 
> 2. Is there a way to use typemaps to avoid wrapping certain pointers? 
> I.e. I have a structure
> 
> struct request {
>    char *data;
>    int size;
> }
> 
> where really I need to access "data" as a C pointer from Chicken. I 
> don't want the pointer to be wrapped; I *especially* don't want data to 
> be treated as a null-terminated string (and thus copied back and forth).
> 

Sure, as seen above, typemaps can match based on argument names
http://www.swig.org/Doc1.3/Typemaps.html#Typemaps_nn16

so you could write a typemap that only is used on arguments

struct request *custom

where not only the type, but also the argument name is taken into
account.  Note, the main reason above I used an argout typemap is so I
could match on the argument name...

John