Re: [Vala] Generating bindings for non-GLib/GObject based API

[Michael Wild <wild ifd mavt ethz ch>]

On 23. Apr, 2010, at 8:06 , Michael Wild wrote:

> On 22. Apr, 2010, at 20:43 , Jan Hudec wrote:
>
> > On Thu, Apr 22, 2010 at 11:47:21 +0200, Michael Wild wrote:
> > > 1) Constructor function: Say I would like to use a C function as the
> > > constructor for a class (as e.g. newwin is used in curses.vapi). However,
> > > that C function does not return the allocated and initialized object in the
> > > return value, but as the first argument. The return value is an error code
> > > instead. How would I handle this?
> >
> > I believe you'll have to declare it as a static function and wrap it as
> > constructor in the .vapi itself.
> >
> > I suppose you have a function like:
> >
> > int thing_init(Thing **self, int param);
> >
> > (allocate+return by reference implies pointer-to-pointer, right?)
> >
> > Than in the class you first declare the function itself, like
> >
> > class Thing {
> >   private static init(out Thing self, int param);
> >
> > Than you need to define the constructor, which is where I am a bit lost.
> > It does work for normal methods, but constructor is a bit special.
> > I would guess this might work:
> >
> >   public Thing(int param) throws ThingError {
> >      int status = init(this, param);
> >      if(status != 0)
> >          throw new ThingError.FAILED(
> >              "Thing construction failed with status %d", status);
> >   }
> >
> > But I am not sure whether you need to initialize this or create normal
> > variable and return it or employ some other trick. Maybe defining another
> > static function and than rebinding it as constructor. Something like
> >
> >   public static Thing new(int param) throws ThingError {
> >      Thing self;
> >      int status = init(self, param);
> >      ...
> >      return self;
> >   }
> >   public Thing(int param) throws (ThingError);
> >
> > You'll have to play with it a bit.
> >
> > Note, that I made it throw an error. I assume if it returns an error code,
> > you'll want to get it somehow and since the allocated object is the return
> > value, an exception is the only option.
>
> Thanks, I was stumbling along the same lines. However, now I'm stuck because I can't convince Vala to 
> generate the C-code for the static functions in the .vapi file.
>
>
> Michael


Looking at gnutls.vapi I noticed that there the same problem exists. Instead of binding it to a constructor, 
static create() methods are used to wrap the C-functions. I'm probably going to do the same.

Another thing where I'm not sure how to best solve it: The basic C-type I'm using is a typedef to a pointer 
whose base-type is implementation dependent (i.e. unknown). E.g. like this:

typedef struct unknown_type* Thing;

Now, in my small test cases, I used the following for the binding:

[SimpleType]
[CCode (cname = "Thing", cheader_filename = "thing.h", free_function = "free")]
struct Thing {
  [CCode (cname = "create_thing")]
  private static int create_imp(Thing* self, int a, int b, double c); 
  [CCode (cname = "create_thing_wrapper")]
  public static Thing create(int a, int b, double c) {
    Thing self;
    int stat = create_imp(&self, a, b, c); 
    return self;
  }
  
  [CCode (cname = "print_thing")]
  public void print();
}

This compiles and runs, but Vala now (quite understandably) thinks that Thing is a value-type and doesn't 
call free() on it. AFAIK for this to happen I'd need to change Thing to be a of compact class type, but then 
everything else would break because Vala starts passing the object around as a pointer (i.e. resulting in 
unknonw_type**, where unknown_type* is expected).

Michael