5.7.1.3 How to Represent Dia Data in Scheme

For all but the most trivial applications, you will probably want to allow some representation of your domain objects to exist on the Scheme level. This is where foreign objects come in, and with them issues of lifetime management and garbage collection.

To get more concrete about this, let’s look again at the example we gave earlier of how application users can use Guile to build higher-level functions from the primitives that Dia itself provides.

(define (change-squares'-fill-pattern new-pattern)
  (for-each-shape current-page
    (lambda (shape)
      (if (square? shape)
          (change-fill-pattern shape new-pattern)))))

Consider what is stored here in the variable shape. For each shape on the current page, the for-each-shape primitive calls (lambda (shape) …) with an argument representing that shape. Question is: how is that argument represented on the Scheme level? The issues are as follows.

• Whatever the representation, it has to be decodable again by the C code for the square? and change-fill-pattern primitives. In other words, a primitive like square? has somehow to be able to turn the value that it receives back into something that points to the underlying C structure describing a shape.
• The representation must also cope with Scheme code holding on to the value for later use. What happens if the Scheme code stores shape in a global variable, but then that shape is deleted (in a way that the Scheme code is not aware of), and later on some other Scheme code uses that global variable again in a call to, say, square??
• The lifetime and memory allocation of objects that exist only in the Scheme world is managed automatically by Guile’s garbage collector using one simple rule: when there are no remaining references to an object, the object is considered dead and so its memory is freed. But for objects that exist in both C and Scheme, the picture is more complicated; in the case of Dia, where the shape argument passes transiently in and out of the Scheme world, it would be quite wrong the delete the underlying C shape just because the Scheme code has finished evaluation. How do we avoid this happening?

One resolution of these issues is for the Scheme-level representation of a shape to be a new, Scheme-specific C structure wrapped up as a foreign object. The foreign object is what is passed into and out of Scheme code, and the Scheme-specific C structure inside the foreign object points to Dia’s underlying C structure so that the code for primitives like square? can get at it.

To cope with an underlying shape being deleted while Scheme code is still holding onto a Scheme shape value, the underlying C structure should have a new field that points to the Scheme-specific foreign object. When a shape is deleted, the relevant code chains through to the Scheme-specific structure and sets its pointer back to the underlying structure to NULL. Thus the foreign object value for the shape continues to exist, but any primitive code that tries to use it will detect that the underlying shape has been deleted because the underlying structure pointer is NULL.

So, to summarize the steps involved in this resolution of the problem (and assuming that the underlying C structure for a shape is struct dia_shape):

• Define a new Scheme-specific structure that points to the underlying C structure:

  struct dia_guile_shape
  {
    struct dia_shape * c_shape;   /* NULL => deleted */
  }
  

• Add a field to struct dia_shape that points to its struct dia_guile_shape if it has one —

  struct dia_shape
  {
    …
    struct dia_guile_shape * guile_shape;
  }
  

  — so that C code can set guile_shape->c_shape to NULL when the underlying shape is deleted.

• Wrap struct dia_guile_shape as a foreign object type.
• Whenever you need to represent a C shape onto the Scheme level, create a foreign object instance for it, and pass that.
• In primitive code that receives a shape foreign object instance, check the c_shape field when decoding it, to find out whether the underlying C shape is still there.

As far as memory management is concerned, the foreign object values and their Scheme-specific structures are under the control of the garbage collector, whereas the underlying C structures are explicitly managed in exactly the same way that Dia managed them before we thought of adding Guile.

When the garbage collector decides to free a shape foreign object value, it calls the finalizer function that was specified when defining the shape foreign object type. To maintain the correctness of the guile_shape field in the underlying C structure, this function should chain through to the underlying C structure (if it still exists) and set its guile_shape field to NULL.

For full documentation on defining and using foreign object types, see Defining New Foreign Object Types.