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17.1.3 How to Represent Dia Data in Scheme
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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 the idea of SMOBs comes in, and with it 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 SMOB. The SMOB is what is passed into and out of
Scheme code, and the Scheme-specific C structure inside the SMOB 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 SMOB. 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 SMOB 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 SMOB type.
* Whenever you need to represent a C shape onto the Scheme level,
create a SMOB instance for it, and pass that.
* In primitive code that receives a shape SMOB 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 SMOB 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 SMOB value, it
calls the "SMOB free" function that was specified when defining the
shape SMOB 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 SMOB types, see
Defining New Types (Smobs).
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