Re: [Chicken-users] define-macro, ffis, and the gc

[felix]

Eric Merritt wrote:
> Guys,
>
>  I pretty new to chicken and have a few questions I hope you can help
> me with. At the bottom of this message you will find a macro. The most
> obvious problem with it is that its way too long. It needs to be
> broken up into smaller functional pieces. However, I am not sure how
> to do that in chicken, considering that functions defined in the file
> are not available at compile time. What is the usual way to handle
> this? 

You can use `eval-when' to define stuff at compile time:

(eval-when (compile eval)  ; <- eval'd in the interpreter as usual, and while 
compiling
  (define (my-helper ...) ...) )

(define-macro (my-macro ...)
  ...
  (my-helper ...)
  ...)


> My other question
> revolves around chicken's gc and malloc/calloc in an ffi. Will the gc
> clean up memory allocated in a foreign-lambda or does it need to
> explicitly released? 

Anything you allocate via [cm]alloc() inside wrapped C code has to be
freed manually. The exception is the `c-string*' result type, which
accepts a malloc'd char * and frees it after copying it into Scheme space.

> If the gc handles the clean up, how do you handle
> issues where special actions need  to be taken to free the memory?

You can use finalizers (`set-finalizer!') to attach a procedure to
a datum, that will be called once the object is not used anymore.
To use this facility with malloc'd C data, you can wrap the data
into a record, like this:


% cat x.scm

(define-record wrapper ptr)

(define-foreign-type wrapper-type c-pointer wrapper-ptr make-wrapper)

(define alloc
  (foreign-lambda* wrapper-type () "return(malloc(42));") )

(define free
  (foreign-lambda void "free" wrapper-type) )

(set-gc-report! #t) ; to see some finalizer output

(define x (alloc))
(set-finalizer! x (lambda (x) (print "finalizin'") (free x)))

(print x)
(print (wrapper-ptr x))

(set! x #f)  ; now remove global reference to it

(gc #t)      ; force all pending finalizers to be run

(print "end.")

% csc x.scm
% x
#<wrapper>
#<pointer 8052640>
GC: 1 item(s) in finalizer table
GC: finalizer table:     size=64,       max=-1
GC: pending finalizers:  1
GC: queueing 1 finalizers
GC: level  1    gcs(minor)  6   gcs(major)  1
GC: stack       bffef7f0        bfffbf60        bffff7f0
GC:  from       404d6008        404ea4fc        40513098           144f4
GC:    to       40498008        40498008        404d5098
finalizin'
GC: level  1    gcs(minor)  0   gcs(major)  2
GC: stack       bffef7f0        bfffe540        bffff7f0
GC:  from       40498008        404ac4e0        404d5098           144d8
GC:    to       404d6008        404d6008        40513098
end.
GC: level  1    gcs(minor)  0   gcs(major)  3
GC: stack       bffef7f0        bfffe610        bffff7f0
GC:  from       404d6008        404ea4e0        40513098           144d8
GC:    to       40498008        40498008        404d5098
%


cheers,
felix