[cffi-devel] Re: First usages of CFFI

[Stephen Compall]

Marko Tapio Manninen <mmstud at gmail.com> writes:
> void (*cb)(struct evbuffer *, size_t, size_t, void *);

This is explained in tutorial format in §3.9 `Calling Lisp from C' in
the official manual, and reference documentation is in §11 `Callbacks'.
The official manual may be found on the website, but I strongly
recommend that you use the one you should have received with CFFI.

> (defconstant HTTP_OK 200)
> (defconstant HTTP_NOCONTENT 204)
> (defconstant HTTP_MOVEPERM 301)
> ...

`defcenum' would be good for this.

> (defcstruct evbuffer
> 	(*buffer :unsigned-char)
> 	(*orig_buffer :unsigned-char)
> 	(misalign size-t)
> 	(totallen size-t)
> 	(off size-t)
> 	(*cb :pointer) ; struct?
> 	(*cbarg :pointer))

Interestingly, this arg is called "closure" in libraries that know what
it hacks around.  You may wish to call it that to make its nature clear
to Lisp-readers.

> (defcfun ("evbuffer_add_printf" %evbuffer-add-printf) :void
> 	(buf evbuffer)
> 	(msg :string)
> 	(uri :string))

I believe this was mentioned on #lisp, but those should not be indented
a full tab-stop.  The communities of Lisp indenters are not nearly as
divergent as with some languages, and I don't think you'll find one that
would encourage that.

> (defcfun ("evhttp_send_reply" %evhttp-send-reply) :void
> 	(req evhttp-request)
> 	(flag :int)
> 	(msg :string)
> 	(buf evbuffer))
>
> ;;; unwind-protect to create evbuffer-new?

Yes, but it's for freeing, not allocating.  See §3.8 `Memory management'
for some tips.

> (defun generic-handler (req &optional (arg nil))
> 	(let ((buf (%evbuffer-new)))
> 		(if (null buf)
> 			(error "Failed to create response buffer")

Unfortunately, you can't signal ERROR here after you convert it to a
real callback; see the aforementioned §3.9 for why.  For generalized
conditions, think of this as a game of volleyball, where the call to
whatever C function calls GENERIC-HANDLER is the net.

-- 
But you know how reluctant paranormal phenomena are to reveal
themselves when skeptics are present. --Robert Sheaffer, SkI 9/2003