Error when passing struct by value

[Marshall Mason]

Hi Liam,
Thanks for the response. It works! Actually, there was one hitch, which I'd
like to explain, if for no other reason than posterity.

I had translate-into-foreign-memory, translate-to-foreign, and
free-translated object defined, which were supposed to unpack a list (using
first, second, third, and fourth), not a plist. The list I sent had two
symbols, a list, and a symbol, as you said. Perhaps I didn't understand the
' quote function, but I thought it was supposed to interpret stuff inside
the list. That's my Lisp ignorance showing.

Because I had those three methods defined, changing to a plist as you
suggested didn't work. I tried just removing the methods entirely, assuming
perhaps what you were getting at here is that CFFI can accept plists by
default, so that the methods only need to be defined if you want to use a
custom mechanism for passing the structs. This worked! I didn't know about
this important feature. I didn't see it in the manual, or any of my Google
searches.

Thanks so much for your help!

Marshall

On Thu, Apr 30, 2015 at 6:27 PM, Liam Healy <lnp at healy.washington.dc.us>
wrote:

> Looks like you're sending it a list of symbols 'read-func etc.
> '(read-func seek-func (null-pointer) tell-func)
> is a list of two symbols, a list, then another symbol. It needs to be
> a plist (property list) with alternating slot names and values
> (list 'read-func read-func 'seek-func seek-func...)
> (At the moment I can't remember if the slot names are in the keyword
> package or not, so you might need :read-func instead of 'read-func
> etc.)
>
> Liam
>
> On Thu, Apr 30, 2015 at 7:28 PM, Marshall Mason
> <marshallmason2 at gmail.com> wrote:
> > Hello,
> > I'm using SBCL version 1.2.4, CFFI version 0.14.0, and libffi version
> 3.1.
> > I've been trying to pass a struct by value using CFFI and libffi. I'm a
> > newbie to both CFFI and Lisp so I had to piece this together from the
> manual
> > as well as some of the unit tests that come with CFFI. I'm getting an
> error
> > that I just can't get past:
> >
> > The value READ-FUNC is not of type SB-SYS:SYSTEM-AREA-POINTER
> >
> > The library I'm trying to wrap is libvorbisfile, which decodes Ogg Vorbis
> > files. It has an init function called ov_open_callbacks:
> >
> > int ov_open_callbacks(void *datasource, OggVorbis_File *vf, const char
> > *initial, long ibytes, ov_callbacks callbacks);
> >
> > The part it's choking on is the last argument, a struct, passed by
> value, of
> > callback functions:
> >
> > typedef struct {
> >     size_t (*read_func)  (void *ptr, size_t size, size_t nmemb, void
> > *datasource);
> >     int    (*seek_func)  (void *datasource, ogg_int64_t offset, int
> whence);
> >     int    (*close_func) (void *datasource);
> >     long   (*tell_func)  (void *datasource);
> > } ov_callbacks;
> >
> > The library is designed to use one of a few static structs defined in a
> > header file. Since it's defined in the header file and not in
> > libvorbisfile.so, I must create CFFI translation code. There were a lot
> of
> > other structs I had to wrap with CFFI as well. Here is my code:
> >
> > http://pastie.org/10122885
> >
> > Does anyone have any clues about what I'm doing wrong here?
> >
> > (Regarding my previous email about getting CFFI working on Debian
> jessie, it
> > turns out the Debian version of cl-alexandria is missing a file and
> cl-cffi
> > is buggy, so I just added the missing file and installed the CFFI source
> by
> > hand. Thanks for the response, Fau.)
> >
> > Marshall
>
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