gmane.lisp.cffi.devel

Double pointers

Hi,

Please excuse the poor use of terminology in the following message.
I'm stabbing around in the dark when
it comes to C.  I'll need to get a textbook someday and learn it properly.

How do you pass a "double pointer" to a C function.  For example,
here's the signature for sqlite's open

int sqlite3_open(
  const char *filename,   /* Database filename (UTF-8) */
  sqlite3 **ppDb          /* OUT: SQLite db handle */
);

I'd like to define a cffi function that lets me call this and keep a
handle on the ppDb object that gets
created as a result.

I've tried various combinations of (foreign-alloc ..) and
(null-pointer) but always got a memory exception
when trying to use the handle in another function.

The sqlite stuff has lots of these functions that modify input
parameters and return error codes
rather than returning some opaque object like the CFFI example does.

Cheers,
Andy

headers

| 1 Sep 17:31

Re: Double pointers

On Mon, Sep 1, 2008 at 3:47 PM, Andy Chambers
<achambers.home <at> googlemail.com> wrote:

> int sqlite3_open(
>  const char *filename,   /* Database filename (UTF-8) */
>  sqlite3 **ppDb          /* OUT: SQLite db handle */
> );

Let's start by declaring this function:

    (defcfun ("sqlite3_open" %sqlite3-open) :int
      (filename :string)
      (db :pointer))

> I'd like to define a cffi function that lets me call this and keep a
> handle on the ppDb object that gets
> created as a result.

  (defun sqlite3-open (filename)
    "Open FILENAME and return a handle to the DB."
    (with-foreign-object (db :pointer)
      (%sqlite-open filename db) ; XXX: do error checking here...
      (mem-ref db :pointer)))

That WITH-FOREIGN-OBJECT form allocates enough memory to hold a
pointer. And the value of DB is a pointer to that newly allocated
memory. We pass that to sqlite3_open() which will fill that memory
with the DB handle (which is a pointer, i.e., an address that points
to some opaque SQLite structure).

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