[cffi-devel] Double pointers

[Luís Oliveira]

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).

Finally, through MEM-REF, we access our newly allocated memory and get
the DB handle.

HTH

-- 
Luís Oliveira
http://student.dei.uc.pt/~lmoliv/