[clisp-list] Re: Clisp FFI question: variable length arrays returned from C fu nction that are not null terminated

[Hoehle, Joerg-C. <Joe...@t-...>]

Hi,

John Small [mailto:jwe...@co...] wrote:

>Hi Joerg, Bruno,
Please send such stuff fo clisp-list directly, where it gets archived, =
becomes searchable for everyone and more people get a change to answer.
=20
>The mysql fetch row function returns a variable length array of =
strings:
>    char **  mysql_fetch_row(result)
[...]
>So I defined my call out as
>        (FFI:DEF-CALL-OUT          mysql_fetch_row
>             ...
>            (:return-type    ffi:c-pointer))
>But then when I go to access row
>        (setf row (mysql_fetch_row result))
>since row is a foreign address and not a place I'm stuck and can't use
>any of the cast , offset, etc. macros.

>[...]which is similar to your article on compression.
Ah, you read some of my stuff. Good!

You indeed need to move from a pointer to a place.

o WITH-FOREIGN-OBJECT / WITH-C-VAR is one of your friends here: you can =
use it to allocate a pointer on the stack initialized to your pointer, =
then cast that to the correct type.
  (with-c-var (p 'c-pointer row-pointer)
    (offset p `(c-array c-string ,actual-size) 0)

o You can use (:return-type (ffi:c-pointer (c-string))) ; limited =
declaration returning a single string
or (ffi:c-pointer (c-array c-string 1))
which will yield a FOREIGN-VARIABLE object, which makes casting (offset =
0 trick) to an array of strings more direct than the above, with the =
help of WITH-C-PLACE.

o You can use the FOREIGN-VARIABLE constructor to create the =
foreign-variable object out of the foreign-pointer, then apply =
WITH-C-PLACE.

Oddly, this constructor is not documented. I thought I had added it to =
CVS?!?

Which you choose may depend on your needs, e.g. whether to dereference =
the whole array at once or only a few elements.

Regards,
	J=F6rg H=F6hle.