How do I access a char ** through ffi in plt-scheme?

Question

I'm mocking about with plt-scheme's ffi and I have a C-function that returns a char ** (array of strings). If I declare my function as (_fun _pointer -> _pointer), how do I convert the result to a list of strings in scheme?

Here are the relevant C-declarations:

typedef char **MYSQL_ROW;   /* return data as array of strings */
// ...
MYSQL_ROW STDCALL mysql_fetch_row(MYSQL_RES *result);

Answer 1

I know it's not exactly what you are looking for, but it might help a little bit. I've done some work on a basic Gambit Scheme FFI for MySQL. I don't know how PLT Scheme and Gambit differ in terms of their FFI implementation (I'd venture with "quite a bit") but maybe you can get something out of it:

http://bunny.jonnay.net/zengarden/trunk/lib/mysql/mysql-ffi.scm

Answer 2

I think that what you want is the cvector:

http://docs.plt-scheme.org/foreign/Derived_Utilities.html#(part._foreign~3acvector)

A cvector of _string/utf-8 or whichever encoding you need seems reasanable.

But that's from a quick survey of the docs - I haven't tried this myself. Please let me know if it works!

Answer 3

Aha, I figured it out myself.

I have to use the _cpointer procedure, described at the page that mike linked to:

(_fun _pointer -> (_cpointer/null 'mysql-row (make-ctype _pointer #f #f)))

It also seems that someone already beat me to creating a ffi to mysqlclient. Not to worry; My main goal is understanding the ffi api, and it's going forward.