Problem with char ** as a output function parameter

[Jacek Bzdak]

Hi,

I'm wrapping a library that contains function that has an API similar to

void GetParameter(int param_id, void** param_text, unsigned int text_length);

and I'm supposed to call it (using C code):

char[8] result;
GetParam(1, &result, 8) //This fills result with some data
printf(result);

I'm having big problems recreating this in cffi code. Maybe I'm missing something obvious.

I have tried (among other things):

data = ffi.new("char[]", 8)
data[0:8] = b' '*8
ffi.addressof(data) # TypeError: expected a struct or union ctype

Any pointers?

JB

[Armin Rigo]

Hi Jacek,

On 9 February 2015 at 12:49, Jacek Bzdak <jbz...@gmail.com> wrote:
> char[8] result;
> GetParam(1, &result, 8) //This fills result with some data
> printf(result);

Hum, are you very sure that this C code is correct? The obvious typos
--- there are 4 of them on 3 lines --- make me think that you didn't
try to execute such C code...

What does GetParameter() do with its "void **" argument? Shouldn't
the C code be rather like this:

void *result;
GetParameter(1, &result, 8); // returns a "void *" by writing it to result
puts((char *)result); // print it by assuming a pointer to
// a null-terminated string

If it is, then the corresponding cffi is:

p_result = ffi.new("void *[1]")
GetParameter(1, p_result, 8)
print ffi.string(ffi.cast("char *", p_result[0]))


A bientôt,

Armin.

[mgr inż. Jacek Bzdak]

Hi Armin,

It was a pseudocode, actual runnable code is like this more complicated (and I wouldn't ever been able to provide runnable example since library I working with is proprietary).

Anyways thanks both for your help, and for the cffi --- which is best tool for working with C code I worked with :)!

JB

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[Armin Rigo]

Hi,

On 9 February 2015 at 19:01, mgr inż. Jacek Bzdak <jbz...@gmail.com> wrote:
> It was a pseudocode, actual runnable code is like this more complicated

The point is that you have to start with "how would I call this in C"
and then translate to CFFI. Your example was not pseudo-code, it was
downright wrong as far as I can tell...


A bientôt,

Armin.