[clisp-list] Re: FFI: array of strings?

[Sam S. <sd...@gn...>]

> * Kaz Kylheku <xn...@nf...g> [2004-02-17 11:00:56 -0800]:
>
> On Tue, 17 Feb 2004, Sam Steingold wrote:
>
>> 2. How do I interface to
>>    <http://www.mathworks.com/access/helpdesk/help/techdoc/apiref/matgetdir.shtml>?
>> char **matGetDir(MATFile *mfp, int *num);
>>    What is the return value?
>>    Also, the documentation appears to indicate that what is actually
>>    returned is (char*[mxMAXNAM]), not (char**): why would they mention
>
> This is not possible; a C function cannot return an array.  :)
>
> I suspect that your actual suspiction is that it might be 
> ``char (*)[mxMAXNAM]''. In other words, the pointer to an array of
> mxMAXNAM characters (or a pointer to the first element of an
> array of arrays, whose elements are arrays of mxMAXNAM characters).
>
>>    mxCalloc() otherwise?
>
> They say:
>
>     matGetDir returns a pointer to an internal array containing
>     pointers to the NULL-terminated names
>
> so it's clear that it's  ``char **'', just like the return type says.
>
> They must do some funny allocation to allow you to blow this away with
> just one call to their mxFree() function. The array of pointers and the
> string data are all allocated in one memory block. Or perhaps the array
> of pointers is dynamic, but the pointers are managed separately; maybe
> the MATFile object owns these strings, and the dynamic array is just
> used as a temporary object to retrieve the pointers in one call.
>
> They probably use their mxCalloc() function like this:
>
>    char **arrayOfPointers = mxCalloc(numElements,
>                                      sizeof arrayOfPointers[0]);
>
> so the use of a calloc-like allocator does not support your 
> suspicion that it's not an array of pointers.

so, what's the bottom line?
how do I call this this via FFI?
it does not appear that I can specify the function to free the data...

-- 
Sam Steingold (http://www.podval.org/~sds) running w2k
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