Re: obtaining a pointer to a pointer for nlffi

Thanks for the help.

The right way is to do exactly as Vesa says, you simply allocate in  
stages, i.e. allocate n (char *0) and then allocate (len(s)) for each  
pointer.

I actually found it in your ml-lib code :-)

Brian

On Dec 31, 2006, at 4:31 AM, Vesa Karvonen wrote:

> Quoting brian <briand-/[email protected]>:
>> I need a char ** for the arg to a function.
>>
>> So I created a char * to actually hold the string
>>
>> stringList = C.alloc C.T.uchar (Word.fromInt (size name + 1))
>>
>> Then I tried the obvious:
>>
>> C.Ptr.|&| stringList
>>
>> but I get an error:
>>
>>    operator domain: ('Z,'Y) C.obj
>>    operand:         (C.uchar,'X) C.obj C.ptr
>>    in expression:
>>      C.Ptr.|&| stringList
>>
>> The error message makes perfect sense, it's trying to create a ptr to
>> an obj, but I already have a ptr, not an obj.  Unfortunately I can't
>> figure out how to make it take the ptr of a ptr.
>
> Here is one way that seems to compile (with MLton):
>
>   (* allocate a char** object - must be deallocated somewhere! *)
>   val cptrptr = C.new (C.T.pointer C.T.uchar)
>
>   (* allocate a char array - must be deallocated somewhere! *)
>   val cptr = C.alloc C.T.uchar (Word.fromInt 10)
>
>   (* set the char** to point to the char array *)
>   val () = C.Set.ptr (cptrptr, cptr)
>
> I believe that in NLFFI you can only take the address of a _C object_.
> A C object is something that lives on the C heap.  When you  
> allocate an
> array using C.alloc, you only allocate a C object to store the array -
> no C object is allocated to store a pointer to the array.  The  
> "pointer"
> that is returned by C.alloc is an ML object (it lives on the ML heap).
> You can't take the address of an ML object, because, for one thing,
> they have no (stable/permanent) addresses - GC can move them around.
>
> -Vesa Karvonen


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