As i understand, in general, the problem that you described is in following:
- you want to pass an address of your buffer contents, but started not from
very first element of your buffer, but somewhere inside a buffer.
In smalltalk you cannot reference an element of array,
only the object (array in that case) as a whole.
The reason why it like so, because VM moves objects around, and you cannot
control directly when that happens,
and also VM responsible for updating all pointers (references) to moved
object(s)
for all interested parties (which could be other objects, stack etc) ,
making sure all references remain consistent upon such move.
So, with such constraints, the only way to validly point to an element
inside array
would be to store two values separately:
- a reference to an object, that represent your buffer (which VM would
update at will)
- an index (or offset) in that object, pointing to element in your buffer
Unfortunately, this is the only way how we could implement such, lets say
'ElementPointer' safely. Which then can be used to pass to C function(s),
converting object reference + offset into simple address just before
invoking a function (and sure thing, knowing that there's no chance
triggering GC, else it will turn into pointer to wrong place, but that's
general problem of passing pointers on object memory heap, not just
exclusively for 'element pointer' and such).
For buffers allocated externally, e.g. outside heap governed by VM,
there's nothing prevents you from having an address that pointing inside
some buffer (or even outside it :)
For NBExternalAddress:
addr := self allocate: somespace.
newAddr := NBExternalAddress value: addr value + someoffset.
or
newAddr := addr copy value: addr value + someoffset
sure, it is up to you then, how to calculate offsets and buffer size(s) as
well as allocating/deallocating memory for buffers you using.
On 8 June 2015 at 16:41, Matthieu Lacaton <matthieu.laca...@gmail.com>
wrote:
> Hello everyone,
>
> I have a small question about NativeBoost : How does the "+" operator when
> applied to a pointer translates into NativeBoost code ?
>
> To give a bit of context, what I want to do is to reallocate some
> non-contiguous bytes in memory to a buffer. Basically, I have an array of
> integers in a buffer and I want to copy some chunks of it in another
> buffer. The chunks are always the same size and the offset between each
> chunk is always the same too.
>
> Because a bit of actual code is easier to understand here is what I'd like
> to do in Pharo :
>
> ...
>
> int i, j;
> int *data = malloc(1000*sizeof(int));
> int *newData = malloc(50*sizeof(int));
>
> // Allocate initial data
> for (i = 0 ; i < 1000, i++) {
> data[i] = i;
> }
>
> //Copy desired chunks into new buffer
> for (i = 0; i < 5; i++ ) {
> memcpy( newData + j*10, data + 200 + j*30, 10*sizeof(int));
> j++;
> }
>
> free(data);
>
> ...
>
> Here basically I'll get in my buffer chunks of 10 integers starting at 200
> with an offset of 30 between chunks, and this 5 times. (200 201 202 ... 208
> 209 230 231 ... 238 239 260 ... 328 329).
>
> I am okay with the malloc, memcpy and free but I don't know how to handle
> the "+" operator in my memcpy function.
>
> Thank you,
>
> Matthieu
>
--
Best regards,
Igor Stasenko.
