Liam Healy | 20 Feb 18:12 2012
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Re: [cffi] FSBV (#2)

Ryan -

I am forwarding this to CFFI-devel and Martin directly (though I'm pretty sure he's subscribed to cffi-devel).  Can you please send future messages to cffi-devel?  I'm not sure many people read the github mailing list; this issue should get wider visibility.   Thanks.

Liam

On Mon, Feb 20, 2012 at 12:02 PM, Ryan Pavlik <reply+i-1614209-ba246666762196459413560690eb7d3a39c7c7ee-838019 <at> reply.github.com> wrote:
I'm not sure what you mean.  It doesn't really matter that a pointer is aggregate or not.  The goal is to get at the Nth member of an array-of-something.  In the case of scalar C types, you're getting the value; in the case of structs it's far more useful to get a pointer, because you probably only want a single value out of the struct, or to **put a value into the struct**.  (The `setf` form works for scalars in the latter case, but not for "member-of-struct-of-index-N"... you most likely want the pointer in these cases.)

It also occurred to me after posting that there is no difference between `(mem-aref ptr '(:pointer (:struct foo)) N)` and just simply `(mem-aref ptr :pointer N)` ... both return a pointer value as if `ptr` is a `void *ptr[k]`.

A struct of more bytes works the same way:

```
(cffi:defcstruct my-struct
 (x :long)
 (y :long)
 (x :long)
 (t :long))

...

Old-ref style:
ptr : #.(SB-SYS:INT-SAP #X7FFFEEFC7FB8)
aref: #.(SB-SYS:INT-SAP #X7FFFEEFC7FD8)
New-ref style:
ptr : #.(SB-SYS:INT-SAP #X7FFFEEFC7FB8)
aref: (T 0 Y 0 X 0)
New-ref with :pointer style:
ptr : #.(SB-SYS:INT-SAP #X7FFFEEFC7FB8)
aref: #.(SB-SYS:INT-SAP #X00000000)
```

Note that on my system, a pointer is 8 bytes, not 4.  This is why I initially found the problem, when trying to access an array of points defined by 2 short; each member is 4 bytes, and it was giving offsets to `sizeof(void*)`.

---
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https://github.com/cffi/cffi/pull/2#issuecomment-4057418

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