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-838...@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*)`. > > --- > Reply to this email directly or view it on GitHub: > https://github.com/cffi/cffi/pull/2#issuecomment-4057418 >
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