From h.wickham at gmail.com  Thu Aug 29 20:37:06 2013
From: h.wickham at gmail.com (Hadley Wickham)
Date: Thu, 29 Aug 2013 13:37:06 -0500
Subject: [Rd] Why does an empty vector occupy 40 bytes?
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> Not to be picky, but that is not the point. The alignment is due to the attrib pointer which is at offset 8 despite the fact that there is only a 4-byte element in front of it. Maybe for better illustration, this is the layout on 64-bit machines:

Ok, thanks for the clarification.

>
> * 4 bytes: sxpinfo header (= 32 bits)
> * 4 bytes: --- padding so next ptr is aligned ---
> * 8 bytes: pointer to attributes
> * 8 bytes: pointer to next node
> * 8 bytes: pointer to previous node
> * 4 bytes: length
> * 4 bytes: true length
>
> = 40 bytes
>
> This is already aligned so the payload alignment doesn't any extra padding so that has no effect at all.

Do pointers always have to be aligned?

Hadley


-- 
Chief Scientist, RStudio
http://had.co.nz/



[Rd] Why does an empty vector occupy 40 bytes?
Hadley Wickham