From ripley at stats.ox.ac.uk  Thu Aug 29 22:17:07 2013
From: ripley at stats.ox.ac.uk (Prof Brian Ripley)
Date: Thu, 29 Aug 2013 21:17:07 +0100
Subject: [Rd] Why does an empty vector occupy 40 bytes?
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On 29/08/2013 20:26, Hadley Wickham wrote:
>>> Do pointers always have to be aligned?
>>
>> Depends on the architecture, e.g., Sparc doesn't allow non-aligned pointers at all (results in a segfault), SV x86_64 ABI specifies pointers to be aligned at 8 bytes, but the x86_64 CPUs will allow misaligned pointers with a performance penalty (the same was true for x86 - i.e. misaligned pointers are tolerated with a penalty).
>
> Ok, so you might not have to align pointers a particular architecture,
> but if you're writing code that you want to work across multiple
> architectures you must.

Yes, and note too that you may or may not need to align doubles, 
independently of pointers (and it may even be a compiler setting: it is 
on Solaris).

>
> Hadley
>


-- 
Brian D. Ripley,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
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[Rd] Why does an empty vector occupy 40 bytes?
Prof Brian Ripley