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Re: are statically allocated structs always aligned to a machine word on x86/x86_64?

On 08/21/2015 05:54 PM, john smith wrote:
> I didn't find any information about alignment requirements for
> statically allocated objects in glibc manual (or I have missed because
> the manual is huge). I noted that sometimes variables such as int are
> not aligned on word boundary in x86 and x86_64 but I have never seen a
> struct that wouldn't be allocated at address that isn't a multiple or
> 4/8. I am asking this question because I would like to know whether
> it's safe to assume that struct will be always assigned at a word
> boundary and therefore it's possible to correctly calculate a struct
> size without running a program.

We looked at a similar question when we assessed the impact of glibc bug
18287.  In the end, we could not find a case where a top-level char
array with automatic storage duration was not aligned to a machine word.
 However, none of the GCC engineers I talked could confirm that this
might never happen, under no circumstances.

So the answer to your question is: no, you cannot assume word alignment.
 Even if you'll never see misaligned variables in practice.

Florian Weimer / Red Hat Product Security

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