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From Jeff Trawick <traw...@gmail.com>
Subject Re: How to align shm in an neat way?
Date Mon, 13 Aug 2012 17:40:16 GMT
On Mon, Aug 13, 2012 at 1:27 PM, Rainer Jung <rainer.jung@kippdata.de> wrote:
> On 13.08.2012 18:32, Jeff Trawick wrote:
>>
>> On Mon, Aug 13, 2012 at 12:30 PM, Rainer Jung <rainer.jung@kippdata.de>
>> wrote:
>>>
>>> Hi,
>>>
>>> PR 53040 reveals, that mod_socache_shmcb has an alignment problem. One of
>>> the three structs mapped into shm contains an apr_time_t member, which at
>>> least on Sparc is 8 Bytes, whereas for 32 bit builds long is only 4
>>> Bytes.
>>>
>>> Currently everything is aligned for 4 Bytes, so we get bus errors/crashes
>>> when trying to assign the apr_time_t to an address that is only divisible
>>> by
>>> 4 instead of 8.
>>>
>>> I can easily reproduce the problem.
>>>
>>> A possible solution is to pad the three structures SHMCBHeader,
>>> SHMCBSubcache and SHMCBIndex to a multiple of 8 Bytes length. For
>>> Subcache
>>> and Index this is already true by coincidence, SHMCBHeader needs another
>>> 4
>>> Bytes.
>>>
>>> I wonder what the right solution is. In the patch
>>>
>>> http://people.apache.org/~rjung/patches/mod_socache_shmcb-padding.patch
>>>
>>> I hard coded the padding, but I don't really like it, because it breaks
>>> if
>>> members are added to the struct. I could add a sizeof() test during
>>> startup
>>> or probably even compilation to warn or err, if the padding is wrong.
>>>
>>> I see several recipes for alignment using pragmas and attribute, but all
>>> of
>>> them are compiler specific.
>>>
>>> One could also wrap the struct in a wrapped struct, so that one could use
>>> the sizeof() of the inner struct to determine the padding of the outer
>>> struct. That would make the code convoluted.
>>>
>>> I checked other parts of the code, but couldn't find a simple solution.
>>> Any
>>> hints how to do this nicely?
>>
>>
>> APR_ALIGN_DEFAULT?
>
>
> I think it doesn't solve this problem, does it? It only gives me a need way
> to round up sizes to multiples of 8 bytes.

It should be possible to use use APR_ALIGN_DEFAULT(sizeof(foo)) in
place of sizeof(foo), instead of adding explicit padding to the
structures. Any other pointer arithmetic which doesn't use sizeof(foo)
would also need to use the macro.


>
> Regards,
>
> Rainer
>



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