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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 16:32:50 GMT
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?

>
> Regards,
>
> Rainer



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