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From Jim Jagielski <...@jaguNET.com>
Subject Re: How to align shm in an neat way?
Date Tue, 14 Aug 2012 21:20:05 GMT
slotmem handles this well, afaict ;)

On Aug 13, 2012, at 12:32 PM, Jeff Trawick <trawick@gmail.com> 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?
> 
>> 
>> Regards,
>> 
>> Rainer
> 
> 
> 
> -- 
> Born in Roswell... married an alien...
> http://emptyhammock.com/
> 


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