httpd-dev mailing list archives

Site index · List index
Message view « Date » · « Thread »
Top « Date » · « Thread »
From Rainer Jung <>
Subject Re: How to align shm in an neat way?
Date Mon, 13 Aug 2012 17:27:15 GMT
On 13.08.2012 18:32, Jeff Trawick wrote:
> On Mon, Aug 13, 2012 at 12:30 PM, Rainer Jung <> 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
>> 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?

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.



View raw message