On 13.08.2012 18:32, Jeff Trawick wrote:
On Mon, Aug 13, 2012 at 12:30 PM, Rainer Jung <rainer.j...@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.
Regards,
Rainer
