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?
Regards,
Rainer
