Starting with Solaris 10 Update 6, on SPARCV8, pthread_mutex_t and pthread_cond_t MUST be aligned on an 8-byte boundary. Misaligned access will result in either SEGV or SIGBUS.
There are numerous places in the multi-threaded version of stdcxx where pthread_mutex_t and/or pthread_cond_t types are contained within an union, but with an enforced alignment different than 8. All these instances must be corrected, and #ifdef-guarded for SPARCV8.
Patches to follow shortly, this is just opening the issue.
Warning: the patchset resolving this issue is very large, and it affects a large number of files.