Solaris 2.6 defines a new interface for interaction between the debugger and the runtime linker. Previous releases don't support any such interface, so debuggers would use unpublished hooks.
The following patch to gdb 4.16 fixes this problem.
--- ./gdb/solib.c.org Sat Mar 30 06:58:51 1996 +++ ./gdb/solib.c Sat Sep 6 14:48:18 1997 @@ -67,6 +67,7 @@#ifdef SVR4_SHARED_LIBS static char *solib_break_names[] = { + "rtld_db_dlactivity", "r_debug_state", "_r_debug_state", "_dl_debug_state",