$ gdb -c core ./ruby
Core was generated by `/home/k_tsj/work/ruby/lib/test/unit/parallel.rb: TestSetTraceFunc#test_class '.
Program terminated with signal 11, Segmentation fault.
#0  0x00002aaab994fde7 in ?? () from /lib/libgcc_s.so.1
(gdb) bt
#0  0x00002aaab994fde7 in ?? () from /lib/libgcc_s.so.1
#1  0x00002aaab995066e in _Unwind_Backtrace () from /lib/libgcc_s.so.1
#2  0x00002aaaabd4037e in backtrace () from /lib/libc.so.6
#3  0x00002aaaaabfb43b in rb_vm_bugreport () at vm_dump.c:773
#4  0x00002aaaaac41bbc in report_bug (
    file=0x2aaaac104370 "/home/k_tsj/work/ruby/lib/test/unit/parallel.rb", line=37, fmt=0x2aaaaac824d7 "Segmentation fault", args=0x2aaaae58a600)
    at error.c:306
#5  0x00002aaaaac41cff in rb_bug (
    fmt=0x2aaaaac824d7 "Segmentation fault") at error.c:325
#6  0x00002aaaaab7dab3 in sigsegv (sig=11, info=0x2aaaae58a870,
    ctx=0x2aaaae58a740) at signal.c:607
#7  <signal handler called>
#8  0x0000000000000025 in ?? ()
#9  0x00002aaaaabfd848 in exec_hooks (th=0x2aaaae3cd870,
    list=0x2aaaaaf5d258, trace_arg=0x2aaaafb3ebd0) at vm_trace.c:291
#10 0x00002aaaaabfd9d4 in rb_threadptr_exec_event_hooks (
    th=0x2aaaae3cd870, event=32, self=46912558579360, id=1336,
    klass=46912501411400) at vm_trace.c:340
#11 0x00002aaaaabe3ed6 in vm_call_cfunc (th=0x2aaaae3cd870,
    reg_cfp=0x2aaaaccd7f60, num=0, recv=46912558579360, blockptr=0x0,
    me=0x2aaaab048de0, defined_class=46912501411400)
    at vm_insnhelper.c:436
#12 0x00002aaaaabe4690 in vm_call_method (th=0x2aaaae3cd870,
    cfp=0x2aaaaccd7f60, num=0, blockptr=0x0, flag=0, id=1336,
    me=0x2aaaab048de0, recv=46912558579360, defined_class=46912501411400)
    at vm_insnhelper.c:568
#13 0x00002aaaaabe9f83 in vm_exec_core (th=0x2aaaae3cd870, initial=0)
    at insns.def:1010
#14 0x00002aaaaabf7a24 in vm_exec (th=0x2aaaae3cd870) at vm.c:1146
#15 0x00002aaaaabf65f1 in invoke_block_from_c (th=0x2aaaae3cd870,
    block=0x2aaaae3cdc90, self=46912501228960, argc=0,
    argv=0x2aaaae63be10, blockptr=0x0, cref=0x0,
    defined_class=46912526173560) at vm.c:618
#16 0x00002aaaaabf6842 in vm_invoke_proc (th=0x2aaaae3cd870,
    proc=0x2aaaae3cdc90, self=46912501228960,
    defined_class=46912526173560, argc=0, argv=0x2aaaae63be10,
    blockptr=0x0) at vm.c:666
#17 0x00002aaaaabf68f2 in rb_vm_invoke_proc (th=0x2aaaae3cd870,
    proc=0x2aaaae3cdc90, argc=0, argv=0x2aaaae63be10, blockptr=0x0)
    at vm.c:685
#18 0x00002aaaaac00db7 in thread_start_func_2 (th=0x2aaaae3cd870,
    stack_start=0x2aaaafb41000) at thread.c:452
#19 0x00002aaaaabffa44 in thread_start_func_1 (th_ptr=0x2aaaae3cd870)
    at thread_pthread.c:688
#20 0x00002aaaab1609ca in start_thread () from /lib/libpthread.so.0
#21 0x00002aaaabd2770d in clone () from /lib/libc.so.6
#22 0x0000000000000000 in ?? ()
(gdb) fr 9
#9  0x00002aaaaabfd848 in exec_hooks (th=0x2aaaae3cd870,
    list=0x2aaaaaf5d258, trace_arg=0x2aaaafb3ebd0) at vm_trace.c:291
291                         (*hook->func)(trace_arg->event, hook->data, trace_arg->self, trace_arg->id, trace_arg->klass);
(gdb) p hook
$1 = (rb_event_hook_t *) 0x2aaaaafc8440
(gdb) p hook->func
$2 = (rb_event_hook_func_t) 0x25
(gdb) rp 0x25
FIXNUM: 18