set_trace_func

static VALUE
thread_set_trace_func_m(VALUE obj, VALUE trace)
{
    rb_thread_t *th;
    GetThreadPtr(obj, th);
    rb_threadptr_remove_event_hook(th, call_trace_func);

    if (NIL_P(trace)) {
        th->tracing = EVENT_RUNNING_NOTHING;
        return Qnil;
    }
    thread_add_trace_func(th, trace);
    return trace;
}