18.c

static void
rb_kill_thread(th, flags)
    rb_thread_t th;
    int flags;
{
    if (th != curr_thread && th->safe < 4) {
	rb_secure(4);
    }
    if (th->status == THREAD_TO_KILL || th->status == THREAD_KILLED)
	return;
    if (th == th->next || th == main_thread) rb_exit(EXIT_SUCCESS);

    rb_thread_ready(th);
    th->flags |= flags;
    th->status = THREAD_TO_KILL;
    if (!rb_thread_critical) rb_thread_schedule();
}

19.c

VALUE
rb_thread_kill(VALUE thread)
{
    rb_thread_t *th;

    GetThreadPtr(thread, th);

    if (th != GET_THREAD() && th->safe_level < 4) {
	rb_secure(4);
    }
    if (th->status == THREAD_TO_KILL || th->status == THREAD_KILLED) {
	return thread;
    }
    if (th == th->vm->main_thread) {
	rb_exit(EXIT_SUCCESS);
    }

    thread_debug("rb_thread_kill: %p (%p)\n", (void *)th, (void *)th->thread_id);

    rb_thread_interrupt(th);
    th->thrown_errinfo = eKillSignal;
    th->status = THREAD_TO_KILL;

    return thread;
}