// Block 数据结构
struct Block_layout {
    // 指向表明该block类型的类
    void *isa;
    // 按bit位表示一些 block 的附加信息，比如判断 block 类型、判断 block 引用计数、判断 block 是否需要执行辅助函数等
    int flags;
    // 保留变量
    int reserved;
    // 函数指针，指向具体的 block 实现的函数调用地址
    void (*invoke)(void *, ...);
    struct Block_descriptor *descriptor;
    /* Imported variables. */
};

// 宏定义
// 触发 block 的实现
#define _dispatch_Block_invoke(bb) \
        ((dispatch_function_t)((struct Block_layout *)bb)->invoke)

// 入口方法
void dispatch_once(dispatch_once_t *val, dispatch_block_t block) {
    dispatch_once_f(val, block, _dispatch_Block_invoke(block));
}

#define DISPATCH_ONCE_DONE ((struct _dispatch_once_waiter_s *)~0l)

struct _dispatch_once_waiter_s {
    //链表下一个节点
    volatile struct _dispatch_once_waiter_s *volatile dow_next;
    // 信号量
    _dispatch_thread_semaphore_t dow_sema;
};

void dispatch_once_f(dispatch_once_t *val, void *ctxt, dispatch_function_t func) {
  	// volatileg 关键字编辑的变量 vval
    // 告诉编译器此指针指向的值随时可能被其他线程改变
    // 从而使得编译器不对此指针进行代码编译优化
    struct _dispatch_once_waiter_s * volatile *vval =
            (struct _dispatch_once_waiter_s**)val;
    struct _dispatch_once_waiter_s dow = { NULL, 0 };
    struct _dispatch_once_waiter_s *tail, *tmp;
  	// 声明信号变量
    _dispatch_thread_semaphore_t sema;

    if (dispatch_atomic_cmpxchg(vval, NULL, &dow, acquire)) {
        _dispatch_client_callout(ctxt, func);

        dispatch_atomic_maximally_synchronizing_barrier();
        // above assumed to contain release barrier
        tmp = dispatch_atomic_xchg(vval, DISPATCH_ONCE_DONE, relaxed);
        tail = &dow;
        while (tail != tmp) {
            while (!tmp->dow_next) {
                dispatch_hardware_pause();
            }
            sema = tmp->dow_sema;
            tmp = (struct _dispatch_once_waiter_s*)tmp->dow_next;
            _dispatch_thread_semaphore_signal(sema);
        }
    } else {
        dow.dow_sema = _dispatch_get_thread_semaphore();
        tmp = *vval;
        for (;;) {
            if (tmp == DISPATCH_ONCE_DONE) {
                break;
            }
            if (dispatch_atomic_cmpxchgvw(vval, tmp, &dow, &tmp, release)) {
                dow.dow_next = tmp;
                _dispatch_thread_semaphore_wait(dow.dow_sema);
                break;
            }
        }
        _dispatch_put_thread_semaphore(dow.dow_sema);
    }
}