Dispatch Queue 之内存中常驻的几个结构

_dispatch_main_q

也就是主要主队列，一定运行在主线程里。

// 6618342 Contact the team that owns the Instrument DTrace probe before renaming this symbol
struct dispatch_queue_s _dispatch_main_q = {
    .do_vtable = &_dispatch_queue_vtable, // 相关的操作函数在这里
    .do_ref_cnt = DISPATCH_OBJECT_GLOBAL_REFCNT,  //表示永远不会被释放
    .do_xref_cnt = DISPATCH_OBJECT_GLOBAL_REFCNT, //表示永远不会被释放
    .do_suspend_cnt = DISPATCH_OBJECT_SUSPEND_LOCK,
    .do_targetq = &_dispatch_root_queues[DISPATCH_ROOT_QUEUE_COUNT / 2],

    .dq_label = "com.apple.main-thread",
    .dq_running = 1,
    .dq_width = 1, // 串行队列
    .dq_serialnum = 1,
};

主队列的目标队列时全局队列其中之一。

对应的do_vtable: _dispatch_queue_vtable

即对应的操作函数。

static const struct dispatch_queue_vtable_s _dispatch_queue_vtable = {
    .do_type = DISPATCH_QUEUE_TYPE,
    .do_kind = "queue",
    .do_dispose = _dispatch_queue_dispose,
    .do_invoke = (void *)dummy_function_r0, //什么都不做，返回 0
    .do_probe = (void *)dummy_function_r0, //什么都不做，返回 0
    .do_debug = dispatch_queue_debug,
};

这样子，在调用do_invoke 和 do_probe 时，会返回0.

_dispatch_root_queues

所有队列最终的target queue。
一共有6个 dispatch_queue_s。

static struct dispatch_queue_s _dispatch_root_queues[] = {
    {
        .do_vtable = &_dispatch_queue_root_vtable,
        .do_ref_cnt = DISPATCH_OBJECT_GLOBAL_REFCNT, //不会被释放
        .do_xref_cnt = DISPATCH_OBJECT_GLOBAL_REFCNT,//不会被释放
        .do_suspend_cnt = DISPATCH_OBJECT_SUSPEND_LOCK,
        .do_ctxt = &_dispatch_root_queue_contexts[0],

        .dq_label = "com.apple.root.low-priority", 
        .dq_running = 2, //偶数，说明不是串行队列
        .dq_width = UINT32_MAX, //最大并发数目
        .dq_serialnum = 4,
    },
....

对应的do_vtable: _dispatch_queue_root_vtable

static const struct dispatch_queue_vtable_s _dispatch_queue_root_vtable = {
    .do_type = DISPATCH_QUEUE_GLOBAL_TYPE,
    .do_kind = "global-queue",
    .do_debug = dispatch_queue_debug,
    .do_probe = _dispatch_queue_wakeup_global,
};

即当调用do_probe时，会调用 _dispatch_queue_wakeup_global 函数，把队列唤醒。

对应的 do_ctxt: _dispatch_root_queue_contexts

dispatch_root_queue_context_s 的定义如下

struct dispatch_root_queue_context_s {
    pthread_workqueue_t dgq_kworkqueue; 
    uint32_t dgq_pending;
    uint32_t dgq_thread_pool_size; // 还可以分配的线程池大小
    dispatch_semaphore_t dgq_thread_mediator; // 信号量
};

生成时如下：

static struct dispatch_root_queue_context_s _dispatch_root_queue_contexts[] = {
    {
        .dgq_thread_mediator = &_dispatch_thread_mediator[0],
        .dgq_thread_pool_size = MAX_THREAD_COUNT,
    },

可见dgq_thread_mediator 对应于另一组常驻内存的变量。

static struct dispatch_semaphore_s _dispatch_thread_mediator[] = {
    {
        .do_vtable = &_dispatch_semaphore_vtable,
        .do_ref_cnt = DISPATCH_OBJECT_GLOBAL_REFCNT,
        .do_xref_cnt = DISPATCH_OBJECT_GLOBAL_REFCNT,
    },