GCD头文件里那些像dispatch_async_f后面带 f 的函数是干嘛的？

我们先来看看源文件中有哪些后面带 f 的函数：

/*!
 * @function dispatch_async_f
 *
 * @abstract
 * Submits a function for asynchronous execution on a dispatch queue.
 *
 * @discussion
 * See dispatch_async() for details.
 *
 * @param queue
 * The target dispatch queue to which the function is submitted.
 * The system will hold a reference on the target queue until the function
 * has returned.
 * The result of passing NULL in this parameter is undefined.
 *
 * @param context
 * The application-defined context parameter to pass to the function.
 *
 * @param work
 * The application-defined function to invoke on the target queue. The first
 * parameter passed to this function is the context provided to
 * dispatch_async_f().
 * The result of passing NULL in this parameter is undefined.
 */
API_AVAILABLE(macos(10.6), ios(4.0))
DISPATCH_EXPORT DISPATCH_NONNULL1 DISPATCH_NONNULL3 DISPATCH_NOTHROW
void
dispatch_async_f(dispatch_queue_t queue,
    void *_Nullable context,
    dispatch_function_t work);

/*!
 * @function dispatch_sync_f
 *
 * @abstract
 * Submits a function for synchronous execution on a dispatch queue.
 *
 * @discussion
 * See dispatch_sync() for details.
 *
 * @param queue
 * The target dispatch queue to which the function is submitted.
 * The result of passing NULL in this parameter is undefined.
 *
 * @param context
 * The application-defined context parameter to pass to the function.
 *
 * @param work
 * The application-defined function to invoke on the target queue. The first
 * parameter passed to this function is the context provided to
 * dispatch_sync_f().
 * The result of passing NULL in this parameter is undefined.
 */
API_AVAILABLE(macos(10.6), ios(4.0))
DISPATCH_EXPORT DISPATCH_NONNULL1 DISPATCH_NONNULL3 DISPATCH_NOTHROW
void
dispatch_sync_f(dispatch_queue_t queue,
    void *_Nullable context,
    dispatch_function_t work);

/*!
 * @function dispatch_barrier_async_f
 *
 * @abstract
 * Submits a barrier function for asynchronous execution on a dispatch queue.
 *
 * @discussion
 * Submits a function to a dispatch queue like dispatch_async_f(), but marks
 * that function as a barrier (relevant only on DISPATCH_QUEUE_CONCURRENT
 * queues).
 *
 * See dispatch_async_f() for details.
 *
 * @param queue
 * The target dispatch queue to which the function is submitted.
 * The system will hold a reference on the target queue until the function
 * has returned.
 * The result of passing NULL in this parameter is undefined.
 *
 * @param context
 * The application-defined context parameter to pass to the function.
 *
 * @param work
 * The application-defined function to invoke on the target queue. The first
 * parameter passed to this function is the context provided to
 * dispatch_barrier_async_f().
 * The result of passing NULL in this parameter is undefined.
 */
API_AVAILABLE(macos(10.7), ios(4.3))
DISPATCH_EXPORT DISPATCH_NONNULL1 DISPATCH_NONNULL3 DISPATCH_NOTHROW
void
dispatch_barrier_async_f(dispatch_queue_t queue,
    void *_Nullable context,
    dispatch_function_t work);

/*!
 * @function dispatch_barrier_sync_f
 *
 * @abstract
 * Submits a barrier function for synchronous execution on a dispatch queue.
 *
 * @discussion
 * Submits a function to a dispatch queue like dispatch_sync_f(), but marks that
 * fuction as a barrier (relevant only on DISPATCH_QUEUE_CONCURRENT queues).
 *
 * See dispatch_sync_f() for details.
 *
 * @param queue
 * The target dispatch queue to which the function is submitted.
 * The result of passing NULL in this parameter is undefined.
 *
 * @param context
 * The application-defined context parameter to pass to the function.
 *
 * @param work
 * The application-defined function to invoke on the target queue. The first
 * parameter passed to this function is the context provided to
 * dispatch_barrier_sync_f().
 * The result of passing NULL in this parameter is undefined.
 */
API_AVAILABLE(macos(10.7), ios(4.3))
DISPATCH_EXPORT DISPATCH_NONNULL1 DISPATCH_NONNULL3 DISPATCH_NOTHROW
void
dispatch_barrier_sync_f(dispatch_queue_t queue,
    void *_Nullable context,
    dispatch_function_t work);

group.h中的相关函数

/*!
 * @function dispatch_group_async_f
 *
 * @abstract
 * Submits a function to a dispatch queue and associates the block with the
 * given dispatch group.
 *
 * @discussion
 * See dispatch_group_async() for details.
 *
 * @param group
 * A dispatch group to associate with the submitted function.
 * The result of passing NULL in this parameter is undefined.
 *
 * @param queue
 * The dispatch queue to which the function will be submitted for asynchronous
 * invocation.
 *
 * @param context
 * The application-defined context parameter to pass to the function.
 *
 * @param work
 * The application-defined function to invoke on the target queue. The first
 * parameter passed to this function is the context provided to
 * dispatch_group_async_f().
 */
API_AVAILABLE(macos(10.6), ios(4.0))
DISPATCH_EXPORT DISPATCH_NONNULL1 DISPATCH_NONNULL2 DISPATCH_NONNULL4
DISPATCH_NOTHROW
void
dispatch_group_async_f(dispatch_group_t group,
    dispatch_queue_t queue,
    void *_Nullable context,
    dispatch_function_t work);

/*!
 * @function dispatch_group_notify_f
 *
 * @abstract
 * Schedule a function to be submitted to a queue when all the blocks
 * associated with a group have completed.
 *
 * @discussion
 * See dispatch_group_notify() for details.
 *
 * @param group
 * The dispatch group to observe.
 * The result of passing NULL in this parameter is undefined.
 *
 * @param context
 * The application-defined context parameter to pass to the function.
 *
 * @param work
 * The application-defined function to invoke on the target queue. The first
 * parameter passed to this function is the context provided to
 * dispatch_group_notify_f().
 */
API_AVAILABLE(macos(10.6), ios(4.0))
DISPATCH_EXPORT DISPATCH_NONNULL1 DISPATCH_NONNULL2 DISPATCH_NONNULL4
DISPATCH_NOTHROW
void
dispatch_group_notify_f(dispatch_group_t group,
    dispatch_queue_t queue,
    void *_Nullable context,
    dispatch_function_t work);

就拿这几个有代表性的函数当做例子吧。

这些函数有一个特点，去掉后面的 _f 就是我们平时常用的GCD以block回调的方式处理多线程的函数。而加上了这个 _f 之后它们会变成什么鬼呢？

看看官方文档中的解释，以 dispatch_async_f 为例：

/*!
 * @function dispatch_async_f
 *
 * @abstract
 * Submits a function for asynchronous execution on a dispatch queue.
 *
 * @discussion
 * See dispatch_async() for details.
 *
 * @param queue
 * The target dispatch queue to which the function is submitted.
 * The system will hold a reference on the target queue until the function
 * has returned.
 * The result of passing NULL in this parameter is undefined.
 *
 * @param context
 * The application-defined context parameter to pass to the function.
 *
 * @param work
 * The application-defined function to invoke on the target queue. The first
 * parameter passed to this function is the context provided to
 * dispatch_async_f().
 * The result of passing NULL in this parameter is undefined.
 */

文档中 @abstract 摘要部分写到:

Submits a function for asynchronous execution on a dispatch queue.

就是说“提交一个在分派队列上异步执行的函数”。再看 @discussion 部分：

See dispatch_async() for details.

哈哈，让我们到 dispatch_async() 去找更多的细节。接下来再看 @param context：

The application-defined context parameter to pass to the function.

要传递给函数的应用程序定义的上下文参数，也就是我们自定义的要传给执行函数的参数。最后看看 @param work ：

The application-defined function to invoke on the target queue. The first parameter passed to this function is the context provided to dispatch_async_f().
 
The result of passing NULL in this parameter is undefined.

要在目标队列上调用的自定义函数。传入到函数的第一个参数是提供给 dispatch_async_f() 函数的上下文参数。如果这个参数传NULL，那么结果是未定义的。

综上所述，我们其实已经可以看得出这些函数是干嘛的了。就是以函数执行具体任务的方式替代GCD以Block执行具体任务的方式。

下面我 给出两个具体代码案例，仅供参考！

- (void)viewDidLoad {
    [super viewDidLoad];
    
    dispatch_async_f(dispatch_get_main_queue(), (void *)123, (void *)async_func);
    
    dispatch_sync_f(dispatch_get_global_queue(0, DISPATCH_QUEUE_PRIORITY_DEFAULT), (void *)456, (void *)sync_func);
}

void async_func(int param) {
    printf("我是给async添加的C语言函数：%d",param);
    printf("\n");
}

void sync_func(int param) {
    printf("我是给sync添加的C语言函数：%d",param);
    printf("\n");
}

控制台：

ok，写这篇博客就是为了记录一下当初的好奇心，为什么会有很多CGD函数有个后面带 _f 的版本，算是一篇笔记了。