/*!
 * @function dispatch_semaphore_wait
 *
 * @abstract
 * Wait (decrement) for a semaphore.
 *
 * @discussion
 * Decrement the counting semaphore. If the resulting value is less than zero,
 * this function waits for a signal to occur before returning.
 *
 * @param dsema
 * The semaphore. The result of passing NULL in this parameter is undefined.
 *
 * @param timeout
 * When to timeout (see dispatch_time). As a convenience, there are the
 * DISPATCH_TIME_NOW and DISPATCH_TIME_FOREVER constants.
 *
 * @result
 * Returns zero on success, or non-zero if the timeout occurred.
 */
intptr_t dispatch_semaphore_wait(dispatch_semaphore_t dsema, dispatch_time_t timeout);

操作：
1、对信号量进行-1
2、判断当前信号量是否小于0，注意是小于0
3、返回0，则不阻塞，非0则阻塞当前线程（超时可继续执行）
作用：
1、方法对信号量-1
2、对信号量-1操作之后，如果信号量小于0，则等待，直到信号量大于等于0