0ull
// 0uLL：unsigned long long 类型的0
// ~：“二进制 按位 否 运算符”
// >>：右移运算符
// <<: 左移运算符
/*!
* @typedef dispatch_time_t
*
* @abstract
* A somewhat abstract representation of time; where zero means "now" and
* DISPATCH_TIME_FOREVER means "infinity" and every value in between is an
* opaque encoding.
*/
typedef uint64_t dispatch_time_t;
#define DISPATCH_TIME_NOW (0ull)
#define DISPATCH_TIME_FOREVER (~0ull)