C++ time statistics

1、clock_gettime

struct timespec nowtime;
clock_gettime(CLOCK_MONOTONIC,&nowtime);
long long t_start = (long long)((nowtime.tv_sec*1000000000ULL + nowtime.tv_nsec)/1000ULL);//μs
/*
 ..................... process code ....................
*/
clock_gettime(CLOCK_MONOTONIC,&nowtime);
long long t_end = (long long)((nowtime.tv_sec*1000000000ULL + nowtime.tv_nsec)/1000ULL);//μs
std::cout<<"time == "<<t_end - t_start<<std::endl;

2、std::chrono::system_clock

auto t1 =  std::chrono::system_clock::now();
/*
 ..................... process code ....................
*/
auto t2 = std::chrono::system_clock::now();
std::cout << "t5 used time = " << std::chrono::duration_cast<std::chrono::milliseconds>(t5 - t4).count() << std::endl;

3、clock()

Inaccurate, not recommended

time_t t1 = clock()