写数据库作业，需要获取纳秒级别的时间，但一开始以为直接 chrono 或者 clock_gettime 就可以，但求出来但全部是微妙级别的时间，和纳秒差了 3 个数量级...

这玩意就，坑了我好久

解决方案如下 https://stackoverflow.com/questions/43451565/store-timestamps-in-nanoseconds-c

#ifdef __MACH__
#include <mach/clock.h>
#include <mach/mach.h>
#endif

inline long long now() {
    timespec timestamp;
#ifdef __MACH__ // OS X does not have clock_gettime, use clock_get_time
    clock_serv_t cclock;
    mach_timespec_t mts;
    host_get_clock_service(mach_host_self(), CALENDAR_CLOCK, &cclock);
    clock_get_time(cclock, &mts);
    mach_port_deallocate(mach_task_self(), cclock);
    timestamp.tv_sec = mts.tv_sec;
    timestamp.tv_nsec = mts.tv_nsec;
#else
    clock_gettime(CLOCK_REALTIME, &timestamp);
#endif
    return (long long)timestamp.tv_sec * 1000000000LL + timestamp.tv_nsec;
}