方案一：

#include <time.h>
#ifdef WIN32
#   include <windows.h>
#else
#   include <sys/time.h>
#endif
#ifdef WIN32
int
gettimeofday(struct timeval *tp, void *tzp)
{
    time_t clock;
    struct tm tm;
    SYSTEMTIME wtm;
    GetLocalTime(&wtm);
    tm.tm_year     = wtm.wYear - 1900;
    tm.tm_mon     = wtm.wMonth - 1;
    tm.tm_mday     = wtm.wDay;
    tm.tm_hour     = wtm.wHour;
    tm.tm_min     = wtm.wMinute;
    tm.tm_sec     = wtm.wSecond;
    tm. tm_isdst    = -1;
    clock = mktime(&tm);
    tp->tv_sec = clock;
    tp->tv_usec = wtm.wMilliseconds * 1000;
    return (0);
}
#endif
 

方案二：

gettimeofday的使用

//copy from muduo
Timestamp Timestamp::now()
{struct timeval tv;gettimeofday(&tv, NULL);//返回1970年至今的秒+微秒int64_t seconds = tv.tv_sec;return Timestamp(seconds * kMicroSecondsPerSecond + tv.tv_usec);
}

gettimeofday要求传入一个timeval和一个时区。因为存在微秒数，显然它比 time_t now = ::time(NULL)更精确。

但是这个函数是linux下的。所以我们需要一个跨平台的实现。

以下是一个实现，使用c++的chrono库。

#include <chrono>
int gettimeofday(struct timeval *__restrict __tv, __timezone_ptr_t __tz)
{
auto now = std::chrono::system_clock::now();
auto now_ticks = std::chrono::duration_cast<std::chrono::microseconds>(now.time_since_epoch());//
__tv->tv_sec = (long)now_ticks.count() / 1000000;
__tv->tv_usec = (long)now_ticks.count() % 1000000;
return 0;
}

now.time_since_epoch()返回的duration比较奇怪,需要转化成微秒。其实主要是windows没有这个函数，那么我们实现它。

//copy from evpp
#ifdef WIN32
int gettimeofday(struct timeval* tp, void* tzp) {uint64_t  intervals;FILETIME  ft;GetSystemTimeAsFileTime(&ft);/** A file time is a 64-bit value that represents the number* of 100-nanosecond intervals that have elapsed since* January 1, 1601 12:00 A.M. UTC.** Between January 1, 1970 (Epoch) and January 1, 1601 there were* 134744 days,* 11644473600 seconds or* 11644473600,000,000,0 100-nanosecond intervals.** See also MSKB Q167296.*/intervals = ((uint64_t)ft.dwHighDateTime << 32) | ft.dwLowDateTime;intervals -= 116444736000000000;tp->tv_sec = (long)(intervals / 10,000,000);tp->tv_usec = (long)((intervals % 10000000) / 10);return (0);
}