mktime显示输出不一致

在尝试编写比给定时间少24小时的代码时， mktime()显示输出不一致。 我计算它类似于： current_time(GMT) - 86400 ，它应该返回正确的值。 我们需要做的就是根据输入时间计算; 我们使用mktime()来改变时间并获得GMT时间，然后进行常规计算。 我在下面提供了我的代码。

 #include  #include  int main() { time_t currentTime, tempTime; struct tm *localTime; time(&currentTime); //localTime = localtime(&currentTime); localTime = gmtime(&currentTime); //get the time in GMT as we are in PDT printf("Time %2d:%02d\n", (localTime->tm_hour)%24, localTime->tm_min); localTime->tm_hour = 19; // Set the time to 19:00 GMT localTime->tm_min = 0; localTime->tm_sec = 0; tempTime = mktime(localTime); //tempTime = mktime(localTime) - timezone; printf("Current time is %ld and day before time is %ld\n", currentTime, (currentTime - 86400)); printf("Current timezone is %ld \n", timezone); printf("New time is %ld and day before time is %ld\n",tempTime, (tempTime - 86400)); } 

但是当我们检查输出时，在调用mktime()之后，它返回错误。 以下是上述程序的输出。

 $ ./a.out Time 11:51 Current time is 1341229916 and day before time is 1341143516 New time is 1341284400 and day before time is 1341198000 $ ./print_gmt 1341229916 Mon Jul 2 11:51:56 2012 $ ./print_gmt 1341143516 Sun Jul 1 11:51:56 2012 $ ./print_gmt 1341284400 Tue Jul 3 03:00:00 2012 $ ./print_gmt 1341198000 Mon Jul 2 03:00:00 2012 $ date Mon Jul 2 04:52:46 PDT 2012 

现在，如果我们取消注释减去时区的行（在time.h中存在），那么输出就是预期的。 以下是上述程序中时区的值

 $ ./a.out . . . Current timezone is 28800 . . . 

那么为什么mktime()存在这种不一致的行为，尽管手册页没有提到时区的这种调整。 在进行此类转换时，我们是否还缺少某些内容？

提前致谢。

Answer 1

我认为问题在于：

The mktime() function shall convert the broken-down time, expressed as local time, in the structure pointed to by timeptr, into a time since the Epoch value...

注意local time这个词。 C标准在mktime()的描述中也有它们：

The mktime function converts the broken-down time, expressed as local time, in the structure pointed to by timeptr into a calendar time value with the same encoding as that of the values returned by the time function.

另一方面， gmtime()以GMT / UTC而不是您的时区产生时间：

The gmtime() function shall convert the time in seconds since the Epoch pointed to by timer into a broken-down time, expressed as Coordinated Universal Time (UTC).

编辑 ：如果您只想要上一个GMT / UTC日的19:00，您可以这样做：

 #include  #include  int main(void) { time_t currentTime; struct tm *brokenDownTime; time(&currentTime); // get the time in GMT as we are in PDT brokenDownTime = gmtime(&currentTime); printf("Current Time (GMT): %2d:%02d\n" " seconds since Epoch: %ld\n", brokenDownTime->tm_hour, brokenDownTime->tm_min, (long)currentTime); // "Unwind" time to 0:00:00 (assuming time_t is an integer): currentTime /= 24 * (time_t)3600; currentTime *= 24 * (time_t)3600; brokenDownTime = gmtime(&currentTime); printf("Time at the beginning of the current GMT day: %2d:%02d\n" " seconds since Epoch: %ld\n", brokenDownTime->tm_hour, brokenDownTime->tm_min, (long)currentTime); // Add 19 hours: currentTime += 19 * (time_t)3600; brokenDownTime = gmtime(&currentTime); printf("Time at 19:00:00 of the current GMT day: %2d:%02d\n" " seconds since Epoch: %ld\n", brokenDownTime->tm_hour, brokenDownTime->tm_min, (long)currentTime); // Subtract 1 day: currentTime -= 24 * (time_t)3600; brokenDownTime = gmtime(&currentTime); printf("Time at 19:00:00 of the previous GMT day: %2d:%02d\n" " seconds since Epoch: %ld\n", brokenDownTime->tm_hour, brokenDownTime->tm_min, (long)currentTime); return 0; } 

输出：

 Current Time (GMT): 13:23 seconds since Epoch: 1341235429 Time at the beginning of the current GMT day: 0:00 seconds since Epoch: 1341187200 Time at 19:00:00 of the current GMT day: 19:00 seconds since Epoch: 1341255600 Time at 19:00:00 of the previous GMT day: 19:00 seconds since Epoch: 1341169200 

Answer 2

mktime(3)使用当前时区执行转换。 这在标准中有点隐秘写入：

应将本地时区信息设置为mktime()称为tzset() 。

Answer 3

mktime规范声明其参数表示“故障时间， 表示为本地时间 ”。 所以你传递给mktime的任何东西都必须在当地时间，而不是GMT。 您的变量localTime在其名称中具有local ，但实际上是从gmtime ，因此其结果描述了GMT中的当前时间。 将其传递给mktime是不一致的。

Answer 4

您没有说明您是在使用POSIX（类似）目标还是某些奇怪的系统，但在前一种情况下，您可以使用POSIX公式将故障时间转换为time_t ：

 tm_sec + tm_min*60 + tm_hour*3600 + tm_yday*86400 + (tm_year-70)*31536000 + ((tm_year-69)/4)*86400 - ((tm_year-1)/100)*86400 + ((tm_year+299)/400)*86400 

资料来源： http ： //pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap04.html#tag_04_15

Answer 5

在某些系统上还有一个timegm函数，它应该做正确的事情。
我刚刚检查了Linux并且它产生了正确的值（例如timegm（gmtime（time））== time）。

以下是在Windows中使用它的方法。
这是一个类似的答案 。