问mktime显示不一致的输出
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Stack Overflow用户

提问于 2012-07-02 12:09:35

回答 5查看 2.9K关注 0票数 1

当尝试编写比给定时间少返回24小时的代码时，mktime()显示不一致的输出。我的计算方法类似于：current_time(GMT) - 86400，它应该返回正确的值。我们所要做的就是根据输入的时间进行计算；我们使用mktime()来改变时间，得到格林尼治标准时间，然后进行常规的计算。我在下面包含了我的代码。

#include <stdio.h>
#include <time.h>

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()之后返回的结果是不正确的。下面是上述程序的输出。

$ ./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中出现)，那么输出就是预期的。下面是上述程序中时区的值

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

那么，为什么mktime()会有如此不一致的行为，尽管手册页并没有提到时区的这种调整。在进行这样的转换时，我们是否遗漏了什么？

提前谢谢。

time

mktime

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回答 5

Stack Overflow用户

回答已采纳

发布于 2012-07-02 12:35:02

我认为问题出在这里：

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...

注意单词local time。C标准在对mktime()的描述中也包含了它们

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()以格林尼治标准时间/协调世界时生成时间，而不是在您的时区：

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).

EDIT：如果您只想要前一天的19:00，您可以这样做：

#include <stdio.h>
#include <time.h>

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

票数 2

Stack Overflow用户

发布于 2012-07-02 14:58:56

您没有说明是在POSIX(-like)目标上工作，还是在某个奇怪的系统上工作，但是在前一种情况下，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