timegm跨平台 [英] timegm cross platform

问题描述

我正在使用Visual Studio c ++编译器（2010），但是该库具有ANSI C和POSIX库功能的不同实现。

I'm using Visual Studio c++ Compiler ( 2010 ), but the library has different implementation of ANSI C and POSIX libraries function.

ANSI C函数和Windows CRT实现之间有什么区别？例如 tzset（）和 _tzset（）或 setenv（）有什么区别 ans _setenv（）？似乎以相同的方式做同样的事情...

What is the difference between ANSI C function and Windows CRT implementation? For example what is the difference between tzset() and _tzset() or setenv() ans _setenv()? It seems the do the same thing in the same way...

我正在使用msvc（2010），我更喜欢Windows CRT实现吗？

I'm using msvc ( 2010 ), have I to prefer the Windows CRT Implementation?

编辑1

我想以可移植的方式转换以UTC表示的struct tm在 time_t 中，但是没有可移植的方法来执行此操作。我必须为不同的平台（Android，Linux，Windows，Windows CE）编写该函数。

Well I want convert in a portable way a struct tm expressed in UTC in a time_t, but there's no portable way to do that. I've to write the function for different platform (Android, Linux, Windows, Windows CE ).

我见过此stackoverflow帖子，其中使用 setenv ， getenv 和 tzset

I've seen this stackoverflow post that uses setenv, getenv and tzset

Edit2

不幸的是，经过一些测试，我发现 getenv（ TZ）在Windows上返回空指针。但是，为什么将UTC时间结构转换成 time_t 如此困难？

Unfortunately after some test I've discovered that getenv("TZ") returns a null pointer on windows. But why is so difficult transform a UTC time struct to a time_t?

编辑3

在Boost中，我在boost / chrono / io / time_point_io.hpp中发现了这段代码。希望这对我有帮助。

From Boost I discovered this fragment of code in boost/chrono/io/time_point_io.hpp. Hope this helps me.

inline int32_t is_leap(int32_t year)
{
  if(year % 400 == 0)
  return 1;
  if(year % 100 == 0)
  return 0;
  if(year % 4 == 0)
  return 1;
  return 0;
}
inline int32_t days_from_0(int32_t year)
{
  year--;
  return 365 * year + (year / 400) - (year/100) + (year / 4);
}
inline int32_t days_from_1970(int32_t year)
{
  static const int days_from_0_to_1970 = days_from_0(1970);
  return days_from_0(year) - days_from_0_to_1970;
}
inline int32_t days_from_1jan(int32_t year,int32_t month,int32_t day)
{
  static const int32_t days[2][12] =
  {
    { 0,31,59,90,120,151,181,212,243,273,304,334},
    { 0,31,60,91,121,152,182,213,244,274,305,335}
  };
  return days[is_leap(year)][month-1] + day - 1;
}

inline time_t internal_timegm(std::tm const *t)
{
  int year = t->tm_year + 1900;
  int month = t->tm_mon;
  if(month > 11)
  {
    year += month/12;
    month %= 12;
  }
  else if(month < 0)
  {
    int years_diff = (-month + 11)/12;
    year -= years_diff;
    month+=12 * years_diff;
  }
  month++;
  int day = t->tm_mday;
  int day_of_year = days_from_1jan(year,month,day);
  int days_since_epoch = days_from_1970(year) + day_of_year;

  time_t seconds_in_day = 3600 * 24;
  time_t result = seconds_in_day * days_since_epoch + 3600 * t->tm_hour + 60 * t->tm_min + t->tm_sec;

  return result;
}

推荐答案

我在Windows：

#define timegm _mkgmtime

与 _mkgmtime 相同。

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