如何在UNIX上使用C日期和时间函数？

Jon Skeet spoke of the complexity of programming dates and times at the 2009 DevDays in London.

Jon Skeet谈到了2009年伦敦DevDays编程日期和时间的复杂性。

Can you give me an introduction to the ANSI C date/time functions on UNIX and indicate some of the deeper issues I should also consider when using dates and times?

您能否介绍一下UNIX上的ANSI C日期/时间函数，并指出在使用日期和时间时我还应该考虑的一些更深层次的问题？

1 个解决方案

#1

Terminology

A date/time can be in two formats:

日期/时间可以采用两种格式：

• calendar time (a.k.a. simpletime) – time as an absolute value typically since some base time, often referred to as the Coordinated Universal Time
• 日历时间（a.k.a.simpletime） - 时间作为绝对值，通常是一些基准时间，通常称为协调世界时
• localtime (a.k.a. broken-down time) – a calendar time made up of components of year, month, day etc. which takes into account the local time zone including Daylight Saving Time if applicable.
• localtime（a.k.a.细分时间） - 由年，月，日等组成的日历时间，考虑到当地时区（包括夏令时）（如果适用）。

Data types

The date/time functions and types are declared in the time.h header file.

日期/时间函数和类型在time.h头文件中声明。

Time can be stored as a whole number or as an instance of a structure:

时间可以存储为整数或结构的实例：

• as a number using the time_t arithmetic type – to store calendar time as the number of seconds elapsed since the UNIX epoch January 1, 1970 00:00:00

  作为使用time_t算术类型的数字 - 将日历时间存储为自UNIX纪元1970年1月1日00:00:00以来经过的秒数

• using the structure timeval – to store calendar time as the number of seconds and nanoseconds elapsed since the UNIX epoch January 1, 1970 00:00:00

  使用结构timeval - 将日历时间存储为自UNIX纪元1970年1月1日00:00:00以来经过的秒数和纳秒数

• using the structure tm to store localtime, it contains attributes such as the following:

  使用结构tm存储localtime，它包含如下属性：

  tm_hour  
  tm_min  
  tm_isdst  
  

The tm_isdst attribute above is used to indicate Daylight Saving Time (DST). If the value is positive it is DST, if the value is 0 it is not DST.

上面的tm_isdst属性用于指示夏令时（DST）。如果值为正，则为DST，如果值为0，则不是DST。

Program to print the current Coordinated Universal Time

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

int main ( int argc, char *argv[] )
{
    time_t now;

    now = time ( NULL );

    printf ( "It’s %ld seconds since January 1, 1970 00:00:00", (long) now );

    return 0;
}

In the program above the function time reads the UNIX system time, subtracts that from January 1, 1970 00:00:00 (the UNIX epoch) and returns its result in seconds.

在上面的程序中，函数时间读取UNIX系统时间，从1970年1月1日00:00:00（UNIX纪元）中减去它，并以秒为单位返回其结果。

Program to print the current local time

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

int main ( int argc, char *argv[] )
{
    time_t now;
    struct tm *lcltime;

    now = time ( NULL );
    lcltime = localtime ( &now );

    printf ( "The time is %d:%d\n", lcltime->tm_hour, lcltime->tm_min );

    return 0;
}

In the program above the function localtime converts the elapsed time in seconds from the UNIX epoch into the broken-down time. localtime reads the UNIX environment TZ (through a call to the tzset function) to return the time relative to the timezone and to set the tm_isdst attribute.

在上面的程序中，函数localtime将从UNIX纪元到耗尽时间的经过时间（以秒为单位）转换。 localtime读取UNIX环境TZ（通过调用tzset函数）返回相对于时区的时间并设置tm_isdst属性。

A typical setting of the TZ variable in UNIX (using bash) would be as follows:

UNIX中TZ变量的典型设置（使用bash）如下：

export TZ=GMT

or

要么

export TZ=US/Eastern

Program to print the current formatted Greenwich Mean Time

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

int main ( int argc, char *argv[] )
{
    time_t now;
    struct tm *gmt;
    char formatted_gmt [50];

    now = time ( NULL );
    gmt = gmtime ( &now );

    strftime ( formatted_gmt, sizeof(formatted_gmt), "%I:%M %p", gmt );
    printf ( "The time is %s\n", formatted_gmt );

    return 0;
}

In the program above the function strftime provides specialised formatting of dates.

在上面的程序中，函数strftime提供了日期的专门格式。

Other issues to consider

• Leap seconds
• 闰秒
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