TLPI-Chapter 10 时间

日历时间Calendat Time

UNIX系统内部对时间的表示均是以自1970年1月1日的零点以来的秒数来度量。日历时间存储与类型time_t的变量中，此类型是由SUSv3定义的整数类型。
系统调用gettimeofday(),可于tv指向的缓冲区中返回日历时间。

#include <sys/time.h>int gettimeofday(struct timeval *tv, struct timezone *tz);          Returns 0 on success, or –1 on error参数tv是指向如下数据结构的一个指针：struct timeval {    __kernel_time_t        tv_sec;        /* seconds */    __kernel_suseconds_t    tv_usec;    /* microseconds */};

在现代X86-32系统上,gettimeofday()可以提供微妙级的准确度。
time函数基于gettimeofday实现，time精度秒级，gettimeofday可以达到微妙。

#include <time.h>time_t time(time_t *timep);        Returns number of seconds since the Epoch,or (time_t) –1 on error

时间转换函数

将time_t转换为可打印格式

#include <time.h>char *ctime(const time_t *timep)

ctime返回的字符串经由静态分配，下一次对ctime()的调用会将其覆盖。
函数gmtime()和localtime()可将time_t类型转换为一个所谓的分解时间。分解时间置于一个经由静态分配的结构中，其地址则作为函数结果返回。
函数gmtime()能够把日历时间转换为一个对应于UTC的分解时间。
函数localtime()需要考虑时区和夏令时设置，返回对应于系统本地时间的一个分解时间。

#include<time.h>struct tm *localtime(const time_t * timep);struct tm *gmtime(const time_t *timep);

函数mktime()将一个本地时区的分解时间转换为time_t，并将结果返回。
函数asctime()则从分解时间转换为打印时间。
char *asctime(const struct tm *timeptr);
在参数tm中提取一个指向分解时间结构的指针,asctime()则会返回一指针，指向经由静态分配的字符串，内含时间，格式则与ctime()相同。

/*************************************************************************\*                  Copyright (C) Michael Kerrisk, 2015.                   **                                                                         ** This program is free software. You may use, modify, and redistribute it ** under the terms of the GNU General Public License as published by the   ** Free Software Foundation, either version 3 or (at your option) any      ** later version. This program is distributed without any warranty.  See   ** the file COPYING.gpl-v3 for details.                                    *\*************************************************************************//* Listing 10-1 *//* calendar_time.c   Demonstrate the use of functions for working with calendar time.   This program retrieves the current time and displays it in various forms.*/#include <locale.h>#include <time.h>#include <sys/time.h>#include "tlpi_hdr.h"#define SECONDS_IN_TROPICAL_YEAR (365.24219 * 24 * 60 * 60)intmain(int argc, char *argv[]){    time_t t;    struct tm *gmp, *locp;    struct tm gm, loc;    struct timeval tv;    /* Retrieve time, convert and display it in various forms */    t = time(NULL);    printf("Seconds since the Epoch (1 Jan 1970): %ld", (long) t);    printf(" (about %6.3f years)\n", t / SECONDS_IN_TROPICAL_YEAR);    if (gettimeofday(&tv, NULL) == -1)        errExit("gettimeofday");    printf("  gettimeofday() returned %ld secs, %ld microsecs\n",            (long) tv.tv_sec, (long) tv.tv_usec);    gmp = gmtime(&t);    if (gmp == NULL)        errExit("gmtime");    gm = *gmp;          /* Save local copy, since *gmp may be modified                           by asctime() or gmtime() */    printf("Broken down by gmtime():\n");    printf("  year=%d mon=%d mday=%d hour=%d min=%d sec=%d ", gm.tm_year,            gm.tm_mon, gm.tm_mday, gm.tm_hour, gm.tm_min, gm.tm_sec);    printf("wday=%d yday=%d isdst=%d\n", gm.tm_wday, gm.tm_yday, gm.tm_isdst);    /* The TZ environment variable will affect localtime().       Try, for example:                TZ=Pacific/Auckland calendar_time    */    locp = localtime(&t);    if (locp == NULL)        errExit("localtime");    loc = *locp;        /* Save local copy */    printf("Broken down by localtime():\n");    printf("  year=%d mon=%d mday=%d hour=%d min=%d sec=%d ",            loc.tm_year, loc.tm_mon, loc.tm_mday,            loc.tm_hour, loc.tm_min, loc.tm_sec);    printf("wday=%d yday=%d isdst=%d\n\n",            loc.tm_wday, loc.tm_yday, loc.tm_isdst);    printf("asctime() formats the gmtime() value as: %s", asctime(&gm));    printf("ctime() formats the time() value as:     %s", ctime(&t));    printf("mktime() of gmtime() value:    %ld secs\n", (long) mktime(&gm));    printf("mktime() of localtime() value: %ld secs\n", (long) mktime(&loc));    exit(EXIT_SUCCESS);}

程序输出结果：

root@ubuntu:~/tlpi-dist/time# ./calendar_time Seconds since the Epoch (1 Jan 1970): 1508659096 (about 47.808 years)  gettimeofday() returned 1508659096 secs, 249229 microsecsBroken down by gmtime():  year=117 mon=9 mday=22 hour=7 min=58 sec=16 wday=0 yday=294 isdst=0Broken down by localtime():  year=117 mon=9 mday=22 hour=15 min=58 sec=16 wday=0 yday=294 isdst=0asctime() formats the gmtime() value as: Sun Oct 22 07:58:16 2017ctime() formats the time() value as:     Sun Oct 22 15:58:16 2017mktime() of gmtime() value:    1508630296 secsmktime() of localtime() value: 1508659096 secsroot@ubuntu:~/tlpi-dist/time# 

程序说明：
函数localtime()需要考虑时区和夏令时设置，因此打印的是本地时间。比gmtime()返回的时间晚整8个小时。
函数gettimeofday确实可以精确到microsecs级别。
1508659096-1508630296=8*3600
函数strftime():
当把一个分解时间转换成打印格式时，函数strftime()可以提供更为精确的控制。
函数currTime()返回当前时间。

/*************************************************************************\*                  Copyright (C) Michael Kerrisk, 2015.                   **                                                                         ** This program is free software. You may use, modify, and redistribute it ** under the terms of the GNU Lesser General Public License as published   ** by the Free Software Foundation, either version 3 or (at your option)   ** any later version. This program is distributed without any warranty.    ** See the files COPYING.lgpl-v3 and COPYING.gpl-v3 for details.           *\*************************************************************************//* Listing 10-2 *//* curr_time.c   Implement our currTime() function.*/#include <time.h>#include "curr_time.h"          /* Declares function defined here */#define BUF_SIZE 1000/* Return a string containing the current time formatted according to   the specification in 'format' (see strftime(3) for specifiers).   If 'format' is NULL, we use "%c" as a specifier (which gives the'   date and time as for ctime(3), but without the trailing newline).   Returns NULL on error. */char *currTime(const char *format){    static char buf[BUF_SIZE];  /* Nonreentrant */    time_t t;    size_t s;    struct tm *tm;    t = time(NULL);    tm = localtime(&t);    if (tm == NULL)        return NULL;    s = strftime(buf, BUF_SIZE, (format != NULL) ? format : "%c", tm);    return (s == 0) ? NULL : buf;}

函数strptime()是strftime()的逆向函数，将包含日期和时间的字符串转换成分解时间。
示例代码：

/*************************************************************************\*                  Copyright (C) Michael Kerrisk, 2015.                   **                                                                         ** This program is free software. You may use, modify, and redistribute it ** under the terms of the GNU General Public License as published by the   ** Free Software Foundation, either version 3 or (at your option) any      ** later version. This program is distributed without any warranty.  See   ** the file COPYING.gpl-v3 for details.                                    *\*************************************************************************//* Listing 10-3 *//* strtime.c   Demonstrate the use of strptime() and strftime().   Calls strptime() using the given "format" to process the "input-date+time".   The conversion is then reversed by calling strftime() with the given   "out-format" (or a default format if this argument is omitted).*/#if ! defined(__sun)#ifndef _XOPEN_SOURCE#define _XOPEN_SOURCE#endif#endif#include <time.h>#include <locale.h>#include "tlpi_hdr.h"#define SBUF_SIZE 1000intmain(int argc, char *argv[]){    struct tm tm;    char sbuf[SBUF_SIZE];    char *ofmt;    if (argc < 3 || strcmp(argv[1], "--help") == 0)        usageErr("%s input-date-time in-format [out-format]\n", argv[0]);    if (setlocale(LC_ALL, "") == NULL)        errExit("setlocale");   /* Use locale settings in conversions */    memset(&tm, 0, sizeof(struct tm));          /* Initialize 'tm' */    if (strptime(argv[1], argv[2], &tm) == NULL)        fatal("strptime");    tm.tm_isdst = -1;           /* Not set by strptime(); tells mktime()                                   to determine if DST is in effect */    printf("calendar time (seconds since Epoch): %ld\n", (long) mktime(&tm));    ofmt = (argc > 3) ? argv[3] : "%H:%M:%S %A, %d %B %Y %Z";    if (strftime(sbuf, SBUF_SIZE, ofmt, &tm) == 0)        fatal("strftime returned 0");    printf("strftime() yields: %s\n", sbuf);    exit(EXIT_SUCCESS);}

执行结果：

如果输入两个参数，则函数strftime使用”%H:%M:%S %A, %d %B %Y %Z”形式，如果输入三个参数，函数strftime则使用输入的格式。

时区

在目录/usr/share/zoneinfo中，每个文件包含一个特定国家或地区内时区制度的相关信息，且往往根据其所描述的时区来加以命名，诸如EST 美国东部 CET欧洲中部 等。
系统的本地时间由时区文件/etc/localtime定义，通常链接到/usr/share/zoneinfo下的一个文件。
设置时区影响ctime() localtime() mktime() strftime(),为了获取当前的时区设置，上述函数都会调用tzset(3),对如下三个全局变量进行初始化：
char *tzname[2], int daylight; long timezone;
函数tzset会首先检查环境变量TZ，如果尚未设置该变量，则使用/etc/localtime中定义的默认时区来初始化时区.
程序执行结果:

root@ubuntu:~/tlpi-book/time# ./show_time ctime() of time() value is:  Sun Oct 22 23:03:27 2017asctime() of local time is:  Sun Oct 22 23:03:27 2017strftime() of local time is: Sunday, 22 October 2017, 23:03:27 CSTroot@ubuntu:~/tlpi-book/time# root@ubuntu:~/tlpi-book/time# root@ubuntu:~/tlpi-book/time# TZ=":Pacific/Auckland" ./show_timectime() of time() value is:  Mon Oct 23 04:03:34 2017asctime() of local time is:  Mon Oct 23 04:03:34 2017strftime() of local time is: Monday, 23 October 2017, 04:03:34 NZDTroot@ubuntu:~/tlpi-book/time#