time、gmtime和localtime，时间、格林威治和本地时间

1、首先从time()函数，返回一个从1970年1月1日 00:00:00到现在的秒数
time_t time(time_t * t); 当参数为NULL时直接返回秒数，当然也会将该值写入t指针指向的地址

2、gmtime():将time函数得到的秒数转换成一个UTC时间的结构体struct tm,该结构体包含什么请自行man

通过此函数gmtime()是0时区，把UTC时间转换成北京时间的话，需要在年数上加1900，月份上加1，小时数加上8

3、同类型的函数还有localtime():得到本地时间，该函数同gmtime函数唯一区别是，在转换小时数不需要加上8了。

localtime是将时区考虑在内了，转出的当前时区的时间。但是注意，有些嵌入式设备上被裁减过的系统，时区没有被设置好，导致二者转出来的时间都是0时区的。

#include 
#include 
#include 
#include 
#include 

#define SECONDS_IN_TROPICAL_YEAR (365.24219 * 24 * 60 * 60)

int main(int argc, char *argv[])
{
    time_t t;
    struct tm *gmp, *locp;
    struct tm gm, loc;
    struct timeval tv;
    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);
    //gettimeofday is a new system call for microsecond  perception time, tv.tv_sec = time(NULL)
    if (gettimeofday(&tv, NULL) == -1)
        perror("gettimeofday\n");
    printf("  gettimeofday() returned %ld secs, %ld microsecs\n",(long) tv.tv_sec, (long) tv.tv_usec);

    gmp = gmtime(&t);
    if (gmp == NULL)
       perror("gmtime\n");
    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);
    locp = localtime(&t);
    if (locp == NULL)
        perror("localtime\n");

    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);
}

Seconds since the Epoch (1 Jan 1970): 1694155147 (about 53.686 years)
  gettimeofday() returned 1694155147 secs, 138826 microsecs
Broken down by gmtime():
  year=123 mon=8 mday=8 hour=6 min=39 sec=7 wday=5 yday=250 isdst=0
Broken down by localtime():
  year=123 mon=8 mday=7 hour=23 min=39 sec=7 wday=4 yday=249 isdst=1

asctime() formats the gmtime() value as: Fri Sep  8 06:39:07 2023
ctime() formats the time() value as:     Thu Sep  7 23:39:07 2023
mktime() of gmtime() value:    1694183947 secs
mktime() of localtime() value: 1694155147 secs