Linux时间操作(time、gettimeofday)

一、time函数   

#include <time.h>
time_t time(time_t *calptr);

返回距计算机元年的秒数
一旦取得这种以秒计的很大的时间值后，通常要调用另一个时间函数将其变换为人们可读的时间和日期

#include     
//calendar time into a broken-down time expressed as UTC
struct tm *gmtime(const time_t *calptr);

//converts the calendar time to the local time, taking into account the local time zone and
//daylight saving time flag
struct tm *localtime(const time_t *calptr);

//converts it into a time_t value
time_t mktime(struct tm *tmptr);

  struct tm {        /* a broken-down time */
     int  tm_sec;     /* seconds after the minute: [0 - 60] */
     int  tm_min;     /* minutes after the hour: [0 - 59] */
     int  tm_hour;    /* hours after midnight: [0 - 23] */
     int  tm_mday;    /* day of the month: [1 - 31] */
     int  tm_mon;     /* months since January: [0 - 11] */
     int  tm_year;    /* years since 1900 */
     int  tm_wday;    /* days since Sunday: [0 - 6] */
     int  tm_yday;    /* days since January 1: [0 - 365] */
     int  tm_isdst;   /* daylight saving time flag: <0, 0, >0 */
   };  

char *asctime(const struct tm *tmptr);
char *ctime(const time_t *calptr);

asctime()和ctime()函数产生形式的26字节字符串，这与date命令的系统默认输出形式类似：    Tue Feb 10 18:27:38 2004\n\0

二、gettimeofday函数得到更精确的时间

#include <sys/time.h>
int gettimeofday(struct timeval *restrict tp, void *restrict tzp);
第二个形参是基于平台实现的，使用的时候最好用NULL

struct timeval{
    time_t tv_sec;        /*** second ***/
    susecond_t tv_usec;    /*** microsecond 微妙***/
}

1秒＝1000毫秒，
1毫秒＝1000微秒，
1微妙＝1000纳秒，
1纳秒＝1000皮秒。
秒用s表现,毫秒用ms,微秒用μs表示，纳秒用ns表示，皮秒用ps表示。

三、内核时间 

内核有两个重要的全局变量:
unsigned long jiffies;
timeval xtime ;

jiffies 是记录着从电脑开机到现在总共的"时钟中断"的次数。
文件linux-2.6.24/kernel/timer.c  
void do_timer(unsigned long ticks)
{
    jiffies_64 += ticks;
    update_times(ticks);
}


xtime 是从cmos电路或rtc芯片中取得的时间，一般是从某一历史时刻开始到现在的时间。
这个就是所谓的"墙上时钟walltimer"，通过它可计算得出操作系统需要的日期时间，它的精确度是微秒。

xtime第一次赋值是在系统启动时调用timekeeping_init或time_init进行的
再调用read_persistent_clock进一步调用get_rtc_time得到的

PS：在/proc/uptime里面的两个数字分别表示:   
the uptime of the system(seconds),
and the amount of time spent in idle process(seconds).   

四、代码示例

“UTC时间字符串”与 “time函数返回值”互换

int64_t TimeToUTC(char *time)
{
    struct tm temp1;
    int rc;
    int year;
    int mon;
    int day;
    int hour;
    int min;
    int sec;

    rc = sscanf(time, "%4d-%2d-%2d %2d:%2d:%2d",
                    &(temp1.tm_year),
                    &(temp1.tm_mon),
                    &(temp1.tm_mday),
                    &(temp1.tm_hour),
                    &(temp1.tm_min),
                    &(temp1.tm_sec));
    sscanf(time, "%4d-%2d-%2d %2d:%2d:%2d",
                    &year,
                    &mon,
                    &day,
                    &hour,
                    &min,
                    &sec);
    if((rc<6)
        || (temp1.tm_year<1900) || (temp1.tm_year>2100)
        || (temp1.tm_mon<1) || (temp1.tm_mon>12)
        || (temp1.tm_mday<1) || (temp1.tm_mday>31)
        || (temp1.tm_hour<0) || (temp1.tm_hour>23)
        || (temp1.tm_min<0) || (temp1.tm_min>59)
        || (temp1.tm_sec<0) || (temp1.tm_sec>59))
    {
        return -1;
    }

    temp1.tm_mon -= 1;
    temp1.tm_year -= 1900;
    struct tm temp3;
    time_t temp2 = mktime(&temp1);
    if (temp2 == -1){
        return -1;
    }
    else{
        localtime_r(&temp2, &temp3);
        if (!((mon == (temp3.tm_mon+1))
            && (day == temp3.tm_mday)
            &&(year == (temp3.tm_year+1900)))){
            return -1;
        }
        else{
                return temp2*1000000LL;
        }
    }
}

void UTCToTime(int64_t utc, char * clock)
{
    struct tm temp1;
    time_t sec;
    float msec;
    int rc;
    sec = utc/1000000;
    msec = utc/1000000.0-sec;
    localtime_r(&sec, &temp1);
    temp1.tm_year += 1900;
    temp1.tm_mon += 1;
    rc = sprintf(clock, "%04d-%02d-%02d %02d:%02d:%02d",
                    temp1.tm_year,
                    temp1.tm_mon,
                    temp1.tm_mday,
                    temp1.tm_hour,
                    temp1.tm_min,
                    temp1.tm_sec);
    clock[rc] = '\0';
}

生成Date

static char * g_weekstr[7] = { "Sun", "Mon", "Tue", "Wed", "Thu", "Fri", "Sat" };
static char * g_monthstr[12] = { "Jan", "Feb", "Mar", "Apr", "May", "Jun",
                "Jul", "Aug", "Sep", "Oct", "Nov", "Dec" };

/*generate the current date string*/
time_t now = time(NULL);
struct tm tt;
gmtime_r( &now, &tt );
char timebuf[64];
snprintf( timebuf, sizeof(timebuf),
        "%s, %02d %s %d %02d:%02d:%02d GMT",
        g_weekstr[tt.tm_wday], tt.tm_mday,
        g_monthstr[tt.tm_mon], tt.tm_year + 1900,
        tt.tm_hour, tt.tm_min, tt.tm_sec );