一、time函数
#include
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
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。
下面是从tcpdump源代码中的一个函数，用来将timeval转化成字符串。
void ts_print(register const struct timeval *tvp)
{
register int s;
struct tm *tm;
time_t Time;
static unsigned b_sec;
static unsigned b_usec;
switch(tflag) {
case 1: /* Default */
s = (tvp->tv_sec + thiszone) % 86400;
(void)printf("%02d:%02d:%02d.%06u ",
s / 3600, (s % 3600) / 60, s % 60,
(unsigned)tvp->tv_usec);
break;
case -1: /* Unix timeval style */
(void)printf("%u.%06u ",
(unsigned)tvp->tv_sec,
(unsigned)tvp->tv_usec);
break;
case -2:
if (b_sec == 0) {
printf("000000 ");
} else {
int d_usec = tvp->tv_usec - b_usec;
int d_sec = tvp->tv_sec - b_sec;
while (d_usec < 0) {
d_usec += 1000000;
d_sec--;
}
if (d_sec)
printf("%d. ", d_sec);
printf("%06d ", d_usec);
}
b_sec = tvp->tv_sec;
b_usec = tvp->tv_usec;
break;
case -3: /* Default + Date*/
s = (tvp->tv_sec + thiszone) % 86400;
Time = (tvp->tv_sec + thiszone) - s;
tm = gmtime (&Time);
if (!tm)
printf("Date fail ");
else
printf("%04d-%02d-%02d ",
tm->tm_year+1900, tm->tm_mon+1, tm->tm_mday);
printf("%02d:%02d:%02d.%06u ",
s / 3600, (s % 3600) / 60, s % 60, (unsigned)tvp->tv_usec);
break;
}
}
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 );
------------------------------ 华丽的分割线 ------------------------------------
关于scanf的返回值
Both scanf and wscanf return the number of fields successfully converted
and assigned; the return value does not include fields that were read but
not assigned. A return value of 0 indicates that no fields were assigned.
The return value is EOF for an error or if the end-of-file character or the
end-of-string character is nocountered in the first attempt to read a character.
如：scanf("%d%d", &a, &b);
如果a和b都被成功读入，那么scanf的返回值就是2
如果只有a被成功读入，返回值为1
如果a和b都未被成功读入，返回值为0
如果遇到错误或遇到end of file，返回值为EOF。
void main()
{
int a;
int b;
int c;
int x;
printf("请输入三个整数：");
x=scanf("%d%d%d",&a,&b,&c);
printf("%d\n%d\n",a,x);
}
输入三个整数：5 6 7则x的值为3；
输入5 6 d（即给c 赋值不正确）则x的值为2；
输入5 t d（即给b和c 赋值不正确）则x的值为1；
scanf()的返回值对我们来说也很有用的，例如可使用if(scanf("%d,%d",&a,&b)＝＝2)这样语句来判断是否正确的给所有的变量赋值了，正确的话才能使用这个变量与运算，这样才能提高我们代码的安全性。
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