time ， ctime ， ftime，gettimeofday
表头文件
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定义函数
int settimeofday ( const struct timeval *tv,const struct timezone *tz);
函数说明
settimeofday()会把目前时间设成由tv所指的结构信息，当地时区信息则设成tz所指的结构 。详细的说明请参考gettimeofday() 。
注意 ， 只有root权限才能使用此函数修改时间 。
返回值
成功则返回0 ， 失败返回－1 ， 错误代码存于errno 。
错误代码
EPERM 并非由root权限调用settimeofday（），权限不够 。
EINVAL 时区或某个数据是不正确的，无法正确设置时间 。
time（取得目前的时间）
相关函数
ctime，ftime，gettimeofday
表头文件
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定义函数
time_t time(time_t *t);
函数说明
此函数会返回从公元1970年1月1日的UTC时间从0时0分0秒算起到现在所经过的秒数 。如果t 并非空指针的话 ，
此函数也会将返回值存到t指针所指的内存 。
返回值
成功则返回秒数，失败则返回((time_t)-1)值，错误原因存于errno中 。
范例
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mian()
{
int seconds= time((time_t*)NULL);
printf("%d\n",seconds);
}
Date and Time Functions: time.h
The header time.h declares types and functions for manipulating date and time. Some functions process local time,
which may differ from calendar time, for example because of time zone. clock_t and time_t are arithmetic types
representing times, and struct tm holds the components
of a calendar time:
int tm_sec;seconds after the minute (0,61)
int tm_min;minutes after the hour (0,59)
int tm_hour;hours since 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
tm_isdst is positive if Daylight Saving Time is in effect, zero if not, and negative if the information is not available.
clock_t clock(void)
clock returns the processor time used by the program since the beginning of execution, or -1 if unavailable.
clock()/CLK_PER_SEC is a time in
seconds.
time_t time(time_t *tp)
time returns the current calendar time or -1 if the time is not available. If tp is not NULL,
the return value is also assigned to *tp.
double difftime(time_t time2, time_t time1)
difftime returns time2-time1 expressed in seconds.
time_t mktime(struct tm *tp)
mktime converts the local time in the structure *tp into calendar time in the same representation used by time.
【信捷屏C语言日期函数 c语言 日期函数】The components will have values
in the ranges shown. mktime returns the calendar time or -1 if it cannot be represented.
The next four functions return pointers to static objects that may be overwritten by other calls.
char *asctime(const struct tm *tp)
asctime*tp into a string of the form
Sun Jan3 15:14:13 1988\n\0
char *ctime(const time_t *tp)
ctime converts the calendar time *tp to local time; it is equivalent to
asctime(localtime(tp))
struct tm *gmtime(const time_t *tp)
gmtime converts the calendar time *tp into Coordinated Universal Time (UTC). It returns NULL if UTC is not available.
The name gmtime has historical significance.
struct tm *localtime(const time_t *tp)
localtime converts the calendar time *tp into local time.
size_t strftime(char *s, size_t smax, const char *fmt, const struct tm *tp)
strftime formats date and time information from *tp into s according to fmt, which is analogous to a printf format.
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