时间函数

UNIX_API

今天学习了一些linux关于时间的函数，还有几个关于文件的时间函数，比如utimes，lutimes,futimes等，不在今天讨论的范围之内。

关于时间的函数，可以进行如下分类

• 返回值是时间字符串，并且是区分时区的，提供线程安全的函数"_r"，返回的结构写入到第二个参数的结构体中

char * ctime(time_t*); 
char* ctime_r(time_t*, char* buf);
char* asctime(struct tm*);
char* asctime(struct tm*, char* buf);

• 返回值是tm结构，里面包含了年月日，时分秒，天数，星期，这两个传递的参数和返回的参数一样，但是gmtime是不区分时区的，localtime是区分时区的，所以一般情况下不采用gmtime/gmtime_r

struct tm * = gmtime(time_t*);
struct tm* = gmtime_r(time_t*, struct tm* result);
struct tm* = localtime(time_t*);
struct tm* = localtime_r(time_t*, struct tm* result);

• 返回time_t时间戳，实际是long，那传递的参数肯定是tm结构的，不会是字符串类型的，是区分时区的，没有mktime_r这个函数，因为返回值就是一个整数。

time_t mktime(struct tm *);

• 返回timeval，是一个用结构体表示的时间戳。直接返回当前的时间和时区

int gettimeofday(struct timeval * __restrict, void * __restrict);

技巧性总结

综上所述，平时常用的就是

• 想获取字符串，根据传递的不同的参数选择使用ctime_r,asctime_r
• 想获取tm结构，就使用localtime_r，不推荐gmtime_r，因为后者不区分时区;
• 想获取time_t时间戳，则实用mktime
• 想获取当前时间的时间戳，则使用gettimeofday，通过参数返回结果，有timeval，timezone

下面是示例代码

#include <iostream>
#include <time.h>
#include <sys/time.h>
using namespace std;
/*
 * ctime(),
 */
int main()
{
    //ctime(const time_t* ),传入一个时间戳,返回一个时间的字符串.但是这个是分时区的，如果传入的
    //是0,那结果就是 Thu Jan  1 08:00:00 1970
    time_t a = 100;
    char * p = ctime(&a);
    cout << "ctime:" << endl;
    cout << "time is " << p << " a is " << a << endl;
    //char *
    //ctime_r(const time_t *clock, char *buf);提供和上面同样的功能,但是结果可以存储在用户提供的buf中
    //因为,返回的是一个志向static内存区域的指针,这个区域可能会被其他线程修改,这个buf起码要26个字符.
    char buf[26] = {0};
    char * p1 = ctime_r(&a, buf);
    cout << "time is " << buf << endl << endl;
    //gmtime(const time_t* ),传入时间戳,返回的是一个tm的结构,年份是距离1900年的时间
    //如果传入0的话,year就是70.
    struct tm* ret = gmtime(&a);
    cout << "gmtime: " << endl;
    cout << "year is " << ret->tm_year << endl;
    cout << "tm_sec is " << ret->tm_sec << endl;
    cout << "tm_wday is " << ret->tm_wday << endl;
    //同样,也有多线程的版本,gmtime(time_t *, tm* );
    struct tm result;
    gmtime_r(&a, &result);
    cout << "year is " << result.tm_year << endl;
    cout << "hour is " << result.tm_hour << endl;
    cout << "min is " << result.tm_min << endl;
    cout << "sec is " << result.tm_sec << endl<< endl;
    //localtime,目测localtime和gmtime用法是一样一样的.但是和gmtime不同的是:区分时区,gmtime返回的是0点,localtime返回的是8点.
    //所以一般情况下,我们使用的是localtime/localtime_r.
    ret = localtime(&a);
    cout << "localtime:" << endl;
    cout << "year is " << ret->tm_year << endl;
    cout << "tm_sec is " << ret->tm_sec << endl;
    cout << "tm_wday is " << ret->tm_wday << endl;
    //同样有多线程的版本
    ret = localtime_r(&a, &result);
    cout << "year is " << result.tm_year << endl;
    cout << "hour is " << result.tm_hour << endl;
    cout << "min is " << result.tm_min << endl;
    cout << "sec is " << result.tm_sec << endl<<endl;
    //asctime是传入的是tm的结构,根据tm结构中的数据,最终返回一个字符串,
    //同时提供多线程安全的版本,多传入一个buf,会写到这个buf中.
    result.tm_sec = 55;
    char * tmp = asctime(&result);
    cout << "asctime:" << endl;
    cout << "ret is " << tmp;
    cout << "year is " << result.tm_year << endl;
    cout << "hour is " << result.tm_hour << endl;
    cout << "min is " << result.tm_min << endl;
    cout << "sec is " << result.tm_sec << endl;
    char asctime_buf[26];
    result.tm_year = 80;
    tmp = asctime_r(&result, asctime_buf);
    cout << "ret is " << asctime_buf;
    cout << "year is " << result.tm_year << endl;
    cout << "hour is " << result.tm_hour << endl;
    cout << "min is " << result.tm_min << endl;
    cout << "sec is " << result.tm_sec << endl<<endl;
    //difftime顾名思义就是时间差,传入的是time_t类型的数据.
    time_t start = 1222;
    time_t end = 1111;
    double diffret = difftime(start,end);
    cout << "difftime:" << endl;
    cout << "diff is " << diffret << endl;
    //mktime顾名思义就是生成time_t 时间戳,这是穿进去的数字是根据自己当前的时区的.
    struct tm mktime_tm;
    mktime_tm.tm_year = 101;
    mktime_tm.tm_hour = 3;
    time_t mktime_ret = mktime(&mktime_tm);
    cout << "ret is " << mktime_ret << endl;
    struct tm* aaa = gmtime(&mktime_ret);
    cout << "hour is " << aaa->tm_hour << endl;
    aaa = localtime(&mktime_ret);
    cout << "hour is " << aaa->tm_hour << endl;
    //最特别的一个,gettimeofday,传递的参数都不同.timeval类型的参数
    //#include <sys/time.h>
    timeval timeofday_a;
    struct timezone timeofzone_a;
    gettimeofday(&timeofday_a, &timeofzone_a);
    cout << "timeofday is " << timeofday_a.tv_sec << endl;
    cout << "time zone is " << timeofzone_a.tz_dsttime << endl;
    cout << "time zone is " << timeofzone_a.tz_minuteswest << endl;
    //相同功能的函数就是time(time_t*);
    return 0;
}