关于时间的相关操作(3)

预备知识：

函数: ctime
　　功 能: 把日期和时间转换为字符串
　　用 法: char *ctime(const time_t *time);


NAME
       asctime,  ctime,  gmtime, localtime, mktime, asctime_r, ctime_r, gmtime_r, local-
       time_r - transform date and time to broken-down time or ASCII

SYNOPSIS
       #include

       char *asctime(const struct tm *tm);
       char *asctime_r(const struct tm *tm, char *buf);

       char *ctime(const time_t *timep);
       char *ctime_r(const time_t *timep, char *buf);

       struct tm *gmtime(const time_t *timep);
       struct tm *gmtime_r(const time_t *timep, struct tm *result);

       struct tm *localtime(const time_t *timep);
       struct tm *localtime_r(const time_t *timep, struct tm *result);

       time_t mktime(struct tm *tm);

DESCRIPTION
       The ctime(), gmtime() and localtime() functions all take an argument of data type
       time_t  which  represents  calendar  time.   When interpreted as an absolute time
       value, it represents the number of seconds elapsed since 00:00:00 on  January  1,
       1970, Coordinated Universal Time (UTC).

       The  asctime()  and mktime() functions both take an argument representing broken-
       down time which is a representation separated into year, month, day, etc.

       Broken-down time is stored in the structure tm which is defined  in    as
       follows:

              struct tm {
                  int tm_sec;         /* seconds */
                  int tm_min;         /* minutes */
                  int tm_hour;        /* hours */
                  int tm_mday;        /* day of the month */
                  int tm_mon;         /* month */
                  int tm_year;        /* year */
                  int tm_wday;        /* day of the week */
                  int tm_yday;        /* day in the year */
                  int tm_isdst;       /* daylight saving time */
              };
             



       The call ctime(t) is equivalent to asctime(localtime(t)).  It converts the calen-
       dar time t into a string of the form

              "Wed Jun 30 21:49:08 1993\n"
            
            

       The gmtime() function converts the calendar time timep to broken-down time repre-
       sentation, expressed in Coordinated Universal Time (UTC). It may return NULL when
       the  year  does not fit into an integer.  The return value points to a statically
       allocated struct which might be overwritten by subsequent calls  to  any  of  the
       date  and  time functions.  The gmtime_r() function does the same, but stores the
       data in a user-supplied struct.

       The localtime() function converts the calendar time timep to  broken-time  repre-
       sentation,  expressed relative to the user's specified time zone.    The function
       acts as if it called tzset(3) and sets the external variables tzname with  infor-
       mation  about the current time zone, timezone with the difference between Coordi-
       nated Universal Time (UTC) and local standard time in seconds, and daylight to  a
       non-zero value if daylight savings time rules apply during some part of the year.
       The return value points to a statically allocated struct which might be overwrit-
       ten by subsequent calls to any of the date and time functions.  The localtime_r()
       function does the same, but stores the data in a user-supplied  struct.  It  need
       not set tzname.

       The  asctime() function converts the broken-down time value tm into a string with
       the same format as ctime().  The return value points to  a  statically  allocated
       string which might be overwritten by subsequent calls to any of the date and time
       functions.  The asctime_r() function does the same, but stores the  string  in  a
       user-supplied buffer of length at least 26.

       The  mktime()  function converts a broken-down time structure, expressed as local
       time, to calendar time representation.  The function ignores the  specified  con-
       tents  of  the structure members tm_wday and tm_yday and recomputes them from the
       other information in the broken-down time structure.  If  structure  members  are
       outside  their legal interval, they will be normalized (so that, e.g., 40 October
       is changed into 9 November).  Calling mktime() also sets  the  external  variable
       tzname  with  information  about the current time zone.  If the specified broken-
       down time cannot be represented as  calendar  time  (seconds  since  the  epoch),
       mktime()  returns  a  value  of  (time_t)(-1)  and does not alter the tm_wday and
       tm_yday members of the broken-down time structure.

程序清单：
 

#include <stdio.h>
#include <time.h>

int main(int argc,char **argv)
{
    time_t tm;
    time(&tm);
    printf("Today's date and time:%sn",ctime(&tm));
    return 0;
}

编译并运行源程序：
obe-240 eagle/test> gcc -o time3 time3.c
obe-240 eagle/test> ./time3
Today's date and time:Mon Dec 13 19:20:12 2010