C/C++ localtime

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C/C++ localtime

  • #include <time.h>

參數

  • timer - pointer to a time_t object to convert
  • buf - pointer to a struct tm object to store the result 
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    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 */
    };

    Return value

  • 1)
    • pointer to a static internal tm object on success
    • or null pointer otherwise. The structure may be shared between gmtime, localtime, and ctime and may be overwritten on each invocation.
  • 2-3)
    • copy of the buf pointer
    • or null pointer on error (which may be a runtime constraint violation or a failure to convert the specified time to local calendar time)

      usage

  1. Converts given time since epoch (a time_t value pointed to by timer) into calendar time, expressed in local time, in the struct tm format. The result is stored in static storage and a pointer to that static storage is returned.
  2. Same as (1), except that the function uses user-provided storage buf for the result.
  3. Same as (1), except that the function uses user-provided storage buf for the result and that the following errors are detected at runtime and call the currently installed constraint handler function:
    • timer or buf is a null pointer

localtime

  • struct tm *localtime ( const time_t *timer );
  • localtime用来获取系统时间，精度为秒

localtime_r

  • struct tm *localtime_r(const time_t *timep, struct tm *result);
  • localtime_r也是用来获取系统时间，运行于linux平台下

reference

localtime_s

  • struct tm *localtime_s( const time_t *restrict timer, struct tm *restrict buf );
  • localtime_s也是用来获取系统时间，运行于windows平台下，与localtime_r只有参数顺序不一样

e.g.

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time_t t = time(NULL);
printf("UTC:       %s", asctime(gmtime(&t)));
printf("local:     %s", asctime(localtime(&t)));
// POSIX-specific
putenv("TZ=Asia/Singapore");
printf("Singapore: %s", asctime(localtime(&t)));

// UTC:       Fri Sep 15 14:22:05 2017
// local:     Fri Sep 15 14:22:05 2017
// Singapore: Fri Sep 15 22:22:05 2017

reference