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文字

在头文件<time.h>中定义



char * asctime(const struct tm * time_ptr);

(1)


errno_t asctime_s(char * buf,rsize_t bufsz,const struct tm * time_ptr);

(2)

(自C11以来)

1)将给定的日历时间转换tm为以下固定的25个字符格式的文本表示形式:Www Mmm dd hh:mm:ss yyyy\n

  • Www-本周三字母缩写英文从当天time_ptr->tm_wday的一个MonTueWedThuFriSatSun

  • Mmm-三个字母的缩写英文月份名称从time_ptr->tm_mon,一JanFebMarAprMayJunJulAugSepOctNovDec

  • dd- 月份的2位数日期,timeptr->tm_mday如同sprintf使用时一样打印%2d

  • hh- 从使用timeptr->tm_hour时打印两位小时sprintf%.2d

  • mm-从2位分钟timeptr->tm_min仿佛印刷通过sprintf使用%.2d

  • ss-来自2位第二timeptr->tm_sec仿佛印刷通过sprintf使用%.2d

  • yyyy- 从使用timeptr->tm_year + 1900时打印出来的4位数年份sprintf%4d

如果任何成员*time_ptr超出其正常范围,则行为不确定

如果指示的日历年份time_ptr->tm_year超过4位或小于1000 年,则行为不明确。

该函数不支持本地化,并且不能删除换行符。

该函数修改静态存储并且不是线程安全的。

2)与(1)相同,不同之处在于消息被复制到用户提供的存储器中buf,并保证以null结尾,并且在运行时检测到以下错误并调用当前安装的约束处理函数:

  • buf或者time_ptr是空指针

  • bufsz 小于26或大于 RSIZE_MAX

  • 并非所有成员*time_ptr都在正常范围内

  • 指示的年份time_ptr->tm_year小于0或大于9999

作为所有的边界检查函数,asctime_s只有在被__STDC_LIB_EXT1__实现定义并且用户在包含之前定义__STDC_WANT_LIB_EXT1__为整数常量时1才能保证可用time.h

参数

time_ptr

-

指向指定打印时间的tm对象的指针

BUF

-

指向用户提供的缓冲区的指针,长度至少为26个字节

bufsz

-

用户提供的缓冲区的大小

返回值

1)指向一个静态的以null结束的字符串的指针,该字符串保存如上所述的日期和时间的文本表示。该字符串可以在asctime和之间共享ctime,并且可以在每次调用这些函数时被覆盖。

2)成功为零,失败时为非零,在这种情况下buf[0]设置为零(除非buf是空指针或者bufsz零或大于RSIZE_MAX)。

注释

该函数返回一个指向静态数据的指针,并且不是线程安全的。POSIX将此功能标记为废弃,并推荐strftime使用。C标准也建议strftime不要使用asctimeasctime_s因为strftime它更灵活且对语言环境敏感。

POSIX限制未定义的行为仅限于输出字符串长度超过25个字符,timeptr->tm_wdaytimeptr->tm_mon不在预期范围内或timeptr->tm_year超过时INT_MAX-1990

一些实现处理timeptr->tm_mday==0意味着前一个月的最后一天。

#define __STDC_WANT_LIB_EXT1__ 1#include <time.h>#include <stdio.h>
 int main(void){
    struct tm tm = *localtime(&(time_t){time(NULL)});    printf("%s", asctime(&tm));
 #ifdef __STDC_LIB_EXT1__
    char str[26];    asctime_s(str, sizeof str, &tm);    printf("%s", str);#endif}

可能的输出:

Tue May 26 21:51:50 2015Tue May 26 21:51:50 2015

参考

  • C11标准(ISO / IEC 9899:2011):

    • 7.27.2.1 asctime函数(p:392-393)

    • K.3.8.2.1 asctime_s函数(p:624-625)

  • C99标准(ISO / IEC 9899:1999):

    • 7.23.3.1 asctime函数(p:341-342)

  • C89 / C90标准(ISO / IEC 9899:1990):

    • 4.12.3.1 asctime函数

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