C语言标准宏__DATE__格式化YY/MM/DD，不能用C语言标准库函数

C语言标准宏__DATE__的格式为"MMM DD YYYY"，其中MMM为英文月份缩写，DD为日期，YYYY为年份。要将其格式化为YY/MM/DD的形式，可以使用下面的代码：

#include <stdio.h>

#define MONTH_LENGTH 3

int main() {
    char date[] = __DATE__;
    char month[MONTH_LENGTH + 1];
    int day, year;
    sscanf(date, "%s %d %d", month, &day, &year);

    // 将月份缩写转换为数字
    int month_num = 0;
    if (strncmp(month, "Jan", MONTH_LENGTH) == 0) {
        month_num = 1;
    } else if (strncmp(month, "Feb", MONTH_LENGTH) == 0) {
        month_num = 2;
    } else if (strncmp(month, "Mar", MONTH_LENGTH) == 0) {
        month_num = 3;
    } else if (strncmp(month, "Apr", MONTH_LENGTH) == 0) {
        month_num = 4;
    } else if (strncmp(month, "May", MONTH_LENGTH) == 0) {
        month_num = 5;
    } else if (strncmp(month, "Jun", MONTH_LENGTH) == 0) {
        month_num = 6;
    } else if (strncmp(month, "Jul", MONTH_LENGTH) == 0) {
        month_num = 7;
    } else if (strncmp(month, "Aug", MONTH_LENGTH) == 0) {
        month_num = 8;
    } else if (strncmp(month, "Sep", MONTH_LENGTH) == 0) {
        month_num = 9;
    } else if (strncmp(month, "Oct", MONTH_LENGTH) == 0) {
        month_num = 10;
    } else if (strncmp(month, "Nov", MONTH_LENGTH) == 0) {
        month_num = 11;
    } else if (strncmp(month, "Dec", MONTH_LENGTH) == 0) {
        month_num = 12;
    }

    // 输出YY/MM/DD格式的日期
    printf("%02d/%02d/%02d\n", year % 100, month_num, day);

    return 0;
}

该代码将__DATE__宏的值通过sscanf()函数解析为月份字符串、日期和年份，然后根据月份字符串确定月份的数字值。最后输出YY/MM/DD格式的日期。注意，需要使用strncmp()函数来比较字符串，因为月份缩写的长度可能不足3个字符。