将c转换为小写; 仅限ASCII

#include <stdio.h>

int lower(int c){
    if (c >= 'A' && c <= 'Z')
        return c + 'a' - 'A';
    else
        return c;
}

int main()
{
    int c;

    while ((c = getchar()) != EOF)
        putchar(lower(c));
    return 0;
}