[NarutoUA]

Проверяйте.

Code:

void UpperToLower(char *str)
{
    for (unsigned char ch = *str; ch; str++)
    {
        ch = *str;
        if ((ch >= 'A' && ch <= 'Z') || (ch > 191 /*А*/ && ch < 224 /*Я*/))
            *str += 32;
        else if (ch == 168 /*Ё*/)
            *str = 184;
    }
}