reverse-str.c

int length(char *str) {
    int s = 0;
    for (; *str; ++str) ++s;
    return s;
}

char *reverse(char *str) {
    int size = length(str);
    for (int i = 0; i < size / 2; ++i) {
        str[i] ^= str[size - i - 1];
        str[size - i - 1] ^= str[i];
        str[i] ^= str[size - i - 1];
    }
    return str;
}