void reverse(char * c)
{
    int i = 0;
    int j = 0;
    while (*(c+j)!=NULL)
    {
        j++;
    }

    char temp;
    for (i = 0, j = j - 1; i < j; i++,j--)
    {
        temp = *(c+i);
        *(c+i) = *(c+j);
        *(c+j) = temp;
    }
}

int main(int argc, char *argv[])
{
    char * s = "abcdef";
    printf("%s\n", s);
    reverse(s);
    printf("%s\n", s);
    return 0;
}