Buffer-overflow attacks began two decades ago and persist today. Over that time, a number of researchers have proposed many solutions. Their targets were either to prevent or to protect against buffer-overflow attacks. As defenses improved, attacks adapted and became more complicated. Given the maturity of field and the fact that some solutions now exist that can prevent most buffer-overflow attacks, it is time to examine these schemes and their critical issues. In this paper, approaches were categorized into three board categories to provide a basis for understanding buffer-overflow protection schemes.