You want to evaluate p in: p = m n If that is gonna overflow, p > SIZE_MAX. Then it would be that (by substitution) m n > SIZE_MAX so m > SIZE_MAX/n

You want to evaluate p in: p = m n If that is gonna overflow, p > SIZE_MAX. Then it would be that (by substitution) m n > SIZE_MAX so m > SIZE_MAX/n except if n = 0. In the latter case, m can be as big as it wants--p will always evaluate to 0. And (size_t)-1 is a silly way to write SIZE_MAX (but it saves you a dependency).