Is it safe to use printf("%*s", len, ptr) where len is an integer and ptr is non-null terminated?

Question

Is it safe to use printf("%*s", len, ptr) where len is an integer and ptr is non-null terminated? If not safe, what will happen in the worst case? And then how can I make it safe?

thank you

Answer 1

No not safe at all. "%*s" only sets a minimum display width, it does not enforce input string length at all.

But assuming you actually mean "%.*s"

Yes, that would be safe.

This format of printf does not look at chars in/at ptr past the n_th (index n-1)

So assuming that n holds an appropriate value there will be no access outside of the allocation unit of ptr and thus no risk of out-of-range pointer undefined behaviour.