strncpy clarify result may not be null terminated

[Paul Eggert]

On 2023-11-09 03:38, Alejandro Colomar wrote:
> If you are consistent in checking the return value of strlcpy(3) and
> reporting an error, it's the best standard alternative nowadays.

Not necessarily. strlcpy is subject to denial-of-service attacks if the 
attacker has control of the source string and can attack by using long 
source strings. strncpy, as bad as it is, does not have this problem.

Instead of this:

    if (strlcpy (dst, src, dstsize) == dstsize)
      return failure;

applications that want want to copy a string into a small nonempty 
fixed-size buffer, failing if the string doesn't fit, should do 
something like this:

    if (strncpy (dst, src, dstsize)[dstsize - 1])
      return failure;

This avoids the denial-of-service attack and is portable all the way 
back to K&R C.

It's unfortunate that strlcpy was misdesigned but here we are.