strnlen_s takes a parameter for the maximum number of characters to scan. This way, it won’t overflow the buffer you provide it if there’s no trailing null byte.

What if there is no null?

Then you probably shouldn't feed it to the str* functions at all. Use the mem* functions instead.

That's the same as saying you should never use the str* functions. Nothing in C guarantees a string must have a null byte. As you're well aware, C-strings aren't a separate type that's distinct from char arrays.

Sure you can say that good programmers will always ensure a C-string is a C-string but there's decades of programming history that shows that's not true in practice.

Then use a language other than C that doesn't shoehorn string handling into char , as I've mentioned in other comments. If you're have a char with no terminating null byte and you hand it to a function with "str" and its name, you're going to have a bad time.

If there isn't a NUL, then you're asking for a property relevantly distict from length of a datum relevantly distinct from a C-string. Use `strchr(s,0x00)` or `memchr(s,0x00,zs)`.