Is using strlen() in the loop condition slower than just checking for the null character?

Question

I have read that use of strlen is more expensive than such testing like this:

We have a string x 100 characters long.

I think that<

Answer 1

The first code checks length of x in every iteration of i and it takes O(n) to find the last 0, so it takes O(n^2), the second case is O(n)