There are though, when it checks for '\0'.

`starts()` looks like it's not checking if len(source) < len(query), but if, say, query="foobar" and source="foo", when i = 3 the line

   if (source[i] != query[i])
      return false;

will evaluate to

   if ('\0' != 'b')
      return false;

so `starts()` will correctly return false.

Always if len(source) < len(query), we'll return false when we get to i=len(source), because source[len(source)] != query[len(source)] as source[len(source)] == '\0' and query[len(source)] != '\0' since len(query) > len(source).

There is a dereference past the bounds of the query in one case in the last code sample, but there is no deference beyond the bounds of the source string.

You're probably thinking in C# or Java; remember that in C the convention is that a zero char ends strings. If the source string is shorter than the query string then the code will encounter a zero char in the source string at the same time as it encounters a non-zero char in the query string, and the inequality will end the loop before the beyond-bounds dereference.

There are other defects; can you find them?

No null checks in find. If source is null, dereferencing source[i] in the while condition will cause undefined behaviour. If just query is null, the same will occur in the first call to starts.