`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;

   if ('\0' != 'b')
      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).

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?