To make a useful kernel we require it to perform tasks like setting a memory chunk to zero, or copying the memory from a source address to destination address. It may have to understand what a boolean is or what NULL is. So, for that we require the "C Standard Library" and hence we need to implement it (or the functions it contains as abstractions to commonly used functions).

For this, in day 4 I utilized clang's internal C library as well as resorted to writing the functions all by myself.