C - Design your own free( ) function

Question

Today, I appeared for an interview and the interviewer asked me this,

1. Tell me the steps how will you design your own f

Answer 1

You can't blindly design free() without knowing how malloc() works under the hood because your implementation of free() would need to know how to manipulate the bookkeeping data and that's impossible without knowing how malloc() is implemented.

So an unswerable question could be how you would design malloc() and free() instead which is not a trivial question but you could answer it partially for example by proposing some very simple implementation of a memory pool that would not be equivalent to malloc() of course but would indicate your presence of knowledge.