NULL Has Two Meanings

my_realloc(p, 0) → intentional free, NULL.

my_realloc(p, positive_size) → NULL on failure; p survives.

This is our custom API. Do not use its zero-size rule as a portable standard-C realloc rule.

Predict ownership

Live 12-byte block; request 20 bytes; allocation fails.

Who owns the old bytes?

The caller still does. No free and no metadata change occurred.

Successful growth

Allocate 20 → copy 12 → free old → return new.

Copying 20 overreads the source. Freeing first allows the allocator to reuse the source too soon.

Shrink is not necessarily a split

Capacity 12; request 5; return the same pointer.

The exercise keeps recorded capacity 12. The first five bytes remain. Splitting requires enough room for another aligned header and useful payload.

Failure begins below realloc

An unchecked header + SIZE_MAX can wrap to a tiny allocation.

Check metadata addition, alignment padding, and the signed sbrk range before changing the heap.

calloc on dirty reused memory

Check size > SIZE_MAX/count before multiplication, with zero arguments handled separately.

Then allocate and clear count*size bytes. Recycling a free block does not clear its previous contents.

Exit check

my_realloc(NULL, 0) returns NULL and allocates nothing.

A NULL result after freeing a live pointer is not a growth failure. The requested size tells the caller which ownership rule applies.