Align First, Then Reuse Capacity

Work aligned allocation before splitting, coalescing, and diagnostics. Each exercise changes a different part of the byte layout.

Trace the aligned payload

Raw address 1000, pointer size 8, alignment 64, request 20. Reserve 20+63+8=91 bytes. Start at 1008; align to 1024. Saved pointer: [1016,1024). User payload: [1024,1044). Paired free recovers 1000; the interior address is not a separate allocation.

Check both arithmetic boundaries

Check padding and saved-pointer additions before allocating. The base allocator also checks rounding, header addition, and signed growth. Toy maximum 255: size 240 + padding 15 fits, but the 8-byte slot does not. Reject without consuming a free block or changing a live allocation.

Account for the new header

Capacity 120, header 24, alignment 8, request 33.

Round request to 40. What usable payload remains after splitting?

Reverse the arithmetic

120 - 40 - 24 = 56.

Merging the neighbors later gives 40 + 24 + 56 = 120.

The second header occupies bytes that used to be payload.

Split threshold

Minimum remainder payload 16: require capacity - aligned_request >= 24 + 16.

First check the request fits. Capacity 72 and aligned request 40 leave 32: no split.

Alignment has an owner

Reserve padding and an original-pointer slot. Return an aligned interior pointer.

Paired free retrieves and frees the original allocation. Do not free the interior address directly.

Links are not adjacency

Merge only if both blocks are free and A + header + A.capacity == B.

Skipping over a live block would hand its storage to another allocation.

Exit check

A magic value matches at a computed address. Does that prove any caller pointer is valid?

No. Reading an untrusted address may already be invalid; diagnostics need trustworthy ownership and bounds.