Lesson 22 · Realloc that relocates - Build a Memory Allocator

Realloc that relocates

When an allocation cannot grow in place, realloc falls back to moving it - allocate a new block, copy the bytes over, free the old one. Today you complete realloc with that relocate-and-copy path.

The goal

Relocate a growing allocation that cannot expand in place, preserving its contents.

Start here - the target

TO DO

Scenario: A growing allocation that cannot expand is moved

Givena 96-byte heap with (0, 24, allocated), (24, 24, allocated), (48, 48, free), the byte 0xAB stored at offset 8, reallocating the first block (payload 8)

WhenRealloc(8, 24) is called (needs a 40-byte block, but the next block is allocated)

Thenit allocates a new block from the free region and returns a different payload offset, 56

Andthe contents are copied - the byte at offset 56 is 0xAB - and the old block at 0 is now free

Background

If an allocation cannot grow where it sits, realloc does what a caller expects: it finds space elsewhere, copies the old contents into it, releases the old block, and returns the new offset. The copy length is the old payload’s capacity, so every byte the caller had written survives the move. This is the general fallback, and it is why a realloc’d pointer can change - the classic footgun in C, made explicit here as a changed offset.

With this, realloc is complete: it shrinks in place, grows in place when a free neighbour allows, and relocates when it must. Notice how it is assembled entirely from pieces you already built - Malloc, Free, and byte copying - which is the mark of a clean allocator core. One convenience allocation remains before the demo: calloc, which allocates and zeroes.

Make it work

// fall-through when in-place growth is impossible:
newPayload, err := a.Malloc(n)      // find space elsewhere
if err != nil { return -1, err }
oldPayloadCap := size - overhead
copy(a.buf[newPayload:newPayload+oldPayloadCap], a.buf[payload:payload+oldPayloadCap])
a.Free(payload)                     // release the old block
return newPayload, nil

CheckpointDONE

Realloc is complete - shrink, grow in place, or relocate. Commit and stop here.