Lesson 23 · Calloc that zeroes - Build a Memory Allocator

Calloc that zeroes

Calloc allocates like malloc but hands back zeroed memory. Because freed blocks keep their old bytes, this matters - a reused block can be full of stale data, and calloc must scrub it.

The goal

Allocate and zero a block's payload, even when it reuses dirty memory.

Start here - the target

TO DO

Scenario: Calloc returns zeroed memory over a dirty block

Givena 64-byte heap where Malloc(16) gave offset 8, the bytes 0xFF were written at offsets 8 and 9, then the block was freed (coalescing the heap back to one free block)

WhenCalloc(16) is called

Thenit returns offset 8 (reusing that space) and the payload is zeroed: the bytes at offsets 8 and 9 are 0x00

Andthe returned offset behaves exactly like a Malloc result (it can be freed and reused)

Background

Free does not erase a block’s bytes - it only flips the allocated flag - so the space handed back still holds whatever was written there. That is fine for Malloc, whose callers are expected to initialize what they take, but it means a reused block is full of stale data. Calloc exists for callers who need a clean slate: it allocates exactly like malloc, then zeroes the payload before returning.

The test makes the staleness visible: write 0xFF into a block, free it so the space returns to the pool, then Calloc the same size and watch those bytes come back as 0x00. Real calloc also multiplies a count by an element size (and guards that multiply against overflow); here the single-size version is enough to show the zeroing guarantee, which is the part that actually matters for correctness.

Make it work

func (a *Allocator) Calloc(n int) (int, error) {
  payload, err := a.Malloc(n)
  if err != nil { return -1, err }
  // scrub the requested bytes - reused memory is dirty
  for i := 0; i < n; i++ { a.buf[payload+i] = 0 }
  return payload, nil
}

CheckpointDONE

Calloc allocates zeroed memory. Commit and stop here.