4.  Reference Counting Garbage Collection

4.1.  Reference Counters

• Block header
  • nWords = The number of words in the block, block size (header + data area)
  • refCount = The number of pointers pointing to this block (initialized to 0 at allocation)

• To assign a pointer, old value = block R, new value = block S:
  Increment S.refCount
  Delete (R)
  Pointer = S

• To delete block P:
  Decrement P.refCount
  If P.refCount == 0
      For every pointer Q in P’s data area
          Delete (Q)
      Put P on the free list

• Pros
  • Very simple, non-compacting garbage collection
  • Heap maintenance spread throughout program execution (instead of suspending the program when the garbage collector runs)

• Cons
  • Extra word in block header to hold reference count
  • Fragile; if you forget to adjust reference counts on any pointer assignment (including passing pointers as subroutine arguments), disaster can happen
  • Major problem: Cannot garbage collect circularly linked data structures

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Heaps and Garbage Collection

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Copyright © 2001 Rochester Institute of Technology. All rights reserved. Last updated 06-Dec-2001. Please send comments to ark­@­cs.rit.edu.