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C++ Academy · Lesson

Hazard Pointers and the ABA Problem

Recognize the ABA problem and mitigate it with hazard pointers or tagged pointers.

Lesson 4 of 415 steps

Hazard Pointers and the ABA Problem is a free C++ Academy lesson on CoddyKit. This is lesson 4 of 4. Any 3 lessons of this learning path are free to read in full — after that, CoddyKit PRO unlocks every lesson, plus hands-on practice with a built-in code editor and a 24/7 AI tutor. It is part of the C++ Academy learning path, and your progress syncs across the web and the CoddyKit app. The C++ Academy course includes 4 lessons in total.

The ABA Problem

In lock-free code with pointers, a value may be A, change to B, then back to A. A CAS comparing only the value cannot tell anything changed in between.

Classic ABA Scenario

Thread 1 reads pointer A and prepares to CAS. Meanwhile Thread 2 deletes A, allocates a new node that happens to reuse A s address, and Thread 1 s CAS incorrectly succeeds.

Tagged Pointers

One solution: pack a counter alongside the pointer. The combined value changes on every operation, so ABA is detectable.

struct TaggedPtr {
    Node* ptr;
    uint64_t tag;
};
std::atomic<TaggedPtr> head;

Limitations of Tags

Tags require double-width atomics (e.g., CMPXCHG16B on x86). They consume more space and may not be available on every platform.

Memory Reclamation Challenge

The fundamental issue: how can you delete a node that other threads may still be accessing? Lock-free code cannot use a simple delete.

Hazard Pointers

Each thread publishes the pointer it is currently accessing into a global hazard list. Before reclaiming memory, a thread scans the hazard list — if anyone is using the pointer, defer the delete.

Hazard Pointer Workflow

For each access:

  1. Read the pointer atomically
  2. Publish it as a hazard
  3. Re-read to confirm it has not changed (avoid race)
  4. Use it
  5. Clear the hazard

Retire Lists

When a node is no longer reachable, add it to a per-thread retire list. Periodically scan all hazards; nodes not present in any hazard are safe to delete.

Epoch-Based Reclamation

An alternative: divide time into epochs. A node becomes deletable once all threads have advanced past the epoch when it was retired. Used in Crossbeam (Rust) and concurrent data structures.

Reference Counting Per Node

Increment a per-node counter on read, decrement after use. Simple but slower than hazards — every operation adds atomic increments and decrements.

When ABA Matters

ABA is a problem only when you have a "free list" or reuse memory addresses. With safe reclamation (hazards, epochs), the issue often dissolves.

Practical Advice

Lock-free data structures are minefields. Use proven implementations from Folly, Boost, or Crossbeam (Rust). Roll your own only for educational purposes or specific performance-critical paths after careful measurement.

Tooling

ThreadSanitizer detects data races but not all lock-free correctness issues. Formal verification tools (CDSChecker, Relacy) explore interleavings systematically.

Quick Check

What does the ABA problem refer to in lock-free programming?

Answer it in the interactive lesson

Recap

The ABA problem strikes lock-free code that reuses memory addresses. Tagged pointers, hazard pointers, and epoch-based reclamation are the canonical solutions. Lock-free correctness is hard — prefer proven libraries over hand-rolled implementations.

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Frequently Asked Questions

Is the “Hazard Pointers and the ABA Problem” lesson free?

Yes — any 3 lessons of the C++ Academy learning path, including “Hazard Pointers and the ABA Problem”, are free to read in full here on the web. After that, CoddyKit PRO unlocks every lesson, plus interactive practice with a built-in code editor and a 24/7 AI tutor. The C++ Academy course includes 4 lessons in total.

What will I learn in “Hazard Pointers and the ABA Problem”?

Recognize the ABA problem and mitigate it with hazard pointers or tagged pointers. You practise C++ Academy with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.

Do I need any experience to start C++ Academy?

No prior experience is required. C++ Academy on CoddyKit is structured for beginners through advanced learners, so you can start here or from the beginning and move at your own pace. This is lesson 4 of 4.

How long does the “Hazard Pointers and the ABA Problem” lesson take?

Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.

Can I write and run code in this C++ Academy lesson?

Yes. Every C++ Academy lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.

All lessons in this course

  1. std::atomic and Memory Orders
  2. Compare-and-Swap CAS Patterns
  3. Lock-Free Queue Implementation
  4. Hazard Pointers and the ABA Problem
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