Citadel·Software Engineer·Technical Phone Screen·Senior

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May 2026

Summary

Citadel software engineer interview with a low-level systems coding question. The problem was scoped but had some nuance around thread safety that they wanted you to think through after the main implementation.

Questions Asked (1)

Implement a fixed-capacity ring buffer in C++ for a single-producer, multiple-consumer scenario. The class should support push, pop, empty, full, and size operations, all in O(1) time using circular indexing on a preallocated array. After coding it up, explain how you'd extend the design to be truly thread-safe.

System DesignTechnical Trade-offsAlgorithms & Data Structures

Author's notes

The core ring buffer wasn't too bad.

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AI HintsAI Generated

Suggested Approach

Start by clarifying requirements and constraints, then implement the ring buffer with a preallocated array and head/tail indices, ensuring O(1) operations. After coding, discuss thread-safety extensions, focusing on synchronization primitives and lock-free techniques for SPSC or MPMC scenarios.

Pro tip: Mention that for true thread-safety, you'd use atomic operations with acquire-release semantics and consider the ABA problem, but also note that a mutex-based solution might be sufficient depending on contention levels.

1. Clarify Requirements

Ask about the exact threading model (single-producer, multiple-consumer), expected contention, and whether lock-free is required. Confirm that push and pop should be O(1) and that the buffer is fixed-capacity.

2. Design and Implement

Use a preallocated array with head and tail indices, and a size counter or full flag to distinguish empty/full. Implement push, pop, empty, full, and size as inline O(1) operations.

3. Test and Validate

Walk through edge cases: empty buffer pop, full buffer push, wrap-around, and concurrent access scenarios. Mention unit tests and stress tests for correctness.

4. Extend to Thread-Safety

Discuss synchronization options: mutexes for simplicity, or lock-free with atomics and memory ordering. For SPSC, a lock-free queue with atomic head/tail is common; for MPMC, consider more complex algorithms or mutexes.

5. Analyze Trade-offs

Compare performance, complexity, and correctness of different approaches. Mention that lock-free is not always faster due to contention and cache effects, and that mutexes may be preferable under high contention.

Key Points to Mention

Circular indexing with modulo or conditional wrap-around to achieve O(1) operations.
Use of head, tail, and size (or full flag) to manage buffer state without ambiguity.
Thread-safety via mutexes (std::mutex) for simplicity, or atomics with acquire-release semantics for lock-free.
For SPSC, a lock-free ring buffer using atomic indices and no locks is efficient and common.
For MPMC, consider using a mutex or more advanced lock-free algorithms (e.g., Dmitry Vyukov's bounded MPMC queue).
Memory ordering and the ABA problem in lock-free designs; use of std::atomic and memory fences.

AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.