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Arista Networks·Software Engineer·Technical Phone Screen·Intermediate

Intermediate
Jun 2026

Summary

Arista Networks SWE interview with a debugging-focused question that had a pretty nasty follow-up. The main question was manageable but the follow-up is where things got interesting.

Questions Asked (2)

Q1

You're given a buggy code snippet where the root cause is an out-of-bounds memory access. Find the bug and explain how to fix it.

Root Cause AnalysisTechnical Trade-offs
Author's notes

Pretty standard debugging exercise on the surface.

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AI HintsAI Generated

Suggested Approach

Start by carefully reading the code and identifying the array or buffer being accessed. Trace the indices and loop bounds to spot where the access exceeds the allocated size. Then explain the fix, such as correcting the loop condition or adding bounds checks, and discuss how to prevent similar issues.

Pro tip: Demonstrate that you understand the memory layout and the consequences of out-of-bounds access, such as undefined behavior or security vulnerabilities. Mention tools like AddressSanitizer or Valgrind that can help detect such bugs.

1. Understand the code

Read the code snippet thoroughly to understand its purpose and identify all memory accesses, especially array indexing and pointer arithmetic.

2. Locate the out-of-bounds access

Check loop bounds, array sizes, and index calculations to find where an index exceeds the valid range. Consider both off-by-one errors and incorrect size assumptions.

3. Explain the root cause

Articulate why the access is out-of-bounds, such as using <= instead of < in a loop, or failing to account for the array size.

4. Propose a fix

Suggest a concrete correction, like adjusting the loop condition, adding a bounds check, or using safer data structures. Explain how the fix prevents the out-of-bounds access.

5. Discuss prevention and trade-offs

Mention strategies to avoid similar bugs, such as static analysis, code reviews, or using safe libraries. Discuss any performance or readability trade-offs of the fix.

Key Points to Mention

  • Common causes of out-of-bounds: off-by-one errors, incorrect loop conditions, and miscalculated buffer sizes.
  • Consequences: undefined behavior, crashes, data corruption, and security vulnerabilities like buffer overflows.
  • Debugging tools: AddressSanitizer, Valgrind, and static analyzers can detect out-of-bounds accesses.
  • Fix strategies: correct loop bounds, use bounds checking, prefer standard library containers with bounds checking (e.g., std::vector::at).
  • Preventive measures: code reviews, unit tests with edge cases, and defensive programming.
  • Trade-offs: bounds checking may add overhead; choose between safety and performance based on context.

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

Q2

The out-of-bounds write doesn't cause a segfault here, it causes an infinite loop instead. Why?

Root Cause AnalysisTechnical Trade-offsAlgorithms & Data Structures
Author's notes

This is the one that got me.

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AI HintsAI Generated

Suggested Approach

Explain that an out-of-bounds write causes undefined behavior, which can manifest as an infinite loop if the write corrupts a loop control variable or a pointer used in the loop condition. Emphasize that the specific outcome depends on memory layout and compiler optimizations, and that segfaults are not guaranteed.

Pro tip: Mention that tools like AddressSanitizer or Valgrind can catch such bugs deterministically, and that relying on segfaults to detect memory errors is dangerous because behavior is unpredictable.

1. Define undefined behavior

Clarify that out-of-bounds writes are undefined behavior in C/C++, meaning the program can do anything, including not crashing.

2. Explain why no segfault

A segfault occurs only when accessing an unmapped page; if the out-of-bounds write lands in valid memory (e.g., another variable or heap metadata), no fault is raised.

3. Describe how an infinite loop arises

The write may corrupt a loop counter, a pointer used in the loop condition, or a flag, causing the loop to never terminate.

4. Discuss compiler and memory layout factors

Mention that stack layout, variable ordering, and optimizations can determine whether the corruption affects loop control, and that behavior may vary across builds.

5. Conclude with detection and prevention

State that such bugs are best caught with memory sanitizers or static analysis, and that one should never rely on crashes to detect memory errors.

Key Points to Mention

  • Undefined behavior means no guaranteed outcome; segfault is just one possibility.
  • Out-of-bounds write may corrupt adjacent memory, such as a loop variable or pointer.
  • Segfault requires an invalid page access; writing to valid but unintended memory does not fault.
  • Compiler optimizations can change memory layout and expose or hide the bug.
  • Infinite loops from memory corruption are often non-deterministic and hard to debug.
  • Use tools like AddressSanitizer, Valgrind, or static analysis to detect out-of-bounds writes.

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