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Meta·Software Engineer·Technical Phone Screen·Intermediate

IntermediatePrefer not to say
May 2026

Summary

Meta SWE coding round, pretty stripped down. Just one question about reimplementing a standard C library function from scratch.

Questions Asked (1)

Q1

Implement the memcpy() function in C.

Algorithms & Data StructuresTechnical Trade-offs
Author's notes

Looks trivial on paper but I fumbled the edge cases a bit.

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AI HintsAI Generated

Suggested Approach

Start by clarifying the function signature and expected behavior, then discuss the naive byte-by-byte implementation and its limitations. Propose an optimized version using word-sized copies and alignment handling, and address edge cases like overlapping memory and null pointers.

Pro tip: Mention that memcpy is undefined for overlapping regions and that memmove should be used instead; this shows awareness of standard library semantics and attention to correctness.

1. Clarify requirements and constraints

Ask about the function signature, return value, and whether overlapping memory needs to be handled. Confirm that memcpy assumes non-overlapping regions.

2. Implement a basic byte-by-byte version

Write a simple loop that copies each byte from source to destination. This establishes correctness and serves as a baseline.

3. Optimize for performance

Discuss copying in word-sized chunks (e.g., using size_t or uintptr_t) when alignment allows, and handling unaligned head and tail bytes separately.

4. Handle edge cases and safety

Consider null pointers, zero length, and potential overlapping memory (though undefined for memcpy). Mention using restrict qualifiers for optimization.

5. Test and validate

Describe how to test the implementation with various sizes, alignments, and edge cases, possibly comparing against the standard library's memcpy.

Key Points to Mention

  • Function signature: void *memcpy(void *dest, const void *src, size_t n)
  • Byte-by-byte copy is simple but inefficient; word-sized copy improves performance
  • Alignment considerations: copying in larger chunks requires aligned addresses
  • Overlapping memory is undefined behavior for memcpy; memmove handles it
  • Use of restrict qualifiers to indicate non-overlapping pointers
  • Edge cases: n=0, null pointers, and return value (dest)

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