Amazon·Software Engineer·Technical Phone Screen·Senior

Senior

Jul 2026

Summary

Amazon embedded interview, one meaty coding question about reimplementing memcpy from scratch. Felt like a low-level systems deep dive more than a typical coding screen, which I wasn't fully prepared for.

Questions Asked (1)

Implement the C standard library memcpy function: copy n bytes from src to dest and return dest. The buffers may be misaligned and must not overlap. Optimize by aligning the destination to a word boundary first, then copying word-by-word in bulk, and handling remaining trailing bytes separately. Also discuss alignment, strict aliasing, and why memmove exists.

System DesignTechnical Trade-offsAlgorithms & Data Structures

Author's notes

I started with the naive byte-by-byte loop and they let me finish before asking about performance.

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AI HintsAI Generated

Suggested Approach

Start by clarifying the requirements and constraints, then present a correct baseline implementation before optimizing with alignment and word-wise copying. Discuss the trade-offs and edge cases, including alignment, strict aliasing, and the need for memmove.

Pro tip: Mention that the compiler often optimizes memcpy better than hand-written code, so the exercise is about understanding the principles, not beating the compiler. Also, emphasize that correctness and portability come before micro-optimizations.

1. Clarify requirements and constraints

Confirm that the function signature is void *memcpy(void *dest, const void *src, size_t n), that buffers must not overlap, and that alignment is not guaranteed. Ask about performance expectations and target architecture.

2. Implement a correct baseline

Write a simple byte-by-byte copy loop that handles n=0 and returns dest. This ensures correctness before optimization.

3. Optimize with alignment and word-wise copying

Align the destination pointer to a word boundary by copying leading bytes individually. Then copy full words using a word-sized type (e.g., uintptr_t) until fewer than one word remains. Finally, copy trailing bytes individually.

4. Address strict aliasing and portability

Explain that casting to a word type may violate strict aliasing; use a char* or memcpy-based approach for portability, or rely on compiler extensions. Mention that the standard allows any implementation as long as behavior is correct.

5. Discuss memmove and overlapping buffers

Explain that memcpy assumes non-overlapping buffers; if they overlap, behavior is undefined. memmove handles overlap by checking direction and copying appropriately, often with a temporary buffer or backward copy.

Key Points to Mention

Alignment: accessing misaligned data can cause faults or performance penalties on some architectures; aligning destination first improves efficiency.
Strict aliasing: accessing the same memory through different types is undefined behavior; use char* or memcpy to avoid violations.
Word-wise copying: copy in chunks of the machine word size (e.g., 4 or 8 bytes) for speed, but handle leading/trailing bytes separately.
memmove: exists because memcpy does not handle overlapping memory; memmove detects overlap and copies safely, often with a temporary buffer or direction-aware copy.
Performance trade-offs: hand-optimized memcpy may not beat compiler built-ins; consider using built-in functions or SIMD for large copies.
Edge cases: n=0, null pointers (if n=0, pointers may be null), and ensuring return value is dest.

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