This one tripped me up a bit because I kept thinking about it from a correctness angle instead of a performance angle.
Start by explaining why strncpy is suboptimal for copying known-length, non-overlapping data: it checks for null bytes and pads with zeros, adding unnecessary overhead. Then describe how memcpy can be used to perform a faster bulk copy, and discuss potential further optimizations like alignment, word-sized transfers, or SIMD. Finally, emphasize the importance of correctness and safety when choosing memcpy.
Pro tip: Mention that in performance-critical code, you should also consider using compiler intrinsics or platform-specific functions like __builtin_memcpy or std::memcpy with restrict pointers to enable better optimization. Also, be prepared to discuss how you would verify the copy's correctness and handle edge cases like zero-length copies.
Explain that strncpy is designed for null-terminated strings and will stop at the first null byte or pad with zeros if the source is shorter, which is unnecessary when the exact length is known.
State that memcpy performs a raw byte copy without any null checks or padding, making it more efficient for known-length, non-overlapping memory regions.
Mention that memcpy is often optimized by the compiler or standard library to use word-sized or SIMD instructions, and that ensuring proper alignment can further improve performance.
Emphasize that memcpy requires the caller to guarantee non-overlapping regions and sufficient destination buffer size, and that using it incorrectly can lead to undefined behavior.
Summarize that while memcpy is faster, it shifts responsibility to the programmer; thus, it should be used judiciously with proper bounds checking and alignment considerations.
AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.