Processor Endianness: Detect and Convert Between Little-Endian and Big-Endian Byte Orders
Question Details
Problem
Write a function in C that detects at runtime whether the current processor is little-endian or big-endian, and implement functions to swap the byte order of 16-bit, 32-bit, and 64-bit integers.
c
#include <stdint.h>
#include <stdbool.h>
bool is_little_endian(void);
uint16_t bswap16(uint16_t x);
uint32_t bswap32(uint32_t x);
uint64_t bswap64(uint64_t x);
// Convert a 32-bit value from host byte order to network byte order (big-endian)
uint32_t hton32(uint32_t x);
Example:
// On a little-endian x86 machine:
is_little_endian() -> true
bswap32(0x12345678) -> 0x78563412
hton32(0x12345678) -> 0x78563412 // must swap on little-endian host
// On a big-endian machine:
hton32(0x12345678) -> 0x12345678 // no swap needed
Follow-ups
- Describe the union-based trick for detecting endianness at runtime without undefined behavior.
- Why does network protocol code (sockets) always use
htonl/ntohl, and what standard defines this? - How do modern compilers (GCC, Clang) optimize
bswap32-- what intrinsic or instruction do they emit? - What is mixed-endian (PDP-endian) and on what historical hardware did it appear?
Full Details
Problem
Write a function in C that detects at runtime whether the current processor is little-endian or big-endian, and implement functions to swap the byte order of 16-bit, 32-bit, and 64-bit integers.
c
#include <stdint.h>
#include <stdbool.h>
bool is_little_endian(void);
uint16_t bswap16(uint16_t x);
uint32_t bswap32(uint32_t x);
uint64_t bswap64(uint64_t x);
// Convert a 32-bit value from host byte order to network byte order (big-endian)
uint32_t hton32(uint32_t x);
Example:
// On a little-endian x86 machine:
is_little_endian() -> true
bswap32(0x12345678) -> 0x78563412
hton32(0x12345678) -> 0x78563412 // must swap on little-endian host
// On a big-endian machine:
hton32(0x12345678) -> 0x12345678 // no swap needed
Follow-ups
- Describe the union-based trick for detecting endianness at runtime without undefined behavior.
- Why does network protocol code (sockets) always use
htonl/ntohl, and what standard defines this? - How do modern compilers (GCC, Clang) optimize
bswap32-- what intrinsic or instruction do they emit? - What is mixed-endian (PDP-endian) and on what historical hardware did it appear?
About This Question
This is a reported interview question from a anduril interview during the phone round.
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Solve similar problems on LeetCode under timed conditions (25-35 minutes per medium difficulty). The goal is pattern recognition: recognize the underlying technique (sliding window, two-pointer, BFS, memoized recursion, etc.) within 60-90 seconds of reading. Strong candidates verbalize their hypothesis out loud before coding, then iterate based on feedback. Weak candidates dive into implementation immediately, lose time on the wrong approach, and run out of time for follow-ups.
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Apply the standard interview round template: clarify requirements (2-3 minutes), state your approach out loud and confirm direction with the interviewer (3-5 minutes), code with narration (15-25 minutes), test with concrete examples including edge cases (5 minutes), discuss optimization or trade-offs if time permits (5 minutes). This template is universally accepted across FAANG and adjacent companies; deviating from it produces weaker interviewer feedback signal.
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