Systems Programming & Low-Level Concepts Questions

Systems programming concepts including memory management, pointers, memory layout, CPU architecture considerations, concurrency primitives, OS interactions, and performance optimization in low-level languages (C, C++). Covers how languages expose low-level resources, toolchains, and platform-specific behaviors; excludes high-level application development.

HardSystem Design

84 practiced

Design a bounded queue in C++ for one producer thread and one consumer thread on the same machine, where millions of handoffs per second are expected and latency matters more than perfect generality. What invariants would your design need to preserve, and what synchronization and memory-safety issues would you pay closest attention to?

MediumTechnical

42 practiced

A binary file format is being parsed by the same C++ code on x86 servers and ARM-based test machines. One test only fails on ARM after someone rewrote the parser to cast a byte buffer directly to a struct. What would you inspect, and how would you make the parser safe across platforms?

Sample Answer

**I would inspect three things immediately: alignment, endianness, and object layout.** Casting a raw byte buffer to a struct is fragile because the file format and the C++ struct are not guaranteed to match. x86 often tolerates unaligned access, while ARM may fault or behave differently, which is why the bug can appear only on ARM.

**What I would check:**- Whether the struct has padding inserted by the compiler.- Whether the buffer address satisfies the struct's alignment requirements.- Whether the file format uses little-endian or big-endian byte order.- Whether the code violates strict aliasing by treating bytes as a different type.

**Concrete example:** if the file stores a 32-bit length as bytes `01 00 00 00`, that means `1` in little-endian. A direct struct cast assumes the host layout matches the file, but that is not safe across platforms.

**Safer approach:** parse fields from bytes explicitly, or copy bytes into fixed-width integers and convert them. For example:

cpp

bool parse_len(const unsigned char* b, std::uint32_t& len) {
    len = std::uint32_t(b[0]) |
          (std::uint32_t(b[1]) << 8) |
          (std::uint32_t(b[2]) << 16) |
          (std::uint32_t(b[3]) << 24);
    return true;
}

That keeps the parser portable, alignment-safe, and explicit about byte order.

MediumTechnical

56 practiced

In a C or C++ Unix program, you spawn a child process, redirect its stdin and stdout, and sometimes the parent hangs waiting for output that never arrives. What OS-level mistakes would you look for in how file descriptors are created, duplicated, and closed?

EasyTechnical

84 practiced

A C function in a service occasionally crashes when a caller uses the pointer it returns, but only after the function has finished executing. How would you debug this, and what would you look for in the code and runtime behavior to confirm the root cause?

MediumTechnical

51 practiced

A request-handling thread needs to read and update a small shared state object on every request. A mutex keeps the code simple, but latency is too high under load. How would you decide whether to keep the lock, change the data layout, or use a different synchronization strategy?

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