Question 1

What is cache coherence in multi-core systems and why does it matter for C programmers?

Accepted Answer

The property ensuring all CPU caches maintain a consistent view of shared memory — without it, one core's write may not be visible to another core reading the same address. MESI protocol maintains coherence: Modified, Exclusive, Shared, Invalid states per cache line. False sharing occurs when two threads modify different variables on the same 64-byte cache line — causing invalidation storms. Fix: pad structs to align each hot variable to its own cache line.

Question 2

What are the key differences between mmap() and malloc() for memory allocation?

Accepted Answer

mmap() maps files or anonymous memory directly into the process's virtual address space — useful for large allocations, IPC, and file I/O; malloc() manages a heap for smaller, frequent allocations. malloc() manages a heap using sbrk/mmap internally — ideal for general allocation. mmap(MAP_ANONYMOUS) maps large blocks directly; mmap(fd) maps a file — reads/writes go directly to file-backed memory. Shared mappings enable zero-copy IPC.

Question 3

What is a memory barrier (fence) and when is it needed in C?

Accepted Answer

A CPU instruction and compiler directive preventing reordering of memory operations across it — required in lock-free code to ensure correct ordering between threads. Both compilers and CPUs reorder memory operations for performance. Without barriers, a flag store signaling 'data is ready' may be reordered before the data store. __atomic_thread_fence(memory_order_seq_cst) or C11 atomics provide the needed ordering guarantees.

Question 4

What is a lock-free data structure?

Accepted Answer

A data structure where threads can always make progress without acquiring locks — using atomic operations such as Compare-And-Swap (CAS). CAS: atomically compare memory value with expected; if equal, swap with new value. Used to implement lock-free queues, stacks, and counters. Avoids lock contention and deadlocks but requires careful ABA problem handling.

Question 5

What is AddressSanitizer (ASan) and when should you use it?

Accepted Answer

A runtime memory error detector (compile with -fsanitize=address) that catches heap/stack buffer overflows, use-after-free, use-after-scope, and memory leaks with minimal code changes. ASan instruments memory accesses at compile time. Overhead: ~2x slowdown, ~3x memory. Use it in CI and development. Combine with UBSan (-fsanitize=undefined) and TSan (-fsanitize=thread) for comprehensive sanitization. Catches bugs that are nearly impossible to find otherwise.

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