13eholder/m07-concurrency

C++ Concurrency

Core Question

How do threads communicate?

• Shared State: Mutexes (std::mutex) or Atomics (std::atomic).
• Coordination: Condition Variables (std::condition_variable) or Latches/Semaphores (C++20).
• Tasks: std::async or Coroutines (C++20).

Error → Design Question

| Issue | Design Question | | ----------------- | ------------------------------------------------------------------ | | Data Race | Are two threads accessing memory without a Happens-Before edge? | | Deadlock | Did you lock Mutex A then B, while another thread locked B then A? | | False Sharing | Are independent atomics sitting on the same cache line? | | Live Lock | Are threads spinning without progress? |

Thinking Prompt

1. Do I need a thread?

   • Short task? → std::async or Thread Pool.
   • Long background? → std::jthread.

2. Is it shared state?

   • Yes? → Protect with std::mutex.
   • Is it small/primitive? → std::atomic.

3. Locks or Atomics?

   • Logic requiring multiple steps? → Mutex (Atomic is only for single instructions).
   • Simple flag/counter? → Atomic.

Trace Up / Down

• Trace Up:
  • Issue: "Random update values in counter."
  • Cause: counter++ is Read-Modify-Write, not atomic. Data race.
  • Fix: std::atomic<int>.

Quick Reference

| Tool | C++ Version | Use When | | ----------------------- | ----------- | ----------------------------- | | std::jthread | C++20 | Standard thread (auto-join). | | std::atomic | C++11 | Lock-free counters/flags. | | std::mutex | C++11 | Locking critical sections. | | std::shared_mutex | C++17 | Read-heavy workloads. | | std::latch | C++20 | Waiting for N tasks to start. | | co_await | C++20 | Async I/O (requires library). |