By name: m07-concurrency description: "Mastering C++ Concurrency. Triggers: std::thread, jthread, atomic, mutex, deadlock, race condition, memory model, coroutine, async."
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::asyncor 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
Do I need a thread?
- Short task? →
std::asyncor Thread Pool. - Long background? →
std::jthread.
- Short task? →
Is it shared state?
- Yes? → Protect with
std::mutex. - Is it small/primitive? →
std::atomic.
- Yes? → Protect with
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). |