這一篇用一些 concurreny 的範例來學習 C++17 的 thread library

關於 concurrency 的定義可以參考 Better Embedded system software 20 章。

關鍵字

• pthread
• thread id
• address space
• join
• lock free
• Synchronization

正文開始

Since C++ 11, there's thread library available to use, before we had to use 3rd party libs like pthread, some comes with more powerful abstractions like boost, POCO, ... etc.

bits of pthread

• We have to cast thread input/output or use global variable, which we would like to avoid all the time. Why? Once two tasks access a resource concurrently, it must be locked or handled with atomic measure to prevent hazard.

Pthread provides in/out value, but we have to cast them with reinterpret_cast from void.

   pthread_create(&thread1, nullptr, mythread, &in_value);
   pthread_join(thread1, &out_value);
   pthread_self(); // return id
   sleep(1); // for better performance, battery life
   pthread_yield(); // not on unix, often scheduler does a better job than you

Sync

• Race conditions, two tasks working on same resources. Example of race condition: link, here used 8000 thread to show it.

To avoid that, we either use mutex(mutual exclusion, example) to protect a critical section.

pthread_mutexattr_settype(&attr, PTHREAD_MUTEX_RECURSIVE); // to allow lock more than once in a thread.
//conditional variable
pthread_cond_t cond = PTHREAD_COND_INITIALIZER;
while(!predicate) {
        pthread_cond_wait(&cond, &lock);
    }// to make threads run in desired order.

// what we do in other thread
predicate = true; pthread_mutex_unlock(&lock); pthread_cond_signal(&cond);

std threads

• type safety， example

  std::packaged_task<int(int)> task1(some_function); // no more void!
  auto f1 = task1.get_future();
  std::thread t1(std::move(task1), 42);
  t1.join();
  ···
  <threads> also provides supports like:
  

• yield,
• sleep_for(1s)
• std::lock_guard lock(mutex) RAII, while task's out of scope, release the lock automaticllay
• while(!mutex.try_lock()); do something else while witout lock

• lockguard for try_lock

  while(!lock.try_lock()) 
  { 
  std::this_thread::sleep_for(1s);
  }