V1089. Waiting on condition variable without predicate. A thread can wait indefinitely or experience a spurious wake-up.

This diagnostic rule is based on the CP.42 CppCoreGuidelines.

The analyzer has detected one of the non-static member functions of the 'std::condition_variable' class template — 'wait', 'wait_for' or 'wait_until' — is called without a predicate. This can lead to a spurious wakeup of a thread or a thread hanging.

Let's consider the example N1 leading to a potential hanging:

#include <iostream>
#include <thread>
#include <mutex>
#include <condition_variable>

std::mutex mtx;
std::condition_variable cond;

void consumer()
{
  std::unique_lock<std::mutex> lck { mtx };
  std::cout << "Waiting... " << std::endl;
  cond.wait(lck);                           // <=
  std::cout << "Working..." << std::endl;
}

void producer()
{
  {
    std::lock_guard<std::mutex> _ { mtx };
    std::cout << "Preparing..." << std::endl;
  }

  cond.notify_one();
}

int main() 
{
  std::thread c { consumer };
  std::thread p { producer };

  c.join();
  p.join();
}

The example contains a race condition. The program can hang if it runs in the following order:

To fix this, we should modify the code as follows:

Here is the fixed example:

#include <iostream>
#include <thread>
#include <mutex>
#include <condition_variable>

std::mutex mtx;
std::condition_variable cond;

bool pendingForWorking = false; // <=

void consumer()
{
  std::unique_lock<std::mutex> lck { mtx };
  std::cout << "Waiting... " << std::endl;
  
  cond.wait(lck, [] { return pendingForWorking; }); // <=
  std::cout << "Working..." << std::endl;
}

void producer()
{
  {
    std::lock_guard<std::mutex> _ { mtx };
    pendingForWorking = true;                 // <=
    std::cout << "Preparing..." << std::endl;
  }

  cond.notify_one();
}

int main() 
{
  std::thread c { consumer };
  std::thread p { producer };

  c.join();
  p.join();
}

Let's consider the example N2 where a spurious wakeup can happen:

#include <iostream>
#include <fstream>
#include <sstream>
#include <queue>
#include <thread>
#include <mutex>
#include <condition_variable>

std::queue<int> queue;
std::mutex mtx;
std::condition_variable cond;

void do_smth(int);

void consumer()
{
  while (true)
  {
    int var;

    {
      using namespace std::literals;
      std::unique_lock<std::mutex> lck { mtx };
      if (cond.wait_for(lck, 10s) == std::cv_status::timeout) // <=
      {
        break;
      }

      var = queue.front();
      queue.pop();
    }

    do_smth(var);
  }
}

void producer(std::istream &in)
{
  int var;
  while (in >> var)
  {
    {
      std::lock_guard<std::mutex> _ { mtx };
      queue.push(var);
    }

    cond.notify_one();
  }
}

void foo(std::ifstream &fin, std::istringstream &sin)
{
  std::thread p1 { &producer, std::ref(fin) };
  std::thread p2 { &producer, std::ref(sin) };
  std::thread p3 { &producer, std::ref(std::cin) };

  std::thread c1 { &consumer };
  std::thread c2 { &consumer };
  std::thread c3 { &consumer };

  p1.join(); p2.join(); p3.join();
  c1.join(); c2.join(); c3.join();
}

A spurious wakeup happens when a waiting thread wakes up and discovers that the condition it was expecting has not been met. This can occur in two scenarios:

In the example N2, a spurious wakeup can occur in threads 'c1', 'c2', and 'c3'. As a result of such a wakeup, the queue may be empty — accessing it, we can get an undefined behavior.

To fix this, we should also call the 'std::condition_variable::wait_for' overload that accepts the predicate. Inside the predicate, we need to check whether the queue is empty or not:

void consumer()
{
  while (true)
  {
    int var;

    {
      using namespace std::literals;
      std::unique_lock<std::mutex> lck { mtx };
      bool res = cond.wait_for(lck,
                               10s,
                               [] { return !queue.empty(); }); // <=
      if (!res)
      {
        break;
      }

      // no spurious wakeup
      var = queue.front();
      queue.pop();
    }

    do_smth(var);
  }
}

This diagnostic is classified as:

You can look at examples of errors detected by the V1089 diagnostic.