A condition variable is a synchronization primitive that can be used to block a thread, or multiple threads at the same time,
until another thread both modifies a shared variable (the condition), and notifies the condition variable.
Waiting for a condition variable without a condition can lead to spurious wake-ups or to wait forever.
Noncompliant code example
#include <iostream>
#include <thread>
#include <condition_variable>
std::mutex mutex;
std::condition_variable condVar;
void consumer() {
std::cout << "Waiting for work" << std::endl;
std::unique_lock<std::mutex> lck(mutex);
condVar.wait(lck); // noncompliant: can wait forever as the order between t1 and t2 is not guaranteed
std::cout << "Doing some work" << std::endl;
}
void producer() {
std::cout << "Work submited" << std::endl;
condVar.notify_one(); // this can be executed before or after the wait in consumer, no guarantee
}
int main() {
std::thread t1(consumer);
std::thread t2(producer);
t1.join();
t2.join();
}
Compliant solution
#include <iostream>
#include <thread>
#include <condition_variable>
std::mutex mutex;
std::condition_variable condVar;
bool pendingWork{false};
void consumer() {
std::cout << "Waiting for work" << std::endl;
std::unique_lock<std::mutex> lck(mutex);
condVar.wait(lck, []{ return pendingWork; }); // compliant: if this is called after producer in t2, the call will not block thanks to the condition
std::cout << "Doing some work" << std::endl;
}
void producer() {
{
std::lock_guard<std::mutex> lck(mutex);
pendingWork = true;
}
std::cout << "Work submitted" << std::endl;
condVar.notify_one();
}
int main(){
std::thread t1(consumer);
std::thread t2(producer);
t1.join();
t2.join();
}
