通过多个线程提升条件变量

Question

In my program, I have two basic threads. The first one is the main thread and the second is a Tcp server thread. The TCP server will listen for requests ,and for each request it will create a corresponding thread, each of the newly created threads should start working until they reach a certain point where they have to wait for an indication from the main thread. To solve this issue I am implementing a condition variable using Boost 1.49.

My main problem is whenever any of the newly created threads reach the point of the condition variable my whole program freezes.

For more information, please check: Boost 1.49 Condition Variable issue

Until now I didn't receive any positive response, and I am not able to solve the problem.

Thanks a lot.

Answer 1

I haven't looked at your other question (too much code)

In general, you have to await/signal a condition under the corresponding mutex, though.

Here's a demonstration using a group of 10 workers that await a start signal:

• See it Live On Coliru

   #include <boost/thread.hpp> #include <boost/optional/optional_io.hpp> ///////////////////////// // start condition logic boost::mutex mx; boost::condition_variable cv; static bool ok_to_start = false; void await_start_condition() { boost::unique_lock<boost::mutex> lk(mx); cv.wait(lk, [] { return ok_to_start; }); } void signal_start_condition() { boost::lock_guard<boost::mutex> lk(mx); ok_to_start = true; cv.notify_all(); } ///////////////////////// // workers static boost::optional<int> shared_secret; void worker(int id) { await_start_condition(); // demo worker implementation static boost::mutex console_mx; boost::lock_guard<boost::mutex> lk(console_mx); std::cout << "worker " << id << ": secret is " << shared_secret << "\\n"; } int main() { boost::thread_group threads; for (int i = 0; i<10; i++) threads.create_thread(boost::bind(worker, i)); // demo - initialize some state before thread start shared_secret = 42; // signal threads can start signal_start_condition(); // wait for all threads to finish threads.join_all(); } 

• In case of C++03 you can replace the lambda with a hand-written predicate: Live On Coliru

   namespace /* anon detail */ { bool ok_to_start_predicate() { return ok_to_start; } } void await_start_condition() { boost::unique_lock<boost::mutex> lk(mx); cv.wait(lk, ok_to_start_predicate); } 

• Or you can use Boost Lambda/Boost Phoenix to do the trick for you: Live On Coliru

   #include <boost/phoenix.hpp> void await_start_condition() { boost::unique_lock<boost::mutex> lk(mx); cv.wait(lk, boost::phoenix::cref(ok_to_start)); }