C++ Cookbook

12.3. Notifying One Thread from Another

Problem

You are using a pattern where one thread (or group of threads) does something and it needs to let another thread (or group of threads) know about it. You may have a master thread that is handing out work to slave threads, or you may use one group of threads to populate a queue and another to remove the data from it and do something useful.

Solution

Use mutex and condition objects, declared in boost/thread/mutex.hpp and boost/thread/condition.hpp. You can create a condition for each situation you want threads to wait for, and notify any waiting threads on the condition. Example 12-4 shows how to use signaling in a master/slave threading model.

Example 12-4. Signaling between threads

#include <iostream> #include <boost/thread/thread.hpp> #include <boost/thread/condition.hpp> #include <boost/thread/mutex.hpp> #include <list> #include <string> class Request { /*...*/ }; // A simple job queue class; don't do this, use std::queue template<typename T> class JobQueue { public: JobQueue() {} ~JobQueue() {} void submitJob(const T& x) { boost::mutex::scoped_lock lock(mutex_); list_.push_back(x); workToBeDone_.notify_one(); } T getJob() { boost::mutex::scoped_lock lock(mutex_); workToBeDone_.wait(lock); // Wait until this condition is // satisfied, then lock the mutex T tmp = list_.front(); list_.pop_front(); return(tmp); } private: std::list<T> list_; boost::mutex mutex_; boost::condition workToBeDone_; }; JobQueue<Request> myJobQueue; void boss() ...