{"id":4781,"date":"2016-06-11T14:24:00","date_gmt":"2016-06-11T14:24:00","guid":{"rendered":"https:\/\/www.modernescpp.com\/index.php\/thread-synchronization-with-condition-variables-or-tasks\/"},"modified":"2023-06-26T12:54:36","modified_gmt":"2023-06-26T12:54:36","slug":"thread-synchronization-with-condition-variables-or-tasks","status":"publish","type":"post","link":"https:\/\/www.modernescpp.com\/index.php\/thread-synchronization-with-condition-variables-or-tasks\/","title":{"rendered":"Thread Synchronization with Condition Variables or Tasks"},"content":{"rendered":"

In case you use promise and future to synchronize threads, they have much in common with condition variables<\/a>. But most of the time, tasks are the better choice.<\/p>\n

<\/p>\n

Synchronization of threads<\/h2>\n

To get the big picture, get the facts. The table compares condition variables with tasks (promise and future).<\/p>\n

\"ConditionVariableVersusTask\"<\/p>\n

The benefit of a condition variable to a promise and future is that you can use condition variables to synchronize threads multiple times. In opposite to that, a promise can send its notification only once. So you have to use more promise and future pairs to get the functionality of a condition variable. But if you use the condition variable only for one synchronization, the condition variable is a lot more challenging to use correctly. So a promise and future pair need no shared variable and, therefore, no lock; they are not prone to spurious <\/a>or lost wakeups<\/a>. In addition to that, they can handle exceptions. So there are a lot of reasons to prefer tasks to condition variables.<\/strong><\/p>\n<\/p>\n

How can I use tasks to synchronize threads?<\/p>\n

<\/p>\n

\n\n\n\n
\n
 1\r\n 2\r\n 3\r\n 4\r\n 5\r\n 6\r\n 7\r\n 8\r\n 9\r\n10\r\n11\r\n12\r\n13\r\n14\r\n15\r\n16\r\n17\r\n18\r\n19\r\n20\r\n21\r\n22\r\n23\r\n24\r\n25\r\n26\r\n27\r\n28\r\n29\r\n30\r\n31\r\n32\r\n33\r\n34\r\n35\r\n36\r\n37\r\n38\r\n39\r\n40\r\n41\r\n42\r\n43<\/pre>\n<\/td>\n
\n
\/\/ promiseFutureSynchronize.cpp<\/span>\r\n\r\n#include <future><\/span>\r\n#include <iostream><\/span>\r\n#include <utility><\/span>\r\n\r\n\r\nvoid<\/span> doTheWork(){\r\n  std::cout << \"Processing shared data.\"<\/span> << std::endl;\r\n}\r\n\r\nvoid<\/span> waitingForWork(std::future<void<\/span>>&& fut){\r\n\r\n    std::cout << \"Worker: Waiting for work.\"<\/span> << std::endl;\r\n    fut.wait();\r\n    doTheWork();\r\n    std::cout << \"Work done.\"<\/span> << std::endl;\r\n\r\n}\r\n\r\nvoid<\/span> setDataReady(std::promise<void<\/span>>&& prom){\r\n\r\n    std::cout << \"Sender: Data is ready.\"<\/span>  << std::endl;\r\n    prom.set_value();\r\n\r\n}\r\n\r\nint<\/span> main(){\r\n\r\n  std::cout << std::endl;\r\n\r\n  std::promise<void<\/span>> sendReady;\r\n  auto<\/span> fut= sendReady.get_future();\r\n\r\n  std::thread<\/span> t1(waitingForWork,std::move(fut));\r\n  std::thread<\/span> t2(setDataReady,std::move(sendReady));\r\n\r\n  t1.join();\r\n  t2.join();\r\n\r\n  std::cout << std::endl;\r\n  \r\n}\r\n<\/pre>\n<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<\/div>\n
 <\/p>\n
Quite easy.<\/p>\n
With the help of the promise sendReady<\/span> (line 32), I get the future fut<\/span> (line 34). The promise signalizes, in this case by his return value void (std::promise<void> sendReady)<\/span>, that it can only send notifications. Both communications endpoints are moved into the threads t1<\/span> respectively t2<\/span> (lines 35 and 36). The future is waiting with its call fut.wait()<\/span> (line 15) for the notification of the promise: prom.set_value()<\/span> (line 24).<\/p>\n
Also, the structure as the program’s output matches the program in den post about the condition variable<\/a>.<\/p>\n
\"promiseFutureSynchronize\"<\/p>\n
What’s next?<\/h2>\n
So that was the overview of the multithreading interface. Now it’s time to have a deeper look into multithreading in C++. Although most programmers will and should never use the features of the C++ memory model, it gives you a deeper insight into the challenges of multithreading programming. The next post<\/a> starts with an overview of the C++ memory model.  (Proofreader Alexey Elymanov<\/strong>)<\/p>\n
 <\/p>\n
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\n
 <\/p>\n<\/dt>\n<\/dl>\n
 <\/p>\n<\/p>\n
<\/span> <\/p>\n","protected":false},"excerpt":{"rendered":"
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