Thread Synchronization with Condition Variables or Tasks

Contents[Show]

In case you are using  promise and future to synchronize threads, they have a lot in common with condition variables. But most of the time, task are the better choice.

Synchronization of threads

To get the big picture, get the facts. The table compares condition variables with tasks (promise and future).

ConditionVariableVersusTask

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 in case you use the condition variable only for one synchronization, the condition variable is a lot more difficult to use in the right way. So a promise and future pair needs no shared variable and therefore no lock, they are not prone to spurious wakeups or lost wakeups. In addition to that, they can handle exceptions. So there are a lot of reasons to prefer tasks to condition variables.

How can I use tasks to synchronize threads?

 1
 2
 3
 4
 5
 6
 7
 8
 9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
// promiseFutureSynchronize.cpp

#include <future>
#include <iostream>
#include <utility>


void doTheWork(){
  std::cout << "Processing shared data." << std::endl;
}

void waitingForWork(std::future<void>&& fut){

    std::cout << "Worker: Waiting for work." << std::endl;
    fut.wait();
    doTheWork();
    std::cout << "Work done." << std::endl;

}

void setDataReady(std::promise<void>&& prom){

    std::cout << "Sender: Data is ready."  << std::endl;
    prom.set_value();

}

int main(){

  std::cout << std::endl;

  std::promise<void> sendReady;
  auto fut= sendReady.get_future();

  std::thread t1(waitingForWork,std::move(fut));
  std::thread t2(setDataReady,std::move(sendReady));

  t1.join();
  t2.join();

  std::cout << std::endl;
  
}

 

Quite easy.

With the help of the promise sendReady (line32) I get the future fut (line34). The promise signalises in this case by his return value void (std::promise<void> sendReady), that it is only capable of sending notifications. Both communications endpoints are moved into the thread t1 respectively t2 (line 35 and 36). The future is waiting with its call fut.wait() (line 15) for the notification of the promise: prom.set_value() (line 24).

As well the structure as the output of the program matches the program in den post about the condition variable.

promiseFutureSynchronize

What's next?

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 C++ memory model, it gives you a deeper insight into the challenges of multithreading programming. The next post starts with an overview of the C++ memory model.  (Proofreader Alexey Elymanov)

 

 

 

Thanks a lot to my Patreon Supporters: Matt Braun, Roman Postanciuc, Tobias Zindl, Marko, G Prvulovic, Reinhold Dröge, Abernitzke, Frank Grimm, Sakib, Broeserl, António Pina, Sergey Agafyin, Андрей Бурмистров, Jake, GS, Lawton Shoemake, Animus24, Jozo Leko, John Breland, espkk, Louis St-Amour, Venkat Nandam, Jose Francisco, Douglas Tinkham, Kuchlong Kuchlong, Robert Blanch, Truels Wissneth, Kris Kafka, Mario Luoni, Neil Wang, Friedrich Huber, lennonli, Pramod Tikare Muralidhara, Peter Ware, Tobi Heideman, Daniel Hufschläger, Red Trip, Alexander Schwarz, Tornike Porchxidze, Alessandro Pezzato, Evangelos Denaxas, Bob Perry, Satish Vangipuram, Andi Ireland, Richard Ohnemus, Satish Vangipuram, and Michael Dunsky.

 

Thanks in particular to Jon Hess, Lakshman, Christian Wittenhorst, Sherhy Pyton, Dendi Suhubdy, Sudhakar Belagurusamy, Richard Sargeant, Rusty Fleming, and Said Mert Turkal.

 

 

My special thanks to Embarcadero CBUIDER STUDIO FINAL ICONS 1024 Small

 

Seminars

I'm happy to give online seminars or face-to-face seminars worldwide. Please call me if you have any questions.

Bookable (Online)

German

Standard Seminars (English/German)

Here is a compilation of my standard seminars. These seminars are only meant to give you a first orientation.

New

Contact Me

Modernes C++,

RainerGrimmSmall

 

Comments   

0 #21 Hanna 2017-03-05 19:23
Everything is very open with a very clear description of
the challenges. It was definitely informative. Your website is useful.
Many thanks for sharing!
Quote
0 #22 Katia 2017-09-12 01:25
This iis really interesting, You're a very skilled
blogger. I have joined your rss feed aand look forward to
seeking more of your great post. Also, I have shared your
website in my social networks!
Quote

My Newest E-Books

Course: Modern C++ Concurrency in Practice

Course: C++ Standard Library including C++14 & C++17

Course: Embedded Programming with Modern C++

Course: Generic Programming (Templates)

Course: C++ Fundamentals for Professionals

Interactive Course: The All-in-One Guide to C++20

Subscribe to the newsletter (+ pdf bundle)

Blog archive

Source Code

Visitors

Today 7339

Yesterday 7224

Week 29338

Month 34603

All 6892812

Currently are 188 guests and no members online

Kubik-Rubik Joomla! Extensions

Latest comments