This thread example investigates the program flow from two simultaneously running threads.

The code I implemented after the talk given by Hans Boehm [1].

Technical facts

Application type(s)

Desktop application

Operating system(s) or programming environment(s)

Lubuntu 11.04 (natty)

IDE(s):

Qt Creator 2.0.1

Project type:

Console application

C++ standard:

C++11

Compiler(s):

G++ 4.5.2
Borland BCC32.EXE version 6.0.10.157

Libraries used:

STL: GNU ISO C++ Library, version 4.5.2

Qt project file: CppThreadExample3.pro


#-------------------------------------------------

#

# Project created by QtCreator 2011-07-29T16:22:11

#

#-------------------------------------------------

QT       += core

QT       -= gui

QMAKE_CXXFLAGS += -std=c++0x

TARGET = CppThreadExample3

CONFIG   += console

CONFIG   -= app_bundle

TEMPLATE = app

SOURCES += main.cpp

main.cpp


#include <algorithm>

#include <iostream>

#include <thread>



struct Tasks

{

  //static mutex, because each thread its must use the same mutex

  static std::mutex mutex;

  static int x;

  static int y;



  static void Task1()

  {

    x = 1;

    int r = y;



    ///Acquire access to std::cout

    std::lock_guard<std::mutex> lock(mutex);



    ///Write to std::cout

    std::cout << "thread #1, x: " << x << ", copy of y: " << r << ", ";

  }

  static void Task2()

  {

    y = 1;

    int r = x;



    ///Acquire access to std::cout

    std::lock_guard<std::mutex> lock(mutex);



    ///Write to std::cout

    std::cout << "thread #2, y: " << y << ", copy of x: " << r << ", ";

  }

};



std::mutex Tasks::mutex;

int Tasks::x = 0;

int Tasks::y = 0;



int main()

{

  std::thread t1(Tasks::Task1);

  std::thread t2(Tasks::Task2);

  t1.join();

  t2.join();

  std::cout << '\n';

}

CppThreadExample3.sh


#!/bin/bash

for ((i = 0;i<=1000;i++))

do

../CppThreadExample3-build-desktop/./CppThreadExample3 >> output.csv

done

Screen ouput

Screen output varies, so I used the script 'CppThreadExample3.sh' to let the program run a thousand times, so I could tally the output.


197x thread #1, x: 1, copy of y: 0, thread #2, y: 1, copy of x: 0, 

 73x thread #1, x: 1, copy of y: 0, thread #2, y: 1, copy of x: 1, 

242x thread #1, x: 1, copy of y: 1, thread #2, y: 1, copy of x: 0, 

  0x thread #1, x: 1, copy of y: 1, thread #2, y: 1, copy of x: 1, 

270x thread #2, y: 1, copy of x: 0, thread #1, x: 1, copy of y: 0, 

217x thread #2, y: 1, copy of x: 0, thread #1, x: 1, copy of y: 1, 

  2x thread #2, y: 1, copy of x: 1, thread #1, x: 1, copy of y: 0, 

  0x thread #2, y: 1, copy of x: 1, thread #1, x: 1, copy of y: 1,

References

Google TechTalk 'Getting C++ Threads Right' by Hans Boehm

Go back to Richel Bilderbeek's C++ page.

Go back to Richel Bilderbeek's homepage.

(C++) std::thread example 3: program flow