C++ threads

1. What is a multithread

    Multithreading allows our computer to run two or more programs concurrently. 

    Multithreading is different from multiprogram.

2. How the c++ language and OS support the multithread

    C++ does not contain built-in support for multithreaded applications until C++11. It relies entirely upon POSIX which is provided by OS.

    The std::thread is implemented on top of pthreads in an environment supporting pthreads. The std::thread library includes many abstract features.

3. A bad example

The problem of the following code is that the main thread create a thread t1, and the main thread does not wait for t1 thread termination. It continues its work. The main thread finishes execution, but t1 is still running. So a run time error happens. All the threads must be terminated before the main thread is terminated. 

#include

#include

using namespace std;

void threadFunc(){

  cout << "Thread Func" << endl;

}

int main(){

  thread t1(threadFunc);

  return 0;

}

 

so we need the following code: 

 

int main(){

 

  thread t1(threadFunc);

  t1.join();    // main thread is blocked until t1 is not finished

 

  return 0;

 

}

 

4. Look at some APIs of threads

std::thread join

void join(); Blocks the current thread until the thread identified by *this finishes its execution.

who is the current thread?   The main thread.

who is the "identified by *this" ?  The thread t1

 

5. Creating threads using library thread from C++11( the following code is from https://en.cppreference.com/w/cpp/thread/thread/thread)

    Command to compile the following code in my computer is $ g++ -std=c++17 hello.cpp -lpthread

    #include
    #include
    #include   // for 
    #include
  
    void f1(int n){
    for (int i = 0; i < 5; ++i) {
      std::cout << "Thread 1 executing\n";
      ++n;
      std::this_thread::sleep_for(std::chrono::milliseconds(10));
    }
  }

  void f2(int& n){
    for (int i = 0; i < 5; ++i) {
      std::cout << "Thread 2 executing\n";
      ++n;
      std::this_thread::sleep_for(std::chrono::milliseconds(10));
    }
  }

  class foo{
  public:
    void bar(){
      for (int i = 0; i < 5; ++i) {
        std::cout << "Thread 3 executing\n";
        ++n;
        std::this_thread::sleep_for(std::chrono::milliseconds(10));
      }
    }
    int n = 0;
  };

  class baz{
  public:
    void operator()(){
      for (int i = 0; i < 5; ++i) {
        std::cout << "Thread 4 executing\n";
        ++n;
        std::this_thread::sleep_for(std::chrono::milliseconds(10));
      }
    }
    int n = 0;
  };

  int main(){
    int n = 0;
    foo f;
    baz b;
    std::thread t1;            // t1 is not a thread of execution
    std::thread t2(f1, n + 1);     // pass by value
    std::thread t3(f2, std::ref(n)); // pass by reference
    std::thread t4(std::move(t3));  // t4 is now running f2(). t3 is no longer a thread

    // t5 runs foo::bar() on object f, we need the function ref, and the object ref.
    std::thread t5(&foo::bar, &f); 

    // t6 runs baz::operator() on object b, we need a object and its operator()
    std::thread t6(b); 


    t2.join();
    t4.join();
    t5.join();
    t6.join();
    std::cout << "Final value of n is " << n << '\n';
    std::cout << "Final value of foo::n is " << f.n << '\n';
  }

 

 

6. Creating a thread using POSIX

The API of pthread : https://pubs.opengroup.org/onlinepubs/7908799/xsh/pthread.h.html

A good source talking about the pthread: https://www.cs.cmu.edu/afs/cs/academic/class/15492-f07/www/pthreads.html

 

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