C++ Thread Library

 

introverts example in C++

 

static void recharge() {

   //  cout << is not MT Safe, so we need to use locks

    cout << oslock <<  “I recharge by spending time alone.”  << endl << osunlock;

 

}

 

int main(int argc, char *argv[]) {

    cout << “Let’s hear from 6 introverts. ” << endl;

    thread introverts[6];  // objects with a thread routine defaultly constructed into them

    for (thread &t : introverts) {

        t = thread(recharge);     // construct the thread around the recharge function

    }

 

    for (thread &t : introverts) {

        t.join();

    }

    cout << “Everyone is recharged. ” << endl;

    return 0 ;

}

 

Now, we progress from no argument thread routine to one that takes an argument

Greeters example

 

Each greeter has a given id and says hello, id number of times before ending as a thread.

 

static void greet(size_t id) {

    for( size_t count = 0; count < id; count++) {

        cout << oslock << “Greeter # ” << id << ” says \”hello\”” << endl << osunlock;

        sleep_for(random() % 1000000);  // introduce some variation into wait time

    }

 

    cout << oslock << “Greeter # ” << id << ” all done” << endl << osunlock;

}

 

int main() {

    cout << “Welcome to Greetland!” << endl;

    thread greeters[6];

    for(size_t i = 0; i < 6; i++) {

        greeters[i] = thread(greet, i+1);

    }

    for( thread &t : greeters) {

        t.join();

    }

    cout << “All done! ” << endl;

    return 0;

}

 

 

 

static int numTickets = 100;

 

static void agent(size_t id) {

 

    while(numTickets > 0) {

        numTickets–;   // sell a ticket

        cout << oslock << “Sold a ticket…” , id, numTickets << endl << osunlock;

        if (shouldTakeBreak()) {   // introduce randomness to influence thread scheduler

            takeBreak();

        }

    }

}

 

Problem:  numTickets– is not atomic.  What if a thread commits to sell a ticket by going into the while loop and gets swapped off the processor before completely executing numTickets–;  Thread has not flushed out the new value to the global variable: numTickets;

 

While loop test is separated from the decrement.

 

// boilerplate correct main()

int main() {

    thread agents[10];

    for ( size_t i = 0; i < 10; i++) {

        agents[i] = thread(agent, 101+i);

    }

    for (thread &t : agents) {

        t.join();

    }

   return 0;

}