Simple example of threading in C++

Well, technically any such object will wind up being built over a C-style thread library because C++ only just specified a stock std::thread model in C++0x, which was just nailed down and hasn't yet been implemented.

The problem is somewhat systemic. Technically the existing C++ memory model isn't strict enough to allow for well-defined semantics for all of the 'happens before' cases. Hans Boehm wrote an paper on the topic a while back and was instrumental in hammering out the C++0x standard on the topic.

Threads Cannot be Implemented as a Library

That said, there are several cross-platform thread C++ libraries that work just fine in practice. The Intel thread building blocks contains a tbb::thread object that closely approximates the C++0x standard and Boost has a boost::thread library that does the same.

oneAPI Threading Building Blocks
Chapter 19. Thread (Boost documentation)

Using boost::thread, you'd get something like:

#include <boost/thread.hpp>

void task1() {
    // do stuff
}

void task2() {
    // do stuff
}

int main (int argc, char ** argv) {
    using namespace boost;
    thread thread_1 = thread(task1);
    thread thread_2 = thread(task2);

    // do other stuff
    thread_2.join();
    thread_1.join();
    return 0;
}

Create a function that you want the thread to execute, for example:

void task1(std::string msg)
{
    std::cout << "task1 says: " << msg;
}

Now create the thread object that will ultimately invoke the function above like so:

std::thread t1(task1, "Hello");

(You need to #include <thread> to access the std::thread class.)

The constructor's first argument is the function the thread will execute, followed by the function's parameters. The thread is automatically started upon construction.

If later on you want to wait for the thread to be done executing the function, call:

t1.join();

(Joining means that the thread who invoked the new thread will wait for the new thread to finish execution, before it will continue its own execution.)

The Code

#include <string>
#include <iostream>
#include <thread>

using namespace std;

// The function we want to execute on the new thread.
void task1(string msg)
{
    cout << "task1 says: " << msg;
}

int main()
{
    // Constructs the new thread and runs it. Does not block execution.
    thread t1(task1, "Hello");

    // Do other things...

    // Makes the main thread wait for the new thread to finish execution, therefore blocks its own execution.
    t1.join();
}

More information about std::thread here

On GCC, compile with -std=c++0x -pthread.
This should work for any operating-system, granted your compiler supports this (C++11) feature.

There is also a POSIX library for POSIX operating systems.

Check for compatibility:

#include <stdio.h>
#include <stdlib.h>
#include <pthread.h>
#include <iostream>

void *task(void *argument){
    char* msg;
    msg = (char*)argument;
    std::cout << msg << std::endl;
}

int main(){
    pthread_t thread1, thread2;
    int i1, i2;
    i1 = pthread_create(&thread1, NULL, task, (void*) "thread 1");
    i2 = pthread_create(&thread2, NULL, task, (void*) "thread 2");

    pthread_join(thread1, NULL);
    pthread_join(thread2, NULL);
    return 0;
}

Compile with -lpthread.

POSIX Threads

#include <thread>
#include <iostream>
#include <vector>
using namespace std;

void doSomething(int id) {
    cout << id << "\n";
}

/**
 * Spawns n threads
 */
void spawnThreads(int n)
{
    std::vector<thread> threads(n);
    // spawn n threads:
    for (int i = 0; i < n; i++) {
        threads[i] = thread(doSomething, i + 1);
    }

    for (auto& th : threads) {
        th.join();
    }
}

int main()
{
    spawnThreads(10);
}

Simple example of threading in C++

The Code

Tags:

C++

Multithreading

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