I'm new to c++ and don't know how to let main thread wait for all async threads done. I refered this but makes void consume() not parallel.
#include <iostream>
#include <vector>
#include <unistd.h> // sleep
#include <future>
using namespace std;
class Myclass {
private:
std::vector<int> resources;
std::vector<int> res;
std::mutex resMutex;
std::vector<std::future<void>> m_futures;
public:
Myclass() {
for (int i = 0; i < 10; i++) resources.push_back(i); // add task
res.reserve(resources.size());
}
void consume() {
for (int i = 0; i < resources.size(); i++) {
m_futures.push_back(std::async(std::launch::async, &Myclass::work, this, resources[i]));
// m_futures.back().wait();
}
}
void work(int x) {
sleep(1); // Simulation time-consuming
std::lock_guard<std::mutex> lock(resMutex);
res.push_back(x);
printf("%d be added.---done by %d.\n", x, std::this_thread::get_id());
}
std::vector<int> &getRes() { return res;}
};
int main() {
Myclass obj;
obj.consume();
auto res = obj.getRes();
cout << "Done. res.size = " << res.size() << endl;
for (int i : res) cout << i << " ";
cout <<"main thread over\n";
}
Main thread ends up when res = 0. I want obj.getRes() be be executed when all results be added into res.
Done. res.size = 0
main thread over
4 be added.---done by 6.
9 be added.---done by 11...
You had the right idea with the commented out line: m_futures.back().wait();, you just have it in the wrong place.
As you note, launching a std::async and then waiting for its result right after, forces the entire thing to execute in series and makes the async pointless.
Instead you want two functions: One, like your consume() that launches all the async's, and then another that loops over the futures and calls wait (or get, whatever suits your needs) on them - and then call that from main.
This lets them all run in parallel, while still making main wait for the final result.
Addition to #Frodyne 's answer,
consume() function calls are parallel, and main thread waits for the all consume() s have their work done;
void set_wait(void)
{
for (int i = 0; i < resources.size(); i++) {
m_futures[i].wait();
}
}
And call it here
void consume() {
for (int i = 0; i < resources.size(); i++) {
m_futures.push_back(std::async(std::launch::async, &Myclass::work, this, resources[i]));
// Calling wait() here makes no sense
}
set_wait(); // Waits for all threads do work
}
I created new function for convenience.
You can use std::future:wait after you add task to m_futures. Example.
void consume() {
for (int i = 0; i < resources.size(); i++) {
m_futures.push_back(std::async(std::launch::async, &Myclass::work, this, resources[i]));
//m_futures.back().wait();
}
for(auto& f: m_futures) f.wait();
}
Thread newbie here. In the following code, I want to add elements to the global list using one thread and search for random elements using another thread.
#include <list>
#include <algorithm>
#include <mutex>
#include <thread>
using namespace std;
list<int> some_list;
mutex some_mutex;
void add_to_list(int new_value)
{
lock_guard<mutex> guard(some_mutex);
some_list.push_back(new_value);
}
bool list_contains(int value_to_find)
{
lock_guard<mutex> guard(some_mutex);
return find(begin(some_list), end(some_list), value_to_find) != end(some_list);
}
int main()
{
for(int i = 0; i < 100; ++i)
{
// Add i to some_list through one thread only
thread t(add_to_list, i);
t.detach();
// Search elements in different thread
thread t2(list_contains, i);
t2.detach();
}
return 0;
}
However, when I pass i along with add_to_list then that many no of threads are created. I want to add the elements to the list using single thread only. How to do this? Do, I need to pass vector of elements to the thread instead of variable?
First of all, don't use detach(). In the code above the program starts 200 threads and ends (when main() returns) without waiting for any of them to finish. Use join() instead, this will allow you to wait for the thread completion properly.
Then, if you want add_to_list to be done sequentially, then do that in a separate loop.
For example:
int main() {
vector<thread> threads;
// Add i to some_list through one thread only
threads.emplace_back([] {
for (int i = 0; i < 100; ++i) {
add_to_list(i);
}
});
for (int i = 0; i < 100; ++i) {
// Search elements in different threads
threads.emplace_back([i] {
list_contains(i);
});
}
// Join all threads (waits for their completion)
for (auto& t : threads) {
t.join();
}
}
I've been trying to solve a problem concurrently, which fits the thread pool pattern very nicely. Here I will try to provide a minimal representative example:
Say we have a pseudo-program like this:
Q : collection<int>
while (!Q.empty()) {
for each q in Q {
// perform some computation
}
// assign a new value to Q
Q = something_completely_new();
}
I'm trying to implement that in a parallel way, with n-1 workers and one main thread. The workers will perform the computation in the inner loop by grabbing elements from Q.
I tried to solve this using two conditional variables, work, on which the master threads notifies the workers that Q has been assigned to, and another, work_done, where the workers notify master that the entire computation might be done.
Here's my C++ code:
#include <iostream>
#include <mutex>
#include <condition_variable>
#include <queue>
#include <thread>
using namespace std;
std::queue<int> Q;
std::mutex mut;
std::condition_variable work;
std::condition_variable work_done;
void run_thread() {
for (;;) {
std::unique_lock<std::mutex> lock(mut);
work.wait(lock, [&] { return Q.size() > 0; });
// there is work to be done - pretend we're working on something
int x = Q.front(); Q.pop();
std::cout << "Working on " << x << std::endl;
work_done.notify_one();
}
}
int main() {
// your code goes here
std::vector<std::thread *> workers(3);
for (size_t i = 0; i < 3; i++) {
workers[i] = new std::thread{
[&] { run_thread(); }
};
}
for (int i = 4; i > 0; --i) {
std::unique_lock<std::mutex> lock(mut);
Q = std::queue<int>();
for (int k = 0; k < i; k++) {
Q.push(k);
}
work.notify_all();
work_done.wait(lock, [&] { return Q.size() == 0; });
}
for (size_t i = 0; i < 3; i++) {
delete workers[i];
}
return 0;
}
Unfortunately, after compiling it on OS X with g++ -std=c++11 -Wall -o main main.cpp I get the following output:
Working on 0
Working on 1
Working on 2
Working on 3
Working on 0
Working on 1
Working on 2
Working on 0
Working on 1
Working on 0
libc++abi.dylib: terminating
Abort trap: 6
After a while of googling it looks like a segmentation fault. It probably has to do with me misusing conditional variables. I would appreciate some insight, both architectural (on how to approach this type of problem) and specific, as in what I'm doing wrong here exactly.
I appreciate the help
Your application was killed by std::terminate.
Body of your thread function is infinite-loop, so when these lines are executed
for (size_t i = 0; i < 3; i++) {
delete workers[i];
}
you want to delete threads which are still running (each thread is in joinable state). When you call destructor of thread which is in joinable state the following thing happens (from http://www.cplusplus.com/reference/thread/thread/~thread/)
If the thread is joinable when destroyed, terminate() is called.
so if you want terminate not to be called, you should call detach() method after creating threads.
for (size_t i = 0; i < 3; i++) {
workers[i] = new std::thread{
[&] { run_thread(); }
};
workers[i]->detach(); // <---
}
Just because the queue is empty doesn't mean the work is done.
finished = true;
work.notify_all();
for (size_t i = 0; i < 3; i++) {
workers[i].join(); // wait for threads to finish
delete workers[i];
}
and we need some way to terminate the threads
for (;!finshed;) {
std::unique_lock<std::mutex> lock(mut);
work.wait(lock, [&] { return Q.size() > 0 || finished; });
if (finished)
return;
I'm trying to stop multiple worker threads using a std::atomic_flag. Starting from Issue using std::atomic_flag with worker thread the following works:
#include <iostream>
#include <atomic>
#include <chrono>
#include <thread>
std::atomic_flag continueFlag;
std::thread t;
void work()
{
while (continueFlag.test_and_set(std::memory_order_relaxed)) {
std::cout << "work ";
std::this_thread::sleep_for(std::chrono::milliseconds(10));
}
}
void start()
{
continueFlag.test_and_set(std::memory_order_relaxed);
t = std::thread(&work);
}
void stop()
{
continueFlag.clear(std::memory_order_relaxed);
t.join();
}
int main()
{
std::cout << "Start" << std::endl;
start();
std::this_thread::sleep_for(std::chrono::milliseconds(200));
std::cout << "Stop" << std::endl;
stop();
std::cout << "Stopped." << std::endl;
return 0;
}
Trying to rewrite into multiple worker threads:
#include <iostream>
#include <atomic>
#include <chrono>
#include <thread>
#include <vector>
#include <memory>
struct thread_data {
std::atomic_flag continueFlag;
std::thread thread;
};
std::vector<thread_data> threads;
void work(int threadNum, std::atomic_flag &continueFlag)
{
while (continueFlag.test_and_set(std::memory_order_relaxed)) {
std::cout << "work" << threadNum << " ";
std::this_thread::sleep_for(std::chrono::milliseconds(10));
}
}
void start()
{
const unsigned int numThreads = 2;
for (int i = 0; i < numThreads; i++) {
////////////////////////////////////////////////////////////////////
//PROBLEM SECTOR
////////////////////////////////////////////////////////////////////
thread_data td;
td.continueFlag.test_and_set(std::memory_order_relaxed);
td.thread = std::thread(&work, i, td.continueFlag);
threads.push_back(std::move(td));
////////////////////////////////////////////////////////////////////
//PROBLEM SECTOR
////////////////////////////////////////////////////////////////////
}
}
void stop()
{
//Flag stop
for (auto &data : threads) {
data.continueFlag.clear(std::memory_order_relaxed);
}
//Join
for (auto &data : threads) {
data.thread.join();
}
threads.clear();
}
int main()
{
std::cout << "Start" << std::endl;
start();
std::this_thread::sleep_for(std::chrono::milliseconds(200));
std::cout << "Stop" << std::endl;
stop();
std::cout << "Stopped." << std::endl;
return 0;
}
My issue is "Problem Sector" in above. Namely creating the threads. I cannot wrap my head around how to instantiate the threads and passing the variables to the work thread.
The error right now is referencing this line threads.push_back(std::move(td)); with error Error C2280 'thread_data::thread_data(const thread_data &)': attempting to reference a deleted function.
Trying to use unique_ptr like this:
auto td = std::make_unique<thread_data>();
td->continueFlag.test_and_set(std::memory_order_relaxed);
td->thread = std::thread(&work, i, td->continueFlag);
threads.push_back(std::move(td));
Gives error std::atomic_flag::atomic_flag(const std::atomic_flag &)': attempting to reference a deleted function at line td->thread = std::thread(&work, i, td->continueFlag);. Am I fundamentally misunderstanding the use of std::atomic_flag? Is it really both immovable and uncopyable?
Your first approach was actually closer to the truth. The problem is that it passed a reference to an object within the local for loop scope to each thread, as a parameter. But, of course, once the loop iteration ended, that object went out of scope and got destroyed, leaving each thread with a reference to a destroyed object, resulting in undefined behavior.
Nobody cared about the fact that you moved the object into the std::vector, after creating the thread. The thread received a reference to a locally-scoped object, and that's all it knew. End of story.
Moving the object into the vector first, and then passing to each thread a reference to the object in the std::vector will not work either. As soon as the vector internally reallocates, as part of its natural growth, you'll be in the same pickle.
What needs to happen is to have the entire threads array created first, before actually starting any std::threads. If the RAII principle is religiously followed, that means nothing more than a simple call to std::vector::resize().
Then, in a second loop, iterate over the fully-cooked threads array, and go and spawn off a std::thread for each element in the array.
I was almost there with my unique_ptr solution. I just needed to pass the call as a std::ref() as such:
std::vector<std::unique_ptr<thread_data>> threads;
void start()
{
const unsigned int numThreads = 2;
for (int i = 0; i < numThreads; i++) {
auto td = std::make_unique<thread_data>();
td->continueFlag.test_and_set(std::memory_order_relaxed);
td->thread = std::thread(&work, i, std::ref(td->continueFlag));
threads.push_back(std::move(td));
}
}
However, inspired by Sam above I also figured a non-pointer way:
std::vector<thread_data> threads;
void start()
{
const unsigned int numThreads = 2;
//create new vector, resize doesn't work as it tries to assign/copy which atomic_flag
//does not support
threads = std::vector<thread_data>(numThreads);
for (int i = 0; i < numThreads; i++) {
auto& t = threads.at(i);
t.continueFlag.test_and_set(std::memory_order_relaxed);
t.thread = std::thread(&work, i, std::ref(t.continueFlag));
}
}
Relevant questions:
About C++11:
C++11: std::thread pooled?
Will async(launch::async) in C++11 make thread pools obsolete for avoiding expensive thread creation?
About Boost:
C++ boost thread reusing threads
boost::thread and creating a pool of them!
How do I get a pool of threads to send tasks to, without creating and deleting them over and over again? This means persistent threads to resynchronize without joining.
I have code that looks like this:
namespace {
std::vector<std::thread> workers;
int total = 4;
int arr[4] = {0};
void each_thread_does(int i) {
arr[i] += 2;
}
}
int main(int argc, char *argv[]) {
for (int i = 0; i < 8; ++i) { // for 8 iterations,
for (int j = 0; j < 4; ++j) {
workers.push_back(std::thread(each_thread_does, j));
}
for (std::thread &t: workers) {
if (t.joinable()) {
t.join();
}
}
arr[4] = std::min_element(arr, arr+4);
}
return 0;
}
Instead of creating and joining threads each iteration, I'd prefer to send tasks to my worker threads each iteration and only create them once.
This is adapted from my answer to another very similar post.
Let's build a ThreadPool class:
class ThreadPool {
public:
void Start();
void QueueJob(const std::function<void()>& job);
void Stop();
void busy();
private:
void ThreadLoop();
bool should_terminate = false; // Tells threads to stop looking for jobs
std::mutex queue_mutex; // Prevents data races to the job queue
std::condition_variable mutex_condition; // Allows threads to wait on new jobs or termination
std::vector<std::thread> threads;
std::queue<std::function<void()>> jobs;
};
ThreadPool::Start
For an efficient threadpool implementation, once threads are created according to num_threads, it's better not to
create new ones or destroy old ones (by joining). There will be a performance penalty, and it might even make your
application go slower than the serial version. Thus, we keep a pool of threads that can be used at any time (if they
aren't already running a job).
Each thread should be running its own infinite loop, constantly waiting for new tasks to grab and run.
void ThreadPool::Start() {
const uint32_t num_threads = std::thread::hardware_concurrency(); // Max # of threads the system supports
threads.resize(num_threads);
for (uint32_t i = 0; i < num_threads; i++) {
threads.at(i) = std::thread(ThreadLoop);
}
}
ThreadPool::ThreadLoop
The infinite loop function. This is a while (true) loop waiting for the task queue to open up.
void ThreadPool::ThreadLoop() {
while (true) {
std::function<void()> job;
{
std::unique_lock<std::mutex> lock(queue_mutex);
mutex_condition.wait(lock, [this] {
return !jobs.empty() || should_terminate;
});
if (should_terminate) {
return;
}
job = jobs.front();
jobs.pop();
}
job();
}
}
ThreadPool::QueueJob
Add a new job to the pool; use a lock so that there isn't a data race.
void ThreadPool::QueueJob(const std::function<void()>& job) {
{
std::unique_lock<std::mutex> lock(queue_mutex);
jobs.push(job);
}
mutex_condition.notify_one();
}
To use it:
thread_pool->QueueJob([] { /* ... */ });
ThreadPool::busy
void ThreadPool::busy() {
bool poolbusy;
{
std::unique_lock<std::mutex> lock(queue_mutex);
poolbusy = jobs.empty();
}
return poolbusy;
}
The busy() function can be used in a while loop, such that the main thread can wait the threadpool to complete all the tasks before calling the threadpool destructor.
ThreadPool::Stop
Stop the pool.
void ThreadPool::Stop() {
{
std::unique_lock<std::mutex> lock(queue_mutex);
should_terminate = true;
}
mutex_condition.notify_all();
for (std::thread& active_thread : threads) {
active_thread.join();
}
threads.clear();
}
Once you integrate these ingredients, you have your own dynamic threading pool. These threads always run, waiting for
job to do.
I apologize if there are some syntax errors, I typed this code and and I have a bad memory. Sorry that I cannot provide
you the complete thread pool code; that would violate my job integrity.
Notes:
The anonymous code blocks are used so that when they are exited, the std::unique_lock variables created within them
go out of scope, unlocking the mutex.
ThreadPool::Stop will not terminate any currently running jobs, it just waits for them to finish via active_thread.join().
You can use C++ Thread Pool Library, https://github.com/vit-vit/ctpl.
Then the code your wrote can be replaced with the following
#include <ctpl.h> // or <ctpl_stl.h> if ou do not have Boost library
int main (int argc, char *argv[]) {
ctpl::thread_pool p(2 /* two threads in the pool */);
int arr[4] = {0};
std::vector<std::future<void>> results(4);
for (int i = 0; i < 8; ++i) { // for 8 iterations,
for (int j = 0; j < 4; ++j) {
results[j] = p.push([&arr, j](int){ arr[j] +=2; });
}
for (int j = 0; j < 4; ++j) {
results[j].get();
}
arr[4] = std::min_element(arr, arr + 4);
}
}
You will get the desired number of threads and will not create and delete them over and over again on the iterations.
A pool of threads means that all your threads are running, all the time – in other words, the thread function never returns. To give the threads something meaningful to do, you have to design a system of inter-thread communication, both for the purpose of telling the thread that there's something to do, as well as for communicating the actual work data.
Typically this will involve some kind of concurrent data structure, and each thread would presumably sleep on some kind of condition variable, which would be notified when there's work to do. Upon receiving the notification, one or several of the threads wake up, recover a task from the concurrent data structure, process it, and store the result in an analogous fashion.
The thread would then go on to check whether there's even more work to do, and if not go back to sleep.
The upshot is that you have to design all this yourself, since there isn't a natural notion of "work" that's universally applicable. It's quite a bit of work, and there are some subtle issues you have to get right. (You can program in Go if you like a system which takes care of thread management for you behind the scenes.)
A threadpool is at core a set of threads all bound to a function working as an event loop. These threads will endlessly wait for a task to be executed, or their own termination.
The threadpool job is to provide an interface to submit jobs, define (and perhaps modify) the policy of running these jobs (scheduling rules, thread instantiation, size of the pool), and monitor the status of the threads and related resources.
So for a versatile pool, one must start by defining what a task is, how it is launched, interrupted, what is the result (see the notion of promise and future for that question), what sort of events the threads will have to respond to, how they will handle them, how these events shall be discriminated from the ones handled by the tasks. This can become quite complicated as you can see, and impose restrictions on how the threads will work, as the solution becomes more and more involved.
The current tooling for handling events is fairly barebones(*): primitives like mutexes, condition variables, and a few abstractions on top of that (locks, barriers). But in some cases, these abstrations may turn out to be unfit (see this related question), and one must revert to using the primitives.
Other problems have to be managed too:
signal
i/o
hardware (processor affinity, heterogenous setup)
How would these play out in your setting?
This answer to a similar question points to an existing implementation meant for boost and the stl.
I offered a very crude implementation of a threadpool for another question, which doesn't address many problems outlined above. You might want to build up on it. You might also want to have a look of existing frameworks in other languages, to find inspiration.
(*) I don't see that as a problem, quite to the contrary. I think it's the very spirit of C++ inherited from C.
Follwoing [PhD EcE](https://stackoverflow.com/users/3818417/phd-ece) suggestion, I implemented the thread pool:
function_pool.h
#pragma once
#include <queue>
#include <functional>
#include <mutex>
#include <condition_variable>
#include <atomic>
#include <cassert>
class Function_pool
{
private:
std::queue<std::function<void()>> m_function_queue;
std::mutex m_lock;
std::condition_variable m_data_condition;
std::atomic<bool> m_accept_functions;
public:
Function_pool();
~Function_pool();
void push(std::function<void()> func);
void done();
void infinite_loop_func();
};
function_pool.cpp
#include "function_pool.h"
Function_pool::Function_pool() : m_function_queue(), m_lock(), m_data_condition(), m_accept_functions(true)
{
}
Function_pool::~Function_pool()
{
}
void Function_pool::push(std::function<void()> func)
{
std::unique_lock<std::mutex> lock(m_lock);
m_function_queue.push(func);
// when we send the notification immediately, the consumer will try to get the lock , so unlock asap
lock.unlock();
m_data_condition.notify_one();
}
void Function_pool::done()
{
std::unique_lock<std::mutex> lock(m_lock);
m_accept_functions = false;
lock.unlock();
// when we send the notification immediately, the consumer will try to get the lock , so unlock asap
m_data_condition.notify_all();
//notify all waiting threads.
}
void Function_pool::infinite_loop_func()
{
std::function<void()> func;
while (true)
{
{
std::unique_lock<std::mutex> lock(m_lock);
m_data_condition.wait(lock, [this]() {return !m_function_queue.empty() || !m_accept_functions; });
if (!m_accept_functions && m_function_queue.empty())
{
//lock will be release automatically.
//finish the thread loop and let it join in the main thread.
return;
}
func = m_function_queue.front();
m_function_queue.pop();
//release the lock
}
func();
}
}
main.cpp
#include "function_pool.h"
#include <string>
#include <iostream>
#include <mutex>
#include <functional>
#include <thread>
#include <vector>
Function_pool func_pool;
class quit_worker_exception : public std::exception {};
void example_function()
{
std::cout << "bla" << std::endl;
}
int main()
{
std::cout << "stating operation" << std::endl;
int num_threads = std::thread::hardware_concurrency();
std::cout << "number of threads = " << num_threads << std::endl;
std::vector<std::thread> thread_pool;
for (int i = 0; i < num_threads; i++)
{
thread_pool.push_back(std::thread(&Function_pool::infinite_loop_func, &func_pool));
}
//here we should send our functions
for (int i = 0; i < 50; i++)
{
func_pool.push(example_function);
}
func_pool.done();
for (unsigned int i = 0; i < thread_pool.size(); i++)
{
thread_pool.at(i).join();
}
}
You can use thread_pool from boost library:
void my_task(){...}
int main(){
int threadNumbers = thread::hardware_concurrency();
boost::asio::thread_pool pool(threadNumbers);
// Submit a function to the pool.
boost::asio::post(pool, my_task);
// Submit a lambda object to the pool.
boost::asio::post(pool, []() {
...
});
}
You also can use threadpool from open source community:
void first_task() {...}
void second_task() {...}
int main(){
int threadNumbers = thread::hardware_concurrency();
pool tp(threadNumbers);
// Add some tasks to the pool.
tp.schedule(&first_task);
tp.schedule(&second_task);
}
Something like this might help (taken from a working app).
#include <memory>
#include <boost/asio.hpp>
#include <boost/thread.hpp>
struct thread_pool {
typedef std::unique_ptr<boost::asio::io_service::work> asio_worker;
thread_pool(int threads) :service(), service_worker(new asio_worker::element_type(service)) {
for (int i = 0; i < threads; ++i) {
auto worker = [this] { return service.run(); };
grp.add_thread(new boost::thread(worker));
}
}
template<class F>
void enqueue(F f) {
service.post(f);
}
~thread_pool() {
service_worker.reset();
grp.join_all();
service.stop();
}
private:
boost::asio::io_service service;
asio_worker service_worker;
boost::thread_group grp;
};
You can use it like this:
thread_pool pool(2);
pool.enqueue([] {
std::cout << "Hello from Task 1\n";
});
pool.enqueue([] {
std::cout << "Hello from Task 2\n";
});
Keep in mind that reinventing an efficient asynchronous queuing mechanism is not trivial.
Boost::asio::io_service is a very efficient implementation, or actually is a collection of platform-specific wrappers (e.g. it wraps I/O completion ports on Windows).
Edit: This now requires C++17 and concepts. (As of 9/12/16, only g++ 6.0+ is sufficient.)
The template deduction is a lot more accurate because of it, though, so it's worth the effort of getting a newer compiler. I've not yet found a function that requires explicit template arguments.
It also now takes any appropriate callable object (and is still statically typesafe!!!).
It also now includes an optional green threading priority thread pool using the same API. This class is POSIX only, though. It uses the ucontext_t API for userspace task switching.
I created a simple library for this. An example of usage is given below. (I'm answering this because it was one of the things I found before I decided it was necessary to write it myself.)
bool is_prime(int n){
// Determine if n is prime.
}
int main(){
thread_pool pool(8); // 8 threads
list<future<bool>> results;
for(int n = 2;n < 10000;n++){
// Submit a job to the pool.
results.emplace_back(pool.async(is_prime, n));
}
int n = 2;
for(auto i = results.begin();i != results.end();i++, n++){
// i is an iterator pointing to a future representing the result of is_prime(n)
cout << n << " ";
bool prime = i->get(); // Wait for the task is_prime(n) to finish and get the result.
if(prime)
cout << "is prime";
else
cout << "is not prime";
cout << endl;
}
}
You can pass async any function with any (or void) return value and any (or no) arguments and it will return a corresponding std::future. To get the result (or just wait until a task has completed) you call get() on the future.
Here's the github: https://github.com/Tyler-Hardin/thread_pool.
looks like threadpool is very popular problem/exercise :-)
I recently wrote one in modern C++; it’s owned by me and publicly available here - https://github.com/yurir-dev/threadpool
It supports templated return values, core pinning, ordering of some tasks.
all implementation in two .h files.
So, the original question will be something like this:
#include "tp/threadpool.h"
int arr[5] = { 0 };
concurency::threadPool<void> tp;
tp.start(std::thread::hardware_concurrency());
std::vector<std::future<void>> futures;
for (int i = 0; i < 8; ++i) { // for 8 iterations,
for (int j = 0; j < 4; ++j) {
futures.push_back(tp.push([&arr, j]() {
arr[j] += 2;
}));
}
}
// wait until all pushed tasks are finished.
for (auto& f : futures)
f.get();
// or just tp.end(); // will kill all the threads
arr[4] = *std::min_element(arr, arr + 4);
I found the pending tasks' future.get() call hangs on caller side if the thread pool gets terminated and leaves some tasks inside task queue. How to set future exception inside thread pool with only the wrapper std::function?
template <class F, class... Args>
std::future<std::result_of_t<F(Args...)>> enqueue(F &&f, Args &&...args) {
auto task = std::make_shared<std::packaged_task<std::result_of_t<F(Args...)>()>>(
std::bind(std::forward<F>(f), std::forward<Args>(args)...));
std::future<return_type> res = task->get_future();
{
std::unique_lock<std::mutex> lock(_mutex);
_tasks.push([task]() -> void { (*task)(); });
}
return res;
}
class StdThreadPool {
std::vector<std::thread> _workers;
std::priority_queue<TASK> _tasks;
...
}
struct TASK {
//int _func_return_value;
std::function<void()> _func;
int priority;
...
}
The Stroika library has a threadpool implementation.
Stroika ThreadPool.h
ThreadPool p;
p.AddTask ([] () {doIt ();});
Stroika's thread library also supports cancelation (cooperative) - so that when the ThreadPool above goes out of scope - it cancels any running tasks (similar to c++20's jthread).