boost asio and condition variables -- strange output - c++

Question

Suggest that I have the following code:
#include <boost/asio/io_service.hpp>
#include <boost/thread.hpp>
#include <condition_variable>
#include <iostream>
#include <mutex>
const int THREAD_POOL_SIZE = 2;
std::condition_variable g_cv;
std::mutex g_cv_mutex;
bool g_answer_ready;
void foo()
{
std::cout << "foo \n";
std::unique_lock<std::mutex> lock(g_cv_mutex);
g_answer_ready = true;
g_cv.notify_all();
}
int main()
{
boost::asio::io_service io_service;
for (int i = 0; i < 10; ++i)
{
std::auto_ptr<boost::asio::io_service::work> work(new boost::asio::io_service::work(io_service));
boost::thread_group threads;
for (int i = 0; i < THREAD_POOL_SIZE; ++i)
{
threads.create_thread(boost::bind(&boost::asio::io_service::run, &io_service));
}
std::unique_lock<std::mutex> lock(g_cv_mutex);
io_service.post(foo);
g_answer_ready = false;
g_cv.wait_for(lock, std::chrono::milliseconds(2000));
if (!g_answer_ready)
{
std::cout << "timed_out \n";
}
io_service.stop();
threads.join_all();
}
}
The output will be different between program's launches. For example,
foo
timed_out
foo
foo
However, if I move boost::asio::io_service object construction inside the loop, it works as expected:
foo
foo
foo
foo
foo
foo
foo
foo
foo
foo
Why? What am I doing wrong? How can I fix it?
boost 1.54, MSVC-11.0

Answer 1

If I understand you correctly, you need to fix last lines on your loop (see comments for descriptions):
// io_service.stop();
// threads.join_all();
work.reset(); // <- signal to process all pending jobs and quit from io_service::run function
threads.join_all(); // <- wait for all threads
io_service.reset(); // <- now `io_service` can accept new tasks
So, there were two issues in the original code:
io_service.stop() will cancel posted but not yet processed jobs (usually this is not what a programmer wants),
io_service.reset() is required to change the state of io_service from "stopped" to "ready to accept new jobs".