Question

I tried using the boost deadline_timer in this simple test application, but had some trouble. The goal is for the timer to trigger every 45 milliseconds using the expires_at() member function of the deadline_timer. (I need an absolute time, so I’m not considering expires_from_now(). I am also not concerned about drift at the moment). When I run the program, wait() does not wait for 45 ms! Yet, no errors are reported. Am I using the library incorrectly somehow?

Sample program:

#include <boost/asio.hpp>
#include <boost/thread.hpp>
#include <boost/bind.hpp>
#include <boost/date_time/posix_time/posix_time.hpp>
#include <iostream>
using namespace std;

int main()
{
        boost::asio::io_service Service;
        boost::shared_ptr<boost::thread> Thread;
        boost::asio::io_service::work RunForever(Service);
        Thread = boost::shared_ptr<boost::thread>(new boost::thread(boost::bind(&boost::asio::io_service::run, &Service)));
        boost::shared_ptr<boost::asio::deadline_timer> Timer(new boost::asio::deadline_timer(Service));

        while(1)
        {
                boost::posix_time::time_duration Duration;
                Duration = boost::posix_time::microseconds(45000);
                boost::posix_time::ptime Start = boost::posix_time::microsec_clock::local_time();
                boost::posix_time::ptime Deadline = Start + Duration;
                boost::system::error_code Error;
                size_t Result = Timer->expires_at(Deadline, Error);
                cout << Result << ' ' << Error << ' ';
                Timer->wait(Error);
                cout << Error << ' ';
                boost::posix_time::ptime End = boost::posix_time::microsec_clock::local_time();
                (cout << "Duration = " << (End - Start).total_milliseconds() << " milliseconds" << endl).flush();
        }
        return 0;
}

Answer 1

You are mixing local time with system time. The time that asio is comparing your local time to is most likely some number of hours after the time that you want your deadline set to so wait returns immediately (depending on where you live; this same code could wait for several hours as well). To avoid this point of confusion, absolute times should be derived from asio::time_traits.

#include <boost/asio.hpp>
#include <boost/asio/time_traits.hpp>
#include <boost/thread.hpp> 
#include <boost/bind.hpp> 
#include <boost/date_time/posix_time/posix_time.hpp> 
#include <iostream> 
using namespace std;

typedef boost::asio::time_traits<boost::posix_time::ptime> time_traits_t;  
int main() {         
    boost::asio::io_service Service;         
    boost::shared_ptr<boost::thread> Thread;         
    boost::asio::io_service::work RunForever(Service);         
    Thread = boost::shared_ptr<boost::thread>(new boost::thread(boost::bind(&boost::asio::io_service::run, &Service)));
    boost::shared_ptr<boost::asio::deadline_timer> Timer(new boost::asio::deadline_timer(Service));          
    while(1)         
    {                 
        boost::posix_time::time_duration Duration;
        Duration = boost::posix_time::microseconds(45000);
        boost::posix_time::ptime Start = time_traits_t::now();
        boost::posix_time::ptime Deadline = Start + Duration;
        boost::system::error_code Error;
        size_t Result = Timer->expires_at(Deadline, Error);
        cout << Result << ' ' << Error << ' ';
        Timer->wait(Error);
        cout << Error << ' ';
        boost::posix_time::ptime End = boost::posix_time::microsec_clock::local_time();
        (cout << "Duration = " << (End - Start).total_milliseconds() << " milliseconds" << endl).flush();
     }         
    return 0; 
}

That should work out for you in this case.