Re: [Boost-users] std::future::wait_for() not working in Boost UDP socket async receive operation

Subject: Re: [Boost-users] std::future::wait_for() not working in Boost UDP socket async receive operation
From: TONGARI J (tongari95_at_[hidden])
Date: 2016-11-28 22:26:59

2016-11-29 5:55 GMT+08:00 <jc.francois_at_[hidden]>:

> The good people over at stackoverflow were not able to help me with this
> issue that I expected to be trivial.
>
> I am writing a UDP server app with Boost that should listen on a socket
> for 5 seconds and if no datagram has been received within these 5 seconds,
> move on to do other things.
>
> After some research I decided to try the solution based on std::future.
>
> The problem is that the call to wait_for() always times out as if no data
> was received. But if I set a breakpoint on the line that executes after the
> timeout and that I inspect variables I see that the buffer contains the
> received datagram and the remote_endpoint object contains the address of
> the client. In other words the socket receive works as expected but the
> std::future::wait_for() does not react. why?
>
> Here is my test server code:
>
> {
>     try
>     {
>         boost::asio::io_service io_service;
>         boost::asio::io_service::work work(io_service);
>         std::thread thread([&io_service]() { io_service.run(); });
>
>         udp::socket socket(io_service, udp::endpoint(udp::v4(), 10000));
>         char recv_buf[8];
>
>         for (;;)
>         {
>             ZeroMemory(recv_buf, 8);
>             udp::endpoint remote_endpoint;
>             std::future<std::size_t> recv_length;
>
>             recv_length = socket.async_receive_from(
>                 boost::asio::buffer(recv_buf),
>                 remote_endpoint,
>                 0,
>                 boost::asio::use_future);
>
>             if (recv_length.wait_for(
>                 std::chrono::seconds(5)) == std::future_status::timeout)
>             {
>                 printf("time out. Nothing received.\n");
>             }
>             else
>             {
>                 printf("received something: %s\n", recv_buf);
>             }
>         }
>         io_service.stop();
>         thread.join();
>     }
>     catch (std::exception& e)
>     {
>         printf("Error: %s\n", e.what());
>     }
>
>     return 0;
> }
>

The problem only happens if you have previous timeout.
You can call socket.cancel() in the timeout branch.

I think the problem is that, if you don't call socket.cancel() on timeout,
in the next round you're calling async_receive_from before the previous
call completes, which is not allowed.