ZMQ_DONTWAIT flag does not work?

Question

I am using ZeroMQ for networking in my appliction, the manual states that by providing the ZMQ_DONTWAIT flag to the flags parameter in send or recv causes the function to not block the thread. Yet it doesn't work in my case somehow:

            std::cout << "a";
            if(ToSend.try_pop(send))
            {
                std::cout << "b";
                local.send(send.data(),send.size(),ZMQ_DONTWAIT);
            }
            std::cout << "c";
            if(local.recv(recv.data(),Networking::max_packet,ZMQ_DONTWAIT))
                std::cout << "Received: " << (char*)recv.data() << std::endl;
            std::cout << "d" << std::endl;

this prints:

abcdab

I made a little class to make things easier:

Client class (striped down from all "unused" stuff, for simplification)

class client
{
public:
    client()
    {

    }
    inline bool init(unsigned short threads = 1)
    {
        Running = true;
        context = zmq_init (threads);
        if(context == NULL)
            return false;
        socket = zmq_socket (context, ZMQ_REQ);
        if(socket == NULL)
            return false;
        return true;
    }
    inline int connect(const char * address, unsigned short port)
    {
        return zmq_connect(socket,string_format("tcp://%s:%d",address,port).c_str());
    }
    inline bool send (void *data, size_t len_, int flags_ = 0)
    {
        message_t request (len_);
        memcpy ((void *) request.data (), data, len_);
        int rc = zmq_send (socket, request.data(), request.size(), flags_);
        if (rc >= 0)
            return true;
        if (rc == -1 && zmq_errno () == EAGAIN)
            return false;
        throw error_t ();
    }
    inline bool recv (void * data, size_t len_, int flags_)
    {
        message_t reply(len_);
        int rc = zmq_recv (socket, reply.data(), len_, flags_);
        if (rc >= 0)
        {
            memcpy (data,(void *)reply.data(), reply.size());
            return true;
        }
        if (rc == -1 && zmq_errno () == EAGAIN)return false;
        throw error_t ();
    }
    inline bool IsRunning()
    {
        return Running;
    }
private:
    void * context;
    void * socket;
    std::atomic<bool> Running;
};

and here is the worker thread:

namespace Data
{
    Concurrency::concurrent_queue <message_t> ToSend;
    void Processor(char * address, unsigned short port, unsigned short threads)
    {
        client local;
        if(!local.init(threads))return;
        if(local.connect(address,port) != 0)return;
        message_t recv(Networking::max_packet);
        message_t send(Networking::max_packet);
        while(local.IsRunning())
        {
            std::cout << "a";
            if(ToSend.try_pop(send))
            {
                std::cout << "b";
                local.send(send.data(),send.size(),ZMQ_DONTWAIT);
            }
            std::cout << "c";
            if(local.recv(recv.data(),Networking::max_packet,ZMQ_DONTWAIT))
                std::cout << "Received: " << (char*)recv.data() << std::endl;
            std::cout << "d" << std::endl;
        }
    }
};

The problem exists here somehow. I just don't know why it's not working.

This is how I launch the worker thread:

int Thread(char * address, unsigned short port, unsigned short threads)
{
    std::thread data(Data::Processor,address,port,threads);
    data.detach();
    while(!Data::status){}
    return Data::status;
}
int main(int argc, char* argv[])
{

    std::thread s(Server::RUN);

    Client::message_t tosend(14);
    memcpy((void*)tosend.data(),"Hello World !\0",14);

    Client::Data::ToSend.push(tosend);

    std::cout << Client::Thread("127.0.0.1",5555,1) << std::endl;

    s.join();
    return 0;
}

This all seems to be correct, so why is recv/send blocking my thread? Why doesn't the flag work?

Answer 1

It printed "abcd" and then "ab". This means that the code

std::cout << "a";
if(ToSend.try_pop(send))
{
    std::cout << "b";

was executed twice although the code you showed at the end of the question implies you only did ToSend.push() once.

Did you attach a debugger to see which thread was hung and what it's callstack is?

• Put the std::cout's behind a mutex/critsect so you know only one thread is writing at a time, and report which thread you are.
• Put each output on it's own line so you don't miss a letter due to buffering.
• If you don't block for data how are you expecting to receive it? Do you have a select/poll/WaitForSingleObject call somewhere?