boost::asio::socket thread safety

Question

( This is a simplified version of my original question )

I have several threads that write to a boost asio socket. This seems to work very well, with no problems.

The documentation says a shared socket is not thread safe( here, way down at the bottom ) so I am wondering if I should protect the socket with mutex, or something.

This question insists that protection is necessary, but gives no advice on how to do so.

All the answers to my original question also insisted that what I was doing dangerous, and most urged me to replace my writes with async_writes or even more complicated things. However, I am reluctant to do this, since it would complicate code that is already working and none of the answerers convinced me they knew what they ware talking about - they seemed to have read the same documentation as I and were guessing, just as I was.

So, I wrote a simple program to stress test writing to a shared socket from two threads.

Here is the server, which simply writes out whatever it receives from the client

int main() {     boost::asio::io_service io_service;      tcp::acceptor acceptor(io_service, tcp::endpoint(tcp::v4(), 3001));      tcp::socket socket(io_service);     acceptor.accept(socket);      for (;;)     {         char mybuffer[1256];         int len = socket.read_some(boost::asio::buffer(mybuffer,1256));         mybuffer[len] = '\0';         std::cout << mybuffer;         std::cout.flush();      }    return 0; } 

Here is the client, which creates two threads that write to a shared socket as fast as they can

boost::asio::ip::tcp::socket * psocket;  void speaker1() {     string msg("speaker1: hello, server, how are you running?\n");     for( int k = 0; k < 1000; k++ ) {         boost::asio::write(             *psocket,boost::asio::buffer(msg,msg.length()));     }  } void speaker2() {     string msg("speaker2: hello, server, how are you running?\n");     for( int k = 0; k < 1000; k++ ) {         boost::asio::write(             *psocket,boost::asio::buffer(msg,msg.length()));     }  }  int main(int argc, char* argv[]) {      boost::asio::io_service io_service;    // connect to server      tcp::resolver resolver(io_service);     tcp::resolver::query query("localhost", "3001");     tcp::resolver::iterator endpoint_iterator = resolver.resolve(query);     tcp::resolver::iterator end;     psocket = new tcp::socket(io_service);     boost::system::error_code error = boost::asio::error::host_not_found;     while (error && endpoint_iterator != end)     {         psocket->close();         psocket->connect(*endpoint_iterator++, error);     }       boost::thread t1( speaker1 );     boost::thread t2( speaker2 );      Sleep(50000);  } 

This works! Perfectly, as far as I can tell. The client does not crash. The messages arrive at the server without garbles. They usually arrive alternately, one from each thread. Sometimes one thread get two or three messages in before the other, but I do not think this is a problem so long as there are no garbles and all the messages arrive.

My conclusion: the socket may not be thread safe in some theoretical sense, but it is so hard to make it fail that I am not going to worry about it.

Answer 1

After restudying the code for async_write I am now convinced that any write operation is thread safe if and only if the packet size is smaller than

default_max_transfer_size = 65536; 

What happens is that as soon as an async_write is called an async_write_some is called in the same thread. Any threads in the pool calling some form of io_service::run will keep on calling async_write_some for that write operation until it completes.

These async_write_some calls can and will interleave if it has to be called more than once (the packets are larger than 65536).

ASIO does not queue writes to a socket as you would expect, one finishing after the other. In order to ensure both thread and interleave safe writes consider the following piece of code:

    void my_connection::async_serialized_write(             boost::shared_ptr<transmission> outpacket) {         m_tx_mutex.lock();         bool in_progress = !m_pending_transmissions.empty();         m_pending_transmissions.push(outpacket);         if (!in_progress) {             if (m_pending_transmissions.front()->scatter_buffers.size() > 0) {                 boost::asio::async_write(m_socket,                     m_pending_transmissions.front()->scatter_buffers,                         boost::asio::transfer_all(),             boost::bind(&my_connection::handle_async_serialized_write,                         shared_from_this(),                         boost::asio::placeholders::error,                                        boost::asio::placeholders::bytes_transferred));             } else { // Send single buffer                 boost::asio::async_write(m_socket,                                     boost::asio::buffer(                                            m_pending_transmissions.front()->buffer_references.front(),                          m_pending_transmissions.front()->num_bytes_left),                 boost::asio::transfer_all(),                 boost::bind(                         &my_connection::handle_async_serialized_write,                         shared_from_this(),                         boost::asio::placeholders::error,                         boost::asio::placeholders::bytes_transferred));             }         }         m_tx_mutex.unlock();     }      void my_connection::handle_async_serialized_write(     const boost::system::error_code& e, size_t bytes_transferred) {         if (!e) {             boost::shared_ptr<transmission> transmission;             m_tx_mutex.lock();             transmission = m_pending_transmissions.front();             m_pending_transmissions.pop();             if (!m_pending_transmissions.empty()) {                 if (m_pending_transmissions.front()->scatter_buffers.size() > 0) {             boost::asio::async_write(m_socket,                     m_pending_transmissions.front()->scatter_buffers,                     boost::asio::transfer_exactly(                             m_pending_transmissions.front()->num_bytes_left),                     boost::bind(                             &chreosis_connection::handle_async_serialized_write,                             shared_from_this(),                             boost::asio::placeholders::error,                             boost::asio::placeholders::bytes_transferred));                 } else { // Send single buffer                     boost::asio::async_write(m_socket,                     boost::asio::buffer(                             m_pending_transmissions.front()->buffer_references.front(),                             m_pending_transmissions.front()->num_bytes_left),                     boost::asio::transfer_all(),                     boost::bind(                             &my_connection::handle_async_serialized_write,                             shared_from_this(),                             boost::asio::placeholders::error,                             boost::asio::placeholders::bytes_transferred));                 }             }             m_tx_mutex.unlock();             transmission->handler(e, bytes_transferred, transmission);         } else {             MYLOG_ERROR(             m_connection_oid.toString() << " " << "handle_async_serialized_write: " << e.message());             stop(connection_stop_reasons::stop_async_handler_error);         }     } 

This basically makes a queue for sending one packet at a time. async_write is called only after the first write succeeds which then calls the original handler for the first write.

It would have been easier if asio made write queues automatic per socket/stream.

Answer 2

Use a boost::asio::io_service::strand for asynchronous handlers that are not thread safe.

A strand is defined as a strictly sequential invocation of event handlers (i.e. no concurrent invocation). Use of strands allows execution of code in a multithreaded program without the need for explicit locking (e.g. using mutexes).

The timer tutorial is probably the easiest way to wrap your head around strands.