Simple ASIO TCP client/server example

A server sits on a specified port, and when a client connects, it sends a message and terminates. A client connects to the server, reads from the socket the message, and terminates. Nothing fancy, but it could be a good introduction on how to use ASIO synchronously to create TCP/IP connections.

After five years, the code is getting obsolete. I have reviewed it moving to the (currently - March 2018) latest version of ASIO, please follow the link to the new post. Sorry for the trouble.

You could get the full C++ code for this example on Github. If you run the executable with no parameter, you start the server, otherwise the client.

Server

In this trivial implementation, my server accepts just one connection before terminating, but it is pretty easy to make it run forever. It is just a matter of running this block in an indefinite loop, and not just once as I do here:

{
  boost::asio::ip::tcp::socket socket(aios); // 1
  std::cout << "Server ready" << std::endl;
  acceptor.accept(socket); // 2

  std::string message("Hello from ASIO");
  boost::asio::write(socket, boost::asio::buffer(message)); // 3
}

1. Create a new TCP/IP socket on an already existing ASIO I/O service.
2. Wait for a client connection.
3. Write a message on the socket to the client.

At the end of the block the socket is automatically closed by its dtor.

Before that, I have done some preparatory stuff:

boost::asio::io_service aios; // 1
boost::asio::ip::tcp::acceptor acceptor(aios, // 2
  boost::asio::ip::tcp::endpoint(boost::asio::ip::tcp::v4(), HELLO_PORT)); // 3

1. The I/O service is the first thing required.
2. The acceptor is used to accept calls from the clients.
3. We need to pass to the acceptor the endpoint, that specifies the protocol used (here is TCP/IP version 4), and the port used (here defined in a constant that I set to 50013).

Client

The client code is symmetrical. First step is about preparing the job:

boost::asio::io_service aios;

boost::asio::ip::tcp::resolver resolver(aios); // 1
boost::asio::ip::tcp::resolver::iterator endpoint = resolver.resolve(
  boost::asio::ip::tcp::resolver::query(host, HELLO_PORT_STR)); // 2

1. Resolver is the counterpart for acceptor. Calling resolve() on it, we get an iterator pointing to the first endpoint associated to a specific address. We can use that iterator to open a connection through the server on a socket, as we'll see below.
2. Query for a specific host and port (here I specified localhost and 50013, notice that both are c-strings).

Now I am ready to open the connection on a socket. If you are using a recent version of Boost Asio (I am working with 1.54), this is done in a one-liner:

boost::asio::connect(socket, endpoint);

If no connection could be opened on any endpoint, a boost system system_error is thrown.

On older asio versions, there was not such a connect() overload, and you have to implement its behavior by hand, in a piece of code like this:

boost::system::error_code error = boost::asio::error::host_not_found;
boost::asio::ip::tcp::resolver::iterator end; // 1
while(error && endpoint != end)
{
  socket.close();
  socket.connect(*endpoint++, error); // 2
}
if(error)
  throw boost::system::system_error(error); // 3

1. The default ctor for a resolver iterator returns its "end" on, we use it to loop on all the endpoints returned by resolver::resolve().
2. Try to connect to the current endpoint, in case of failure loop until we have another endpoint to check.
3. Can't find any endpoint, throw an exception.

Once we have a socket connected to the server, it's just a matter of getting the message it sends to us:

for(;;)
{
  std::array<char, 4> buf; // 1
  boost::system::error_code error;
  size_t len = socket.read_some(boost::asio::buffer(buf), error); // 2

  if(error == boost::asio::error::eof)
    break; // 3
  else if(error)
    throw boost::system::system_error(error); // 4

  std::cout.write(buf.data(), len);
  std::cout << '|'; // 5
}
std::cout << std::endl;

1. Usually I would use a much bigger buffer.
2. Partial read of the message, limited by the buffer dimension.
3. Detected end of file, stop looping.
4. In case of error throws an exception.
5. I show the junctions in the message due to the local buffer, usually it is rebuilt seamlessly.

I written this post as a review of a piece of code I conceived at beginning 2011, that is still documented in a couple of posts, one dedicated to the server part, the other to the client part. You may want to have a look a it.

The original source is the Boost Asio tutorial. Here is the slightly different version of their client and of the their server.