Boost ASIO echo TCP synchronous client-server

I think this echo client-server application is a good introduction to ASIO. The server creates a new TCP socket each time it receives a request from a client, and run it in a new thread, where the read-write activity is accomplished in a synchronous way. The client sends some data to the server, gets it back, and then terminates.
The structure is simple, still a few interesting points are touched.


Given io, the app ASIO io_context, and the server hostname as a string, the client tries this block, and eventually just output to console an exception.
namespace ba = boost::asio;
using ba::ip::tcp;
// ...

tcp::socket socket{ io };  // 1
tcp::resolver resolver{ io };
ba::connect(socket, resolver.resolve(host, ECHO_PORT_STR));  // 2

// ...
ba::write(socket, ba::buffer(request, reqLen));  // 3

char reply[CLIENT_MAX_LEN];  // 4
size_t repLen = ba::read(socket, ba::buffer(reply, reqLen));  // 4
// ...
1. Create an ASIO TCP socket and a resolver on the current io_context.
2. Then resolve() the resolver on the host and port of the echo server (in my case, localhost:50014), and use the resulting endpoints to estabilish a connection on the socket.
3. If the connection holds, write to the socket the data we previously put in the char buffer named request, for a size of reqLen.
4. We reserve a confidently large buffer where to store the server reply. Since we are writing a echo application, we know that the size of the data we are about to get from the client should be the same of the size we have just sent. This simplify our code to the point that we can do a single read for the complete data block.
5. Use the socket for reading from the server. We use the buffer, and the size of the data we sent, for what said on (4).

At this point we could do whatever we want with the data we read in reply with size repLen.

Server loop

Once we create an acceptor on the ASIO io_context, specifying as endpoint the IP protocol we want (here I used version 4) and the port number, we loop forever, creating a new socket through a call to accept() on the acceptor each time a request comes from a client, passing it to the session() function that is going to run in a new thread.
tcp::acceptor acceptor{ io, tcp::endpoint(tcp::v4(), ECHO_PORT) };

for (;;)
 std::thread(session, acceptor.accept()).detach();
Notice that each thread created in the loop survives the exiting of the block only because it is detached. This is both handy and frightening. In production code, I would probably push them in a collection instead, so that I could explicitly kill anyone that would stop behave properly.

Server session

Since we don't know the size of the data sent by the client, we should be ready to split it and read it in chunks.
for (;;)
 char data[SERVER_MAX_LEN];  // 1

 bs::error_code error;
 size_t len = socket.read_some(ba::buffer(data), error);  // 2
 if (error == ba::error::eof)
 else if (error)
  throw bs::system_error(error);

 ba::write(socket, ba::buffer(data, len)); // 3
1. To better see the effect, I have chosen a ridiculously small size for the server data buffer.
2. The data coming from the client is split in chunks from read_some() on the socket created by the acceptor. When the read is completed, read_some() sets the passed boost system error to eof error. When we detect it, we know that we could terminate the session. Any other error says that the something went wrong.
3. If read_some() set no error, we use the current chunk of data to do what the server should do. In this case, we just echo it back to the client.

Full C++ code on GitHub. The original source is the official Boost ASIO tutorial, divided in two parts, client and server.

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