Boost ASIO echo UDP synchronous client-server

Close to the previous post. The main difference that there we have seen a TCP-based data exchange while here we see a UDP echo.


This server is simpler than the previous one. Just one connection is served at a time.
udp::socket socket(io, udp::endpoint(udp::v4(), ECHO_PORT));  // 1

for (;;)  // 2
 char data[MAX_LEN];
 udp::endpoint client;
 size_t len = socket.receive_from(ba::buffer(data), client);  // 3

 // ...
 socket.send_to(ba::buffer(data, len), client);  // 4
1. Create an ASIO UDP socket on the app io_context, on a UDP created on the fly where the UDP IP protocol and the port to be used are specified.
2. Forever loop to serve, in strict sequential order, all the requests coming from clients.
3. ASIO blocks here, expecting the socket to receive a connection from a client. Make sure that the buffer data is big enough.
4. Since this is an echo server, nothing exciting happens between receiving and sending. Here we send the data, as received, to the endpoint as set by receive_from().

char request[MAX_LEN];
// ...

udp::socket socket{ io, udp::endpoint(udp::v4(), 0) };  // 1
udp::resolver resolver{ io };
udp::endpoint destination = *resolver.resolve(udp::v4(), host, ECHO_PORT_STR).begin();  // 2
socket.send_to(ba::buffer(request, std::strlen(request)), destination);  // 3

char reply[MAX_LEN];
udp::endpoint sender;
size_t len = socket.receive_from(ba::buffer(reply), sender);  // 4
// ...
1. Create a UDP socket on the ASIO I/O context. Notice that the UDP endpoint passed specify the IP protocol but not a valid port.
2. The destination endpoint, that refers to the server, is generated by the resolver created on the line above, that resolves the specified host and port for the given UDP IP protocol. Then the first result is taken (by the begin iterator and then dereferencing). In case of any trouble we have guarantee an exception is thrown by resolve().
3. Send the data through buffer to the socket, that mediates the connection to the server.
4. Once send_to() has ended its job (notice that it is a blocking function), we get the reply from the server calling receive_from(). The socket knows where to go and get the data, and will fill the passed endpoint (sender) with these information.

I pushed the full C++ code - both client and server in the same source file - to GitHub. I based them on blocking_udp_echo_server.cpp and blocking_udp_echo_client.cpp from the official Boost ASIO Tutorial.

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