.. zephyr:code-sample:: async-sockets-echo
   :name: Asynchronous echo server using poll()
   :relevant-api: bsd_sockets

   Implement an asynchronous IPv4/IPv6 TCP echo server using BSD sockets and poll()

Overview

The sockets/echo-async sample application for Zephyr implements an asynchronous IPv4/IPv6 TCP echo server using a BSD Sockets compatible API with non-blocking sockets and a poll() call. This is an extension of the :zephyr:code-sample:`sockets-echo` sample. It's a more involved application, supporting both IPv4 and IPv6 with concurrent connections, limiting maximum number of simultaneous connections, and basic error handling.

The source code for this sample application can be found at: :zephyr_file:`samples/net/sockets/echo_async`.

Requirements

• :ref:`networking_with_host`
• or, a board with hardware networking (including 6LoWPAN)

Building and Running

Build the Zephyr version of the sockets/echo_async application like this:

.. zephyr-app-commands::
   :zephyr-app: samples/net/sockets/echo_async
   :board: <board_to_use>
   :goals: build
   :compact:

After the sample starts, it expects connections at 192.0.2.1 (IPv4), or 2001:db8::1 (IPv6), port 4242. The easiest way to connect is:

$ telnet 192.0.2.1 4242     # use this for IPv4
$ telnet 2001:db8::1 4242   # or this for IPv6

After a connection is made, the application will echo back any line sent to it. Unlike the above-mentioned :zephyr:code-sample:`sockets-echo` sample, this application supports multiple concurrent client connections. You can open another terminal window and run the same telnet command as above. The sample supports up to three connected clients, but this can be adjusted by changing NUM_FDS defined in the source code.

Wi-Fi

The IPv4 Wi-Fi support can be enabled in the sample with :ref:`Wi-Fi snippet <snippet-wifi-ipv4>`.

Running application on POSIX Host

The same application source code can be built for a POSIX system, e.g. Linux. (Note: if you look at the source, you will see that the code is the same except the header files are different for Zephyr vs POSIX, and there's an additional option to set for Linux to make a socket IPv6-only).

To build:

$ make -f Makefile.host

To run:

$ ./socket_echo

To test:

$ telnet 127.0.0.1 4242   # use this for IPv4
$ telnet ::1 4242         # or this for IPv6

As can be seen, the behavior of the application is the same as the Zephyr version.