Asynchronous Socket Echo Server


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 Socket Echo Server 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: samples/net/sockets/echo_async.


Building and Running

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

# On Linux/macOS
cd $ZEPHYR_BASE/samples/net/sockets/echo_async
mkdir build && cd build
# On Windows
cd %ZEPHYR_BASE%\samples\net\sockets\echo_async
mkdir build & cd build
cmake -GNinja -DBOARD=<board_to_use> ..

board_to_use defaults to qemu_x86. In this case, you can run the application in QEMU using make run. If you used another BOARD, you will need to consult its documentation for application deployment instructions. You can read about Zephyr support for specific boards in the documentation at Supported Boards.

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

$ telnet 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 Socket Echo Server, 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.

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 for a host POSIX OS:

$ make -f Makefile.posix

To run:

$ ./socket_echo

To test:

$ telnet 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.