Detect TCP client disconnect

Kernel
2

heinbali01 wrote on Tuesday, February 12, 2019:

For your application, ipconfigTCP_KEEP_ALIVE will be helpful. It sends mini-packets to the peer, just to test if it is still alive. After 3 times “no answer”, the connection will be broken.

If your TCP server will only have 1 client at the time, consider using FREERTOS_SO_REUSE_LISTEN_SOCKET.

Normally, FreeRTOS_accept() returns a new socket. The server socket itself will never connect and it will only be used to call accept().
With the FREERTOS_SO_REUSE_LISTEN_SOCKET option, a successful FreeRTOS_accept() will return the server socket itself. After the connection has gone, please call FreeRTOS_closesocket() for that server socket.
After that, create a new server socket and call accept() again.

I wrote some code that causes a graceful disconnect here

But normally when the peer takes the initialive to disconnect ( like a telnet server ), you can use this type of code:

    for( ;; )
    {
    BaseType_t xResult;

        xResult = FreeRTOS_recv( xSocket, pcBuffer, sizeof( pcBuffer ), 0 );
        if( ( xResult < 0 ) && ( xResult != -pdFREERTOS_ERRNO_EAGAIN ) )
        {
            break;
        }
		if( xResult > 0 )
		{
			/* Do something with the data received. */
		}
    }

Now when the peer sends a shutdown, FreeRTOS_recv() will return immediately with -pdFREERTOS_ERRNO_ENOTCONN or so.

You can also poll the current status: FreeRTOS_issocketconnected().

Personally I like to use the option ipconfigSOCKET_HAS_USER_SEMAPHORE. That allows you to attach semaphore to all sockets. If any socket has an important event ( RX, TX, error, disconnect ), the semaphore will be given to. When using this technique, the sockets are used in a non-blocking way ( e.g. with FREERTOS_MSG_DONTWAIT ).
The main loop will look like this:

    for( ;; )
    {
		xSemaphoreTake (xMySemaphore, pdMS_TO_TICKS( 10000 ));
		/* Check all sockets in a non-blocking way. */
	}
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