LwIP API - netconn - send TCP packet while process is blocked?

Question

I use FreeRTOS + LwIP to design an Ethernet-based nursing call device. After reading some examples and documents, I want to use the LwIP netconn API to send and receive data in TCP, because I am not familiar with the BSD style API, and the original API can be difficult.

I know that the netconn_accept() function netconn_accept() block the process until a connection request is received from the remote host, and the netconn_recv() function netconn_recv() also block the process while waiting for data to be received ... both of these functions will be block , this is good for the server receiving the data, however, my device must also act as a client who needs to call a “call” to the nurse’s center when the patient presses a button. How can it initiate sending a TCP packet while it is also waiting for a connection (blocking a process) from a remote host?

I need to send TCP because I have to use SIP (Session Initiation Protocol) to initiate a call, it has to be reliable because it is a medical device. Many thanks

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@Kyle Heironimus, @werewindle I add you to my question because I know that you have experience with LwIP, I hope this does not make you inconvenient and I hope you can help, thanks.

Answer 1

You cannot send data to a remote host before it initiates a connection. It just doesn't make any sense. My question is: do you want your device to connect to the remote host, or do you want the remote host to initiate a connection to your device?

You are currently using netconn_accept on your device - this means that you expect the remote host to initiate a connection to your device before your device can signal to the remote host. This is the fully expected behavior for the code you wrote, but you seem to be worried about this. Isn't that your intention? If not, why did you code it? Another alternative is for your device to initiate a connection to the remote host. Here is an example of using netconns here . Of course, this is also due to changes on another device in your system.

So, the moral of this story is that you cannot send any data when there is no connection, and you are waiting for the connection before sending data. You do not want to wait for the connection, so you need to change your software to initiate the connection, and not wait until the other side initiates the connection.

Another problem that you may encounter is that you want to send and receive data at the same time on the same connection. Most of the examples I've seen for lwip include blocking calls waiting for data, and then responding to that data by passing something back. Sometimes you want to convey something without first receiving something. I can help too.

This is what worked for me when I created a netconn listening connection.

First you must enable timeouts by setting:

#define LWIP_SO_RCVTIMEO 1

Then you need to configure your netconn in the same way:

pxTCPListener = netconn_new (NETCONN_TCP);

netconn_bind (pxTCPListener, NULL, 23);

netconn_listen (pxTCPListener);

pxNewConnection = netconn_accept (pxTCPListener); // This blocks until the connection is accepted

// This is an important line!

pxNewConnection-> recv_timeout = 10; // note This is millseconds - lwip works in ms

// This is a loop until the connection is closed

while (! ERR_IS_FATAL (pxNewConnection-> err)) {// Fatal errors include closed connections, reset, aborted, etc.

// This netconn_recv call now waits 10 ms for any new data, and then returns

if ((pxRxBuffer = netconn_recv (pxNewConnection))! = NULL) {

// Processing the received data

}

// This is where any transfers you want are made

} // End of while loop above

This code will allow you to make transfers and receive at the same time, without worrying about blocking.