Off-by-one Problem with TCP/IP Client Receive Block and Python Server Sender

When I send the first value from my py server to the Simulink TCP/IP Client Receive block (non-blocking-mode), nothing happens and the data and status outputs both remain 0. When I send the second value, the first value is received. When I send the third, the second is received. And so on.

I have no idea why this is happening. Does anyone know how to fix this, or do I need to work around this phenomenon?

[The if-block and data store memory are for handling the non-blocking mode]

Python-Server:

import socket
import struct


def start_tcp_server_v3():
    # Initialize socket
    server_socket = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
    server_socket.settimeout(100)  # Set timeout to 100 seconds as specified in Simulink

    # Bind the socket
    host, port = 'localhost', 50000
    server_socket.bind((host, port))

    while True:
        try:
            # Listen for incoming connections
            server_socket.listen(1)
            print("Server is listening...")

            # Establish connection
            client_socket, client_address = server_socket.accept()
            print(f"Connection established with {client_address}")

            # Reset the timeout for the client socket
            client_socket.settimeout(None)

            while True:
                try:
                    # Get user input for the data to send
                    user_input = input("Please enter a double value to send: ")
                    try:
                        double_value = float(user_input)
                    except ValueError:
                        print("Invalid input. Please enter a valid double value.")
                        continue

                    # Prepare data
                    data = struct.pack('>d', double_value)  # Big-endian byte order, type double

                    # Send data
                    client_socket.sendall(data)
                    print("Just sent: [ "+str(double_value)+" ]")

                except (BrokenPipeError, ConnectionResetError, ConnectionAbortedError):
                    print("Client disconnected. Returning to listening mode.")
                    client_socket.close()  # Close the client socket
                    break  # Break the inner loop to go back to listening for new connections

        except socket.timeout:
            print("Server timeout. No client connected.")
            continue

        except KeyboardInterrupt:
            print("Server is shutting down.")
            break

        finally:
            # Close client socket if it exists
            if 'client_socket' in locals():
                client_socket.close()

    # Close the server socket
    server_socket.close()
    print("Server has been closed.")

# -----------------------------------------

start_tcp_server_v3()

Answer 1

[Ayush]

Hi Simon,

I understand that you want to fix your issue of receiving the data over a TCP/IP connection via the Simulink TCP/IP Client Receive block in real-time and not encounter a delay of one time-step every time.

I was able to reproduce the same error on my end with MATLAB R2023b release. It looks to be an issue with the condition that checks the number of bytes in the buffer with the output size set in the block parameters. This is a known bug and expected to be resolved in the upcoming MATLAB R2024a release.

You can achieve the same functionality by downloading the MATLAB R2024a prerelease or wait for the full release if you have the valid license for the same. Please refer to the below link for more information about MATLAB R2024a prerelease.

https://www.mathworks.com/products/new_products/release-highlights.html

Hope it helps,

Regards,

Ayush Misra

[Daniel]

Which record number is this bug? It's not obvious to me from the description.