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UDP receive problem via SysSocket [CODESYS Control Win V3.5]

Engineering 🇬🇧
dino
2021-07-08
2021-07-08

  • dino - 2021-07-08

    Hi, I try to send and receive data using a UDP connection via SysSocket 3.5.17.0. While sending data works fine, I have problems with the receiving part.

    Basically, the code looks like this:

    PROGRAM UdpTest
VAR
    test_string : STRING(25) := 'Test string';

    socket_handle : RTS_IEC_HANDLE;
    socket_address : SOCKADDRESS;
    sys_result : RTS_IEC_RESULT;
    enable_option : DINT := 1;

    bytes_sent : __XINT;
    bytes_received : __XINT;
    bytes_available : DINT;

    receive_buffer : ARRAY [0..63] OF BYTE;
    udp_reply : UDP_REPLY2;
END_VAR

    socket_address.sin_family := SOCKET_AF_INET;
socket_address.sin_port := SysSockHtons(12000);
socket_address.sin_addr.ulAddr := SysSockHtonl(SOCKET_INADDR_ANY);

socket_handle := SysSockCreate(socket_address.sin_family, SOCKET_DGRAM, SOCKET_IPPROTO_UDP, ADR(sys_result));

sys_result := SysSockSetOption(socket_handle, SOCKET_SOL, SOCKET_SO_REUSEADDR, ADR(enable_option), SIZEOF(enable_option));

sys_result := SysSockBind(socket_handle, ADR(socket_address), SIZEOF(socket_address));

    (For better readability, checking of the return values is omitted.)

    The sending part is working well:

    bytes_sent := SysSockSendToUdp(socket_handle, 12010, '127.0.0.1', ADR(test_string), SIZEOF(test_string), ADR(sys_result));

    However, I am not able to receive any data:

    bytes_received := SysSockGetRecvSizeUdp(socket_handle, 0, ADR(sys_result));

    always yields 0.

    sys_result := SysSockIoctl(socket_handle, SOCKET_FIONREAD, ADR(bytes_available));

    also yields 0 for bytes_available all the time.

    Therefore, I made the socket non-blocking:

    sys_result := SysSockIoctl(socket_handle, SOCKET_FIONBIO, ADR(enable_option));

    and tried to receive the data:

    bytes_received := SysSockRecvFromUdp2(socket_handle, ADR(receive_buffer), SIZEOF(receive_buffer), ADR(udp_reply), ADR(sys_result));

    I also tried

    bytes_received := SysSockRecvFrom(socket_handle, ADR(receive_buffer), SIZEOF(receive_buffer), 0, ADR(socket_address), SIZEOF(socket_address), ADR(sys_result));

    with variations of socket_address, but unfortunately without success.

    Only when I use

    socket_handle := SysSockCreateUdp(12000, 12000, ADR(sys_result));

    checking the RecvSize gave me a value different from 0:

    bytes_received := SysSockGetRecvSizeUdp(socket_handle, 0, ADR(sys_result));

    However, for this setup I have to change the port every time I start the application, even though SOCKET_SO_REUSEADDR is set. Thus, this approach is not satisfactory either.

    What am I doing wrong that prevents the socket created with SysSockCreate from receiving data?

     

  • dino - 2021-07-08

    I just realised that

    bytes_received := SysSockGetRecvSizeUdp(socket_handle, 0, ADR(sys_result));

    also works once for each port I specify, i.e. when I define a port, download the application and start it, receiving works. After a restart, bytes_received will remain 0. To make it work again, I need to specify another port or restart the PC.

    Edit:
    It looks like Control Win does not close open ports during a cold restart or application download. Is this intended or a bug? For now, I added a PrepareStop event handler that closes the open port as a workaround.

     

    Last edit: dino 2021-07-09

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