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I am trying to get my head into SDLnet and I am encountering a problem where any UDP packets that I send from the client to the server are being broken up on the space character. I can't see any reason for this happening as I am not explicitly programming this behaviour in - I am literally just sending across a string.

The source code I am using is part of an online example on The Game Programming Wiki

Server

    printf("Fill the buffer\n>");
    scanf("%s", (char *)p->data);

    p->address.host = srvadd.host;  /* Set the destination host */
    p->address.port = srvadd.port;  /* And destination port */

    p->len = strlen((char *)p->data) + 1;
    SDLNet_UDP_Send(sd, -1, p); /* This sets the p->channel */

    /* Quit if packet contains "quit" */
    if (!strcmp((char *)p->data, "quit"))
        quit = 1;

Client

    /* Wait a packet. UDP_Recv returns != 0 if a packet is coming */
    if (SDLNet_UDP_Recv(sd, p))
    {
        printf("UDP Packet incoming\n");
        printf("\tChan:    %d\n", p->channel);
        printf("\tData:    %s\n", (char *)p->data);
        printf("\tLen:     %d\n", p->len);
        printf("\tMaxlen:  %d\n", p->maxlen);
        printf("\tStatus:  %d\n", p->status);
        printf("\tAddress: %x %x\n", p->address.host, p->address.port);

        /* Quit if packet contains "quit" */
        if (strcmp((char *)p->data, "quit") == 0)
            quit = 1;
    }

Output

The output looks like this image.

The operating system I am running on is Windows 7 64-bit and I'm wondering if this could be something OS-related.

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    So what? UDP packets can be split up (and merged) anywhere, not just at space characters. It´s your job to know where one piece of send info ends and the next begins. – deviantfan May 22 '15 at 17:55
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    I did consider this actually. However, I found it unusual to be able to replace the space characters with an underscore and it be transmitted in full instead of split up. Does the space character have any special significance in this case? –  May 22 '15 at 17:58
  • UDP datagrams are received intact or not at all. Unless SLDnet has some layered behaviour over that, your problem lies elsewhere. – user207421 May 22 '15 at 18:08
  • I'll have a look in the source, thanks @EJP –  May 22 '15 at 18:15
  • @deviantfan No, it's UDP's job actually. It's a datagram protocol, not a stream protocol. – user207421 May 22 '15 at 18:23

1 Answers1

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This is not the fault of UDP, it's go to do with the char* being split up when using scanf. ( I'm not a 100% sure about the details here. ) But as a general rule, in C, you shouldn't use scanf

Since you are using C++ ( at least according to the tags), you should do this the C++ way :

std::string msg = "";
std::cout << "Type a message and hit enter\n";

// Let user type a message
std::cin.ignore();
std::getline(std::cin, msg );

// UDPpacket uses Uint8, whereas msg.c_str() gives us a char*
// This simply copies the integer value of the chars into packet->data
memcpy(packet->data, msg.c_str(), msg.length() );

packet->len = msg.length();

Note : The std::cin.ignore(); is there to make sure we stop and wait for the user to type in the message.

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olevegard
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  • Cheers @olevegard, I will look at using your example above tonight and get back to you. I have a feeling I tried `std::string` and `std::cin` but that didn't work either - can't hurt to try again though. –  May 25 '15 at 09:54
  • @danolo That's quite likely. In my code I used `std::string` but had the issue you have. But when I saw this question I looked into it again and found a way to fix it. – olevegard May 25 '15 at 10:08
  • I switched my code to use `std::getline` instead of `scanf` and it works perfectly now, thanks for your help. Your blog is great, by the way! –  Aug 21 '15 at 11:52