recvcfrom() and sendto() ip address to be used

Question

Actually, I want to create an application in C such that 2 people can chat with each other. Let us assume they know their IP (Actually, I think I am making the mistake here. I get my IPs from www.whatismyip.com).

void recv_data(char *from, unsigned short int Port, char *data, int data_length)
{
                WSADATA wsaData;
                SOCKET RecvSocket;
                sockaddr_in RecvAddr;
                char RecvBuf[data_length];
                sockaddr_in SenderAddr;
                int SenderAddrSize = sizeof (SenderAddr);
                WSAStartup(MAKEWORD(2, 2), &wsaData);
                RecvSocket = socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP);
                RecvAddr.sin_family = AF_INET;
                RecvAddr.sin_port = htons(Port);
                   RecvAddr.sin_addr.s_addr = inet_addr(from);
                bind(RecvSocket, (SOCKADDR *) & RecvAddr, sizeof (RecvAddr));
                recvfrom(RecvSocket, RecvBuf, data_length, 0, (SOCKADDR *) & SenderAddr, &SenderAddrSize);
                int i;
            for(i=0;i<=data_length-1;i++)
                *(data+i)=RecvBuf[i];
            WSACleanup();
}

The above is a function to receive what the other person is sending. It works great when "127.0.0.1" is the value of from but when my ip (117.193.52.176) is used, something else appears. Could anyone tell me where I am wrong ?

What do you mean with "something else appears"? Also, check the return values of all functions, and print out any errors you get (if a function fails, check `WSAGetLastError`). — Some programmer dude, Jan 16 '12 at 11:42
Something else means text which I have not even sent from my talker application... Actually a bunch of hearts, faces in DOS — , Jan 16 '12 at 11:44
Do you have any hardware connected between your computer and the jack in the wall? That might be your router. if your router is not configured to pass on the information for the specified port to your computer you will not get a connection. I cannot say what the text is though.. — default, Jan 16 '12 at 11:47
after the `socket()` call, do `if (RecvSocket == INVALID_SOCKET) { // error }` do your code still execute then? — default, Jan 16 '12 at 11:48
Actually, it is wireless. Anyway, how do I "configure" the router to pass on the information for the specified port ? Port Forwarding or something ? — , Jan 16 '12 at 11:49
it depends on your router. Check your router manuals and/or the interface for the router. for instance, try to connect to `192.168.0.1` (the default IP for many routers) in your webbrowser. — default, Jan 16 '12 at 11:56
The `WSAGetLastError` function returns an error code that you can print. — Some programmer dude, Jan 16 '12 at 12:08

selbie · Accepted Answer · 2012-01-16T21:27:04.303

The address you passing to "bind" is likely wrong. Just use the IP of INADDR_ANY (0) for the call to bind. I suspect 117.193.52.176 is likely your external IP address outside of your home NAT. Your PC's real IP address is 192.168.1.2 or something like that. Type "ipconfig /all" from the command line. In any case, just bind to INADDR_ANY so you don't have to know your real IP address.

Other issues with this code:

Not checking return values from socket APIs
Don't call WSAStartup and WSACleanup for every recvfrom call. Just call WSAStartup once in your app, and don't worry about calling WSACleanup.
I'm not entirely sure if the line "char RecvBuf[data_length];" will compile. (Dynamically length static buffer on the stack? Maybe it's a new compiler feature).
Don't create a new socket for every recvfrom call. Create it once and bind to it, then use it for all subsequent send/recv calls.

5.. A more fundamnetal design problem. Unless both you and person you are communicating with are directly connected to the Internet (not NAT and no firewall), sending and receiving UDP packets will be difficult. Read the article on hole-punching here.

In any case, here's a cleaner version of your code:

int g_fWinsockInit = 0;

void initWinsock()
{
    WSADATA wsaData = {};

    if(!g_fWinsockInit)
    {
        WSAStartup(MAKEWORD(2,2), &wsaData);
        g_fWinsockInit = 1;
    }
}

void recv_data(char *from, unsigned short int Port, char *data, int data_length)
{
    SOCKET RecvSocket;
    sockaddr_in RecvAddr = {}; // zero-init, this will implicitly set s_addr to INADDR_ANY (0)

    sockaddr_in SenderAddr = {}; // zero-init
    int SenderAddrSize = sizeof(SendAddr);
    int ret;

    initWinsock();

    RecvSocket = socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP);
    if (RecvSocket == INVALID_SOCK)
    {
        printf("Error - socket failed (err = %x)\n", WSAGetLastError());
        return;
    }

    RecvAddr.sin_family = AF_INET;
    RecvAddr.sin_port = htons(Port);

    ret = bind(RecvSocket, (SOCKADDR *) & RecvAddr, sizeof (RecvAddr));
    if (ret < 0)
    {
       printf("bind failed (error = %x)\n", WSAGetLastError());
       return;
    }

    ret = recvfrom(RecvSocket, data, data_length, 0, (SOCKADDR *) &SenderAddr, &SenderAddrSize);

    if (ret < 0)
    {
       printf("recvfrom failed (error = %x)\n", WSAGetLastError());
    }
    else
    {
        printf("received %d bytes\n");
    }

}

recvcfrom() and sendto() ip address to be used

1 Answers1