Trying to receive incoming UDP packets, empty buffer - c++

Question

I'm trying to read these incoming UDP packets using Wireshark:
I'm using the next code:
struct sockaddr_in si_other;
struct sockaddr_in remaddr; /* remote address */
int slen = sizeof(remaddr);
int s, recvlen;
char buf[BUFLEN];
char message[BUFLEN];
WSADATA wsa;
//Initialise winsock
printf("\nInitialising Winsock...");
if (WSAStartup(MAKEWORD(2, 2), &wsa) != 0)
{
printf("Failed. Error Code : %d", WSAGetLastError());
exit(EXIT_FAILURE);
}
printf("Initialised.\n");
//create socket
if ((s = socket(AF_INET, SOCK_DGRAM, 0)) == SOCKET_ERROR) //IPPROTO_UDP
{
printf("socket() failed with error code : %d", WSAGetLastError());
exit(EXIT_FAILURE);
}
//setup address structure
memset((char *)&si_other, 0, sizeof(si_other));
si_other.sin_family = AF_INET;
si_other.sin_port = htons(PORT);
si_other.sin_addr.S_un.S_addr = inet_addr(SERVER);
if (bind(s, (struct sockaddr *)&si_other, sizeof(si_other)) < 0) {
perror("bind failed");
return 0;
}
u_long nMode = 1; // 1: NON-BLOCKING
if (ioctlsocket(s, FIONBIO, &nMode) == SOCKET_ERROR)
{
closesocket(s);
WSACleanup();
return 0;
}
//start communication
while (1)
{
printf("waiting on port %d\n", PORT);
if (recvfrom(s, buf, BUFLEN, 0, (struct sockaddr *) &si_other, &slen) == SOCKET_ERROR)
{
printf("recvfrom() failed with error code : %d", WSAGetLastError());
exit(EXIT_FAILURE);
}
printf("Done");
puts(buf);
}
The address I'm binding is 192.168.1.1 and the port 1234.
And the given output by WSAGetLastError is an empty buffer error: 10035
I tried to disconnect firewall, antivirus...and I'm running the program as a administrator, and it didn't help.
Why is the buffer empty? I'm clearly seeing that there are packets coming, what could be blocking the incoming data to the socket?

Answer 1

the given output by WSAGetLastError() is an empty buffer error: 10035
No it isn't. 10035 is WSAEWOULDBLOCK. That means the operation would block in blocking mode, i.e. that no data was present to be received at the moment you called recvfrom().
The address I'm binding is 192.168.1.1
The IP 192.168.1.1 is shown as a gateway
These two statements are mutually inconsistent. If it's a gateway it's an external IP address, and if it's an external IP address you can't bind to it.
I tried INADDR_ANY and it didn't help
Not surprising. If you're not 192.168.1.1 you can't receive its messages. Unclear why you would think otherwise.

Answer 2

As #EJP mentioned 10035 is WSAEWOULDBLOCK, i.e., no data is ready. The cause of this is that the receiver is 192.168.1.1 which is your gateway's IP address, thus not a local IP. In this case you cannot receive the packets in a normal manner.
You can receive these packets, even though they are not meant for you, by using an UDP sniffer, e.g., using libpcap.

Answer 3

Thanks for the answers, they pointed me to the right direction. I 've got now the program working and I'm able to receive the incoming UDP packets.
The solution was to "sniff" these UDP packets, winsock allows that using the WSAIoctl function after bind, in my case adding the next code to the above program (after the bind function):
if (WSAIoctl(s, SIO_RCVALL, &j, sizeof(j), 0, 0, (LPDWORD)&si_other, 0, 0) == SOCKET_ERROR)
{
printf("WSAIoctl() failed.\n");
return 1;
}
I hope could help somebody else with similar issue.