Multiple servers with single client - c

Question

Scenerio: several servers are listening, and a single client will send a UDP broadcast to all machines on the network and the servers will reply back. (goal: get all the ip addresses of the servers)
Here is the client code:
int main()
{
struct sockaddr_in connectedSocket;
int length=sizeof(connectedSocket);
int iResult = 0, iOptVal = 0, nOptiontValue = 1;
SOCKET Socket;
char receiveBuffer[1000];
char message[1000];
//Clear the buffer by filling null, it might have previously received data
memset(receiveBuffer,'\0', 1000);
WSADATA wsa;
//Initialise winsock
printf("\nInitialising Winsock...\n");
if (WSAStartup(MAKEWORD(2,2),&wsa) != 0)
{
printf("\nFailed. Error Code : %d",WSAGetLastError());
exit(EXIT_FAILURE);
}
printf("\n.........Initialised.\n");
//Create socket
Socket = socket(AF_INET, SOCK_DGRAM, 0);
if (Socket == SOCKET_ERROR)
{
printf("Create a UDP socket failed with error = %d\n" , WSAGetLastError());
exit(EXIT_FAILURE);
}
//Set socket options to broadcast
iResult = setsockopt(Socket, SOL_SOCKET, SO_BROADCAST,(char *) &iOptVal, sizeof (iOptVal));
if(iResult == SOCKET_ERROR)
{
printf("Set socket options failed with error = %d\n" , WSAGetLastError());
exit(EXIT_FAILURE);
}
//Setup address structure
memset((char *) &connectedSocket, 0, sizeof(connectedSocket));
connectedSocket.sin_family = AF_INET;
connectedSocket.sin_port = htons(PORT);
connectedSocket.sin_addr.s_addr = INADDR_BROADCAST;
while(1)
{
printf("\n\n\nEnter message : ");
gets(message);
//send the message
if (sendto(Socket, message,sizeof(message) , 0 , (struct sockaddr *) &connectedSocket, sizeof(connectedSocket)) == SOCKET_ERROR)
{
printf("\nsendto() failed with error code : %d" , WSAGetLastError());
exit(EXIT_FAILURE);
}
printf("\nMessage Successfully sent to Server");
// fflush(stdout);
if (recvfrom(Socket, receiveBuffer, 1000, 0, (struct sockaddr *) &connectedSocket,&length) == SOCKET_ERROR)
{
printf("\nrecvfrom() failed with error code : %d" , WSAGetLastError());
exit(EXIT_FAILURE);
}
printf("\nServer Says : ");
printf(receiveBuffer,sizeof(receiveBuffer));
}
closesocket(Socket);
WSACleanup();
return 0;
}
When I run this, I get
sendto() failed with error code : 10013
I look up the winsock error and it says
An attempt was made to access a socket in a way forbidden by its access permissions. An example is using a broadcast address for sendto without broadcast permission being set using setsockopt(SO_BROADCAST).
But I am setting the sockopt to SO_BROADCAST. Can anyone tell me why this is happening?

Answer 1

The relevant parts of the code you posted are here:
int iResult = 0, iOptVal = 0, nOptiontValue = 1;
...
//Set socket options to broadcast
iResult = setsockopt(Socket, SOL_SOCKET, SO_BROADCAST,(char *) &iOptVal, sizeof (iOptVal));
if(iResult == SOCKET_ERROR)
{
printf("Set socket options failed with error = %d\n" , WSAGetLastError());
exit(EXIT_FAILURE);
}
Note that the variable iOptVal has been initialized 0. It's not modified anywhere else. Then you pass that variable to the setsockopt() function.
This sets the SO_BROADCAST option to FALSE. (Which is the default value, so actually it doesn't change the value).
It's not sufficient to set it. You have to set it to the correct value. Which would be TRUE.
You can do this by initializing the variable to 1 instead of 0.