WSASend for UDP socket triggers FD_READ when no destination available - c++

Question

I'm writing C++ code for UDP socket class to handle basic operations (such as connect, send and receive data). I try using network events mechanism with WSAEventSelect for these basic operations associated with the socket.
When I use WSASend to send data to a (UDP) destination that receives the data everything goes well.
However, when I use WSASend to send data to a destination that does not exist (UDP) or is not reachable through the network I get the FD_READ event triggered. This of course causes serious problems since there is no actual data to receive !!
I can't explain why is this happening - any ideas ?
Maybe I'm doing something wrong, Here are relevant parts of my code:
WSADATA m_wsaData ;
SOCKET m_Socket ;
WSAEVENT m_SocketEvent ;
if(WSAStartup(MAKEWORD(2,2), &m_wsaData) != 0)
{
// some error
}
// Create a new socket to receive datagrams on
struct addrinfo hints, *res = NULL ;
int rc ;
memset(&hints, 0, sizeof(hints)) ;
hints.ai_family = AF_UNSPEC ;
hints.ai_socktype = SOCK_DGRAM ;
hints.ai_protocol = IPPROTO_UDP ;
rc = getaddrinfo("SomePC", "3030", &hints, &res) ;
if(rc == WSANO_DATA)
{
// some error
}
if ((m_Socket = WSASocket(res->ai_family, res->ai_socktype, res->ai_protocol, NULL, 0, 0)) == INVALID_SOCKET)
{
// some error
}
// create event and associate it with the socket
m_SocketEvent = WSACreateEvent() ;
if(m_SocketEvent == WSA_INVALID_EVENT)
{
// some error
}
// associate only the following events: close, read, write
if(SOCKET_ERROR == WSAEventSelect(m_Socket, m_SocketEvent, FD_CLOSE+FD_READ+FD_WRITE))
{
// some error
}
// connect to a server
int ConnectRet = WSAConnect(m_Socket, (SOCKADDR*)res->ai_addr, res->ai_addrlen, NULL, NULL, NULL, NULL) ;
if(ConnectRet == SOCKET_ERROR)
{
// some error
}
And then, whenever I try to send some data over the socket to a (UDP socket) destination that is not listening or not reachable I always get the FD_READ triggered:
char buf[32] ; // some data to send...
WSABUF DataBuf;
DataBuf.len = 32;
DataBuf.buf = (char*)&buf;
DWORD NumBytesActualSent ;
if( SOCKET_ERROR == WSASend(m_Socket, &DataBuf, 1, &NumBytesActualSent,0,0,0))
{
if(WSAGetLastError() == WSAEWOULDBLOCK) // non-blocking socket - wait for send ok ?
{
// handle WSAEWOULDBLOCK...
}
else
{
// some error
return ;
}
}
int ret = WSAWaitForMultipleEvents(1, &m_SocketEvent, FALSE, INFINITE, FALSE) ;
if(ret == WAIT_OBJECT_0)
{
WSANETWORKEVENTS NetworkEvents ;
ZeroMemory(&NetworkEvents, sizeof(NetworkEvents)) ;
if(SOCKET_ERROR == WSAEnumNetworkEvents(m_Socket, m_SocketEvent, &NetworkEvents))
{
return ; // some error
}
if(NetworkEvents.lNetworkEvents & FD_READ) // Read ?
{
if(NetworkEvents.iErrorCode[FD_READ_BIT] != 0) // read not ok ?
{
// some error
}
else
{
TRACE("Read Event Triggered ! - Why ? ? ? ? ? ? ?\n") ;
}
}
}
Any help or insights would be most appriciated !
Thanks,
Amit C.

Answer 1

The easiest way to inspect what really happens is to use Wireshark to capture packets. I do not have Windows PC nearby to provide you a complete example, but my guess is that this is a normal behaviour - you try to send UDP datagram, it gets dropped by a router (host not found) or rejected by a server (socket closed); an ICMP message is sent back to inform you about the failure, which is what you receive and get an event for. Since there is no actual user data, underlying stack translates ICMP message and provides you an appropriate error message via WSARecv() return code. The "false" FD_READ event is necessary since UDP is connectionless protocol and there are no other means to inform about a network state. Simply add error handling for WSARecv() and your code should work fine.