Question

In my socket server I use WSAEventSelect() and WSAEnumNetworkEvents() to detect FD_ACCEPT and FD_CLOSE events.

Here is the simplified version of my code:

HANDLE sockEv=CreateEvent(NULL,TRUE,FALSE,NULL);
WSAEventSelect(servSocket,sockEv,FD_ACCEPT|FD_CLOSE);
for(;;) {
  if(WSAWaitForMultipleEvents(1,&sockEv,FALSE,INFINITE,FALSE)==WSA_WAIT_EVENT_0) {
    WSANETWORKEVENTS wsaEvents={0};
    WSAEnumNetworkEvents(servSocket,sockEv,&wsaEvents);
    if((wsaEvents.lNetworkEvents & FD_ACCEPT)==FD_ACCEPT) {
      SOCKET clntSock=accept(servSocket,(sockaddr*)clientAddr,&addrlen);
      ....
    }
    else if (wsaEvents.lNetworkEvents==0) {
    // this event occurs after each FD_ACCEPT event
    }
    ....
  }
  ....
}

The problem is that each time a new connection is accepted from a client, I receive 2 events: first with WSANETWORKEVENTS.lNetworkEvents set to FD_ACCEP, then with WSANETWORKEVENTS.lNetworkEvents set to zero. That is the for(;;) loop is executed 2 times each time a new client is accepted. Or maybe the sockEv is not reset by WSAEnumNetworkEvents after detecting FD_ACCEPT.

What does this zero event mean and why it happens?
I didn’t find anything about it in the documentation.

UPDATE: Compilable sample

int _tmain(int argc, _TCHAR* argv[])
{
  WSADATA wsaData;
  WSAStartup(MAKEWORD(2,2),&wsaData);
  try {
    SOCKET servSocket=socket(AF_INET,SOCK_STREAM,IPPROTO_TCP);

    sockaddr_in serv={0};
    serv.sin_family=AF_INET;
    serv.sin_addr.s_addr=htonl(INADDR_ANY);
    serv.sin_port=htons(18081);
    memset(serv.sin_zero,0,sizeof(serv.sin_zero));

    if(bind(servSocket,(sockaddr*)&serv,sizeof(serv))==SOCKET_ERROR)
      throw WSAGetLastError();
    if(listen(servSocket,SOMAXCONN)==SOCKET_ERROR)
      throw WSAGetLastError();
    HANDLE sockEv=CreateEvent(NULL,TRUE,FALSE,NULL);
    if(WSAEventSelect(servSocket,sockEv,FD_ACCEPT|FD_CLOSE)==SOCKET_ERROR) 
      throw WSAGetLastError();

    for(;;) {
      if(WSAWaitForMultipleEvents(1,&sockEv,FALSE,INFINITE,FALSE)!=WSA_WAIT_EVENT_0)
        throw -1;
      WSANETWORKEVENTS wsaEvents={0};
      if(WSAEnumNetworkEvents(servSocket,sockEv,&wsaEvents)==SOCKET_ERROR)
        throw WSAGetLastError();
      if((wsaEvents.lNetworkEvents & FD_ACCEPT)==FD_ACCEPT) {
        SOCKET clntSocket=accept(servSocket,0,0);
        closesocket(clntSocket);
        puts("accept");
      }
      else if(wsaEvents.lNetworkEvents==0) {
        puts("zero event");
      }
    }
  }
  catch(int errorCode) {
    printf("Last Error: %d\n",errorCode);
  }
  WSACleanup();
  return 0;
}

I tried this code on WinXP 32bit.

Answer 1

Answering my own question. It seems I found why and when this happens. This zero event occurs after the client socket (the socket returned by accept()) is closed. It won’t happen if you don’t use FD_CLOSE with WSAEventSelect. So one can just ignore it.
MSDN says nothing about this behavior.