CODEWITHSUNDEEP
c++udpsendto
user3677092
user3677092

Reputation: 31

C++ UDP sendto fails, needs sleep

I have a UDP client C++ code - based on WSA sockets - that works well. The code was originally written in VS6 and I recently recompiled it in VS2010 for 64bit environment, with only little adjustments.

Now, the sendto() fails to send something, if there is no Sleep(..) or any equivalent delay after the sendto() and before closesocket(). "Fails" means, that sendto() returns the proper amount of data, but I see no message on the network (I used wireshark to check this).

This is my code:

void CTest::SendHello()
{
  SOCKET sSocket;
  sSocket = socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP);

  struct sockaddr_in addr;
  addr.sin_family       = AF_INET;
  addr.sin_addr.s_addr  = inet_addr(m_strDstIpAddr);
  addr.sin_port         = htons(m_nTxPort);

  int nMsgLen = 8;
  char pTxBuffer[8];
  *((DWORD*) &pTxBuffer[ 0]) = 0x11223344;
  *((DWORD*) &pTxBuffer[ 4]) = 0;

  int nSent = sendto(sSocket, pTxBuffer, nMsgLen, 0, (struct sockaddr *) &addr, sizeof(addr));
  Sleep(10);   // <- this seems to be necessary

  if (nSent != nMsgLen)
  {
     CString s = "error sending HELO\n";
     AfxMessageBox(s);
  } 
  closesocket(sSocket);

}

Without the Sleep(), the code does not send anything, yet it returns no errors. With the Sleep() it works. Also, this is happens in release version, when compiled for debug, the code also works without the Sleep().

It seems, as if the closesocket() shuts the socket down, before the message is finally sent, but I thought sendto() is a synchronous function. I tried using SO_LINGER, but this is not applicable for SOCK_DGRAM sockets.

Since the code is inside a DLL, I can't create the socket in ctor and delete it in the dtor, because SendHello() might be called from different thread contexts, and I like to avoid to make the code too complicated.

thanks for any help

Upvotes: 2

Views: 4531

Answers (1)

David C. Rankin
David C. Rankin

Reputation: 84642

With UDP there is no ordering of data between the sender and receiver and data sent using UDP datagram sockets isn't guaranteed to arrive. All sleep is doing in your case is practically providing enough time for the data to arrive at the other end. If you want confirmation of receipt and error checking, then you can either code a scheme for UDP or use TCP. In fact, you can turn off the server completely and your client will happily fire UDP packets out without error even though there is nobody listening.

to insure a connection, look at connect(). Nothing prevents using connect with UDP and you can then use send() recv().

Upvotes: 0

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