Call this member function to send data to a specific destination.

      int SendTo(
   const void* lpBuf,
   int nBufLen,
   UINT nHostPort,
   LPCTSTR lpszHostAddress = NULL,
   int nFlags = 0 
int SendTo(
   const void* lpBuf,
   int nBufLen,
   const SOCKADDR* lpSockAddr,
   int nSockAddrLen,
   int nFlags = 0 


A buffer containing the data to be transmitted.


The length of the data in lpBuf in bytes.


The port identifying the socket application.


The network address of the socket to which this object is connected: a machine name such as "," or a dotted number such as "".


Specifies the way in which the call is made. The semantics of this function are determined by the socket options and the nFlags parameter. The latter is constructed by combining any of the following values with the C++ OR operator:

  • MSG_DONTROUTE   Specifies that the data should not be subject to routing. A Windows Sockets supplier can choose to ignore this flag.

  • MSG_OOB   Send out-of-band data (SOCK_STREAM only).


A pointer to a SOCKADDR structure that contains the address of the target socket.


The length of the address in lpSockAddr in bytes.

If no error occurs, SendTo returns the total number of characters sent. (Note that this can be less than the number indicated by nBufLen.) Otherwise, a value of SOCKET_ERROR is returned, and a specific error code can be retrieved by calling GetLastError. The following errors apply to this member function:

  • WSANOTINITIALISED   A successful AfxSocketInit must occur before using this API.

  • WSAENETDOWN   The Windows Sockets implementation detected that the network subsystem failed.

  • WSAEACCES   The requested address is a broadcast address, but the appropriate flag was not set.

  • WSAEINPROGRESS   A blocking Windows Sockets operation is in progress.

  • WSAEFAULT   The lpBuf or lpSockAddr parameters are not part of the user address space, or the lpSockAddr argument is too small (less than the size of a SOCKADDR structure).

  • WSAEINVAL   The host name is invalid.

  • WSAENETRESET   The connection must be reset because the Windows Sockets implementation dropped it.

  • WSAENOBUFS   The Windows Sockets implementation reports a buffer deadlock.

  • WSAENOTCONN   The socket is not connected (SOCK_STREAM only).

  • WSAENOTSOCK   The descriptor is not a socket.

  • WSAEOPNOTSUPP   MSG_OOB was specified, but the socket is not of type SOCK_STREAM.

  • WSAESHUTDOWN   The socket has been shut down; it is not possible to call SendTo on a socket after ShutDown has been invoked with nHow set to 1 or 2.

  • WSAEWOULDBLOCK   The socket is marked as nonblocking and the requested operation would block.

  • WSAEMSGSIZE   The socket is of type SOCK_DGRAM, and the datagram is larger than the maximum supported by the Windows Sockets implementation.

  • WSAECONNABORTED   The virtual circuit was aborted due to timeout or other failure.

  • WSAECONNRESET   The virtual circuit was reset by the remote side.

  • WSAEADDRNOTAVAIL   The specified address is not available from the local machine.

  • WSAEAFNOSUPPORT   Addresses in the specified family cannot be used with this socket.

  • WSAEDESTADDRREQ   A destination address is required.

  • WSAENETUNREACH   The network cannot be reached from this host at this time.

SendTo is used on datagram or stream sockets and is used to write outgoing data on a socket. For datagram sockets, care must be taken not to exceed the maximum IP packet size of the underlying subnets, which is given by the iMaxUdpDg element in the WSADATA structure filled out by AfxSocketInit. If the data is too long to pass atomically through the underlying protocol, the error WSAEMSGSIZE is returned, and no data is transmitted.

Note that the successful completion of a SendTo does not indicate that the data was successfully delivered.

SendTo is only used on a SOCK_DGRAM socket to send a datagram to a specific socket identified by the lpSockAddr parameter.

To send a broadcast (on a SOCK_DGRAM only), the address in the lpSockAddr parameter should be constructed using the special IP address INADDR_BROADCAST (defined in the Windows Sockets header file WINSOCK.H) together with the intended port number. Or, if the lpszHostAddress parameter is NULL, the socket is configured for broadcast. It is generally inadvisable for a broadcast datagram to exceed the size at which fragmentation can occur, which implies that the data portion of the datagram (excluding headers) should not exceed 512 bytes.

To handle IPv6 addresses, use CAsyncSocket::SendToEx.


Header: afxsock.h