WSASendTo

Article
11/18/2015

A version of this page is also available for

4/8/2010

This function sends data to a specific destination, using overlapped I/O where applicable.

Syntax

int WSASendTo(
  SOCKET s,
  LPWSABUF lpBuffers,
  DWORD dwBufferCount,
  LPDWORD lpNumberOfBytesSent,
  DWORD dwFlags,
  const struct sockaddr FAR* lpTo,
  int iToLen,
  LPWSAOVERLAPPED lpOverlapped,
  LPWSAOVERLAPPED_COMPLETION_ROUTINE lpCompletionRoutine
);

Parameters

s
[in] Descriptor identifying a possibly connected socket.

lpBuffers
[in] Pointer to an array of WSABUF structures. Each WSABUF structure contains a pointer to a buffer and the length of the buffer. This array must remain valid for the duration of the send operation.

dwBufferCount
[in] Number of WSABUF structures in the lpBuffers array.

lpNumberOfBytesSent
[out] Pointer to the number of bytes sent by this call if the I/O operation completes immediately.

dwFlags
[in] Indicator specifying the way in which the call is made.

lpTo
[in] Optional pointer to the address of the target socket.

iToLen
[in] Size of the address in the lpTo parameter.

lpOverlapped
[in] Pointer to a WSAOVERLAPPED structure (ignored for nonoverlapped sockets).

lpCompletionRoutine
[in] Pointer to the completion routine called when the send operation has been completed (ignored for nonoverlapped sockets).

Return Value

If no error occurs and the send operation has completed immediately, this function returns zero. The overlapped structures are updated with the receive results, and the associated event is signalled.

In Windows Embedded CE, the completion object will not be signalled. If an error occurs, a value of SOCKET_ERROR is returned, and a specific error code can be retrieved by calling WSAGetLastError. The error code WSA_IO_PENDING indicates that the overlapped operation has been successfully initiated and that completion will be indicated at a later time. Any other error code indicates that the overlapped operation was not successfully initiated and no completion indication will occur. The following table shows a list of possible error codes.

Error code	Description
WSANOTINITIALISED	A successful WSAStartup call must occur before using this function.
WSAENETDOWN	The network subsystem has failed.
WSAEACCES	The requested address is a broadcast address, but the appropriate flag was not set.
WSAEINTR	The socket was closed.
WSAEINPROGRESS	A blocking Winsock call is in progress, or the service provider is still processing a callback function.
WSAEFAULT	The lpBuffers, lpTo, lpOverlapped, lpNumberOfBytesSent, or lpCompletionRoutine parameter is not part of the user address space, or the lpTo argument is too small.
WSAENETRESET	The connection has been broken due to keep-alive activity detecting a failure while the operation was in progress.
WSAENOBUFS	The Windows Sockets provider reports a buffer deadlock.
WSAENOTCONN	The socket is not connected (connection-oriented sockets only).
WSAENOTSOCK	The descriptor is not a socket.
WSAEOPNOTSUPP	MSG_OOB was specified, but the socket is not stream style such as type SOCK_STREAM, out of band (OOB) data is not supported in the communication domain associated with this socket, MSG_PARTIAL is not supported, or the socket is unidirectional and supports only receive operations.
WSAESHUTDOWN	The socket has been shut down. It is not possible to call WSASendTo on a socket after shutdown has been invoked with how set to SD_SEND or SD_BOTH.
WSAEWOULDBLOCK	For overlapped sockets, there are too many outstanding overlapped I/O requests. For nonoverlapped sockets, the socket is marked as nonblocking and the send operation cannot be completed immediately.
WSAEMSGSIZE	The socket is message-oriented, and the message is larger than the maximum supported by the underlying transport.
WSAEINVAL	The socket has not been bound with bind (Windows Sockets), or the socket is not created with the overlapped flag.
WSAECONNABORTED	The virtual circuit was terminated due to a time-out or other failure.
WSAECONNRESET	The virtual circuit was reset by the remote side.
WSAEADDRNOTAVAIL	The remote address is not a valid address (such as ADDR_ANY).
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.
WSA_IO_PENDING	An overlapped operation was successfully initiated and completion will be indicated at a later time.
WSA_OPERATION_ABORTED	The overlapped operation has been canceled due to the closure of the socket.

Remarks

This function provides functionality over and above the standard sendto function in the following two important areas:

It can be used in conjunction with overlapped sockets to perform overlapped send operations.
It allows multiple send buffers to be specified, enabling a scatter/gather type of I/O.

This function is typically used on a connectionless socket specified by socket s to send a datagram contained in one or more buffers to a specific peer socket identified by the lpTo parameter. Even if the connectionless socket has been previously connected using the connect (Windows Sockets) function to a specific address, lpTo overrides the destination address for that particular datagram only. On a connection-oriented socket, the lpTo and iToLen parameters are ignored; in this case, the WSASendTo is equivalent to WSASend.

For overlapped sockets (created using WSASocket with the WSA_FLAG_OVERLAPPED flag), sending data uses overlapped I/O unless both lpOverlapped and lpCompletionRoutine are NULL, in which case the socket is treated as a nonoverlapped socket. A completion indication will occur (invoking the completion routine or setting of an event object) when the supplied buffers have been consumed by the transport. If the operation does not complete immediately, the final completion status is retrieved through the completion routine or WSAGetOverlappedResult.

If both lpOverlapped and lpCompletionRoutine are NULL, the socket in this function will be treated as a nonoverlapped socket.

For nonoverlapped sockets, the last two parameters (lpOverlapped and lpCompletionRoutine) are ignored and WSASendTo adopts the same blocking semantics as send. Data is copied from the supplied buffers into the transport's buffer. If the socket is nonblocking and stream-oriented, and there is not sufficient space in the transport's buffer, WSASendTo returns with only part of the application's buffers having been consumed. Given the same buffer situation and a blocking socket, WSASendTo will block until all of the application's buffer contents have been consumed.

The array of WSABUF structures indicated by the lpBuffers parameter is transient. If this operation is completed in an overlapped manner, it is the service provider's responsibility to capture these WSABUF structures before returning from this call. This enables applications to build stack-based WSABUF arrays.

For message-oriented sockets, care must be taken not to exceed the maximum message size of the underlying transport, which can be obtained by getting the value of socket option SO_MAX_MSG_SIZE. If the data is too long to pass atomically through the underlying protocol, the error WSAEMSGSIZE is returned and no data is transmitted.

The successful completion of a WSASendTo call does not indicate that the data was successfully delivered.

The dwFlags parameter can be used to influence the behavior of the function invocation beyond the options specified for the associated socket. That is, the semantics of this function are determined by the socket options and the dwFlags parameter. The following table shows the values that are used with the bitwise OR operator to construct the dwFlags parameter.

Value	Description
MSG_DONTROUTE	Specifies that the data should not be subject to routing. A Windows Sockets service provider may choose to ignore this flag.
MSG_OOB	Send out of band (OOB) data (stream-style socket such as SOCK_STREAM only).
MSG_PARTIAL	Specifies that lpBuffers only contains a partial message. Note that the error code WSAEOPNOTSUPP will be returned by transports that do not support partial message transmissions.

Overlapped Socket I/O

Note

For Windows Embedded CE, avoid specifying completion routines for overlapped I/O operations. Because Windows Embedded CE does not support asynchronous procedure calls (APCs), which occur in the calling thread, the OS has to spin a thread for each call that specifies a completion routine. With one thread create per function call, using completion routines with overlapped I/O can quickly become very memory-consuming. Using events is recommended instead.

If an overlapped operation completes immediately, WSASendTo returns a value of zero and the lpNumberOfBytesSent parameter is updated with the number of bytes sent. If the overlapped operation is successfully initiated and will complete later, WSASendTo returns SOCKET_ERROR and indicates error code WSA_IO_PENDING. In this case, lpNumberOfBytesSent is not updated. When the overlapped operation completes, the amount of data transferred is indicated either through the cbTransferred parameter in the completion routine (if specified) or through the lpcbTransfer parameter in WSAGetOverlappedResult.

The WSASendTo function can be called from within the completion routine of a previous WSARecv, WSARecvFrom, WSASend, or WSASendTo function. This permits time-sensitive data transmissions to occur entirely within a pre-emptive context.

The lpOverlapped parameter must be valid for the duration of the overlapped operation. If multiple I/O operations are simultaneously outstanding, each must reference a separate WSAOVERLAPPED structure.

If the lpCompletionRoutine parameter is NULL, the hEvent parameter of lpOverlapped is signaled when the overlapped operation completes if it contains a valid event object handle. An application can use WSAGetOverlappedResult to wait or poll on the event object.

If lpCompletionRoutine is not NULL, the hEvent parameter is ignored and can be used by the application to pass context information to the completion routine. A caller that passes a non-NULL lpCompletionRoutine parameter and later calls WSAGetOverlappedResult for the same overlapped I/O request may not set the fWait parameter for that invocation of WSAGetOverlappedResult to TRUE. In this case, the usage of the hEvent parameter is undefined and attempting to wait on the hEvent parameter would produce unpredictable results.

Transport providers allow an application to invoke send and receive operations from within the context of the socket I/O completion routine and guarantee that, for a given socket, I/O completion routines will not be nested. This permits time-sensitive data transmissions to occur entirely within a pre-emptive context.

The following syntax shows the prototype of the completion routine.

void CALLBACK CompletionRoutine(
  IN DWORD dwError, 
  IN DWORD cbTransferred, 
  IN LPWSAOVERLAPPED lpOverlapped, 
  IN DWORD dwFlags
);

The CompletionRoutine function is a placeholder for an application-defined or library-defined function name. The dwError parameter specifies the completion status for the overlapped operation as indicated by lpOverlapped. The cbTransferred parameter specifies the number of bytes sent. Currently there are no flag values defined and dwFlags will be zero. This function does not return a value.

Returning from this function allows invocation of another pending completion routine for this socket. All waiting completion routines are called before the alertable thread's wait is satisfied with a return code of WSA_IO_COMPLETION. The completion routines can be called in any order, not necessarily in the same order in which the overlapped operations are completed. However, the posted buffers are guaranteed to be sent in the same order they are supplied.