The WSASendTo function sends data to a specific destination, using overlapped I/O where applicable.
int WSAAPI WSASendTo( SOCKET s, LPWSABUF lpBuffers, DWORD dwBufferCount, LPDWORD lpNumberOfBytesSent, DWORD dwFlags, const sockaddr *lpTo, int iTolen, LPWSAOVERLAPPED lpOverlapped, LPWSAOVERLAPPED_COMPLETION_ROUTINE lpCompletionRoutine );
A descriptor identifying a (possibly connected) socket.
A pointer to an array of WSABUF structures. Each WSABUF structure contains a pointer to a buffer and the length of the buffer, in bytes. For a Winsock application, once the WSASendTo function is called, the system owns these buffers and the application may not access them. This array must remain valid for the duration of the send operation.
The number of WSABUF structures in the lpBuffers array.
A pointer to the number of bytes sent by this call if the I/O operation completes immediately.
Use NULL for this parameter if the lpOverlapped parameter is not NULL to avoid potentially erroneous results. This parameter can be NULL only if the lpOverlapped parameter is not NULL.
The flags used to modify the behavior of the WSASendTo function call.
An optional pointer to the address of the target socket in the SOCKADDR structure.
The size, in bytes, of the address in the lpTo parameter.
A pointer to a WSAOVERLAPPED structure (ignored for nonoverlapped sockets).
A pointer to the completion routine called when the send operation has been completed (ignored for nonoverlapped sockets).
If no error occurs and the send operation has completed immediately, WSASendTo returns zero. In this case, the completion routine will have already been scheduled to be called once the calling thread is in the alertable state. Otherwise, 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 requested address is a broadcast address, but the appropriate flag was not set.|
|The remote address is not a valid address (such as ADDR_ANY).|
|Addresses in the specified family cannot be used with this socket.|
|For a UDP datagram socket, this error would indicate that a previous send operation resulted in an ICMP "Port Unreachable" message.|
|A destination address is required.|
|The lpBuffers, lpTo, lpOverlapped, lpNumberOfBytesSent, or lpCompletionRoutine parameters are not part of the user address space, or the lpTo parameter is too small.|
|A socket operation was attempted to an unreachable host.|
|A blocking Windows Sockets 1.1 call is in progress, or the service provider is still processing a callback function.|
|A blocking Windows Socket 1.1 call was canceled through WSACancelBlockingCall.|
|The socket has not been bound with bind, or the socket is not created with the overlapped flag.|
|The socket is message oriented, and the message is larger than the maximum supported by the underlying transport.|
|The network subsystem has failed.|
|For a datagram socket, this error indicates that the time to live has expired.|
|The network cannot be reached from this host at this time.|
|The Windows Sockets provider reports a buffer deadlock.|
|The socket is not connected (connection-oriented sockets only).|
|The descriptor is not a socket.|
|The socket has been shut down; it is not possible to WSASendTo on a socket after shutdown has been invoked with how set to SD_SEND or SD_BOTH.|
Overlapped sockets: there are too many outstanding overlapped I/O requests. Nonoverlapped sockets: The socket is marked as nonblocking and the send operation cannot be completed immediately.
|A successful WSAStartup call must occur before using this function.|
|An overlapped operation was successfully initiated and completion will be indicated at a later time.|
|The overlapped operation has been canceled due to the closure of the socket, or the execution of the SIO_FLUSH command in WSAIoctl.|
The WSASendTo function provides enhanced features over the standard sendto function in 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 making it applicable to the scatter/gather type of I/O.
For overlapped sockets (created using WSASocket with flag WSA_FLAG_OVERLAPPED) 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 buffer(s) 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.
For nonoverlapped sockets, the last two parameters (lpOverlapped, lpCompletionRoutine) are ignored and WSASendTo adopts the same blocking semantics as send. Data is copied from the buffer(s) into the transport 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.
If this function is completed in an overlapped manner, it is the Winsock service provider's responsibility to capture the WSABUF structures before returning from this call. This enables applications to build stack-based WSABUF arrays pointed to by the lpBuffers parameter.
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.
If the socket is unbound, unique values are assigned to the local association by the system, and the socket is then marked as bound.
If the socket is connected, the getsockname function can be used to determine the local IP address and port associated with the socket.
If the socket is not connected, the
getsockname function can be used to determine the local port number associated with the socket but the IP address returned is set to the wildcard address for the given protocol (for example, INADDR_ANY or "0.0.0.0" for IPv4 and IN6ADDR_ANY_INIT or "::" for IPv6).
The successful completion of a WSASendTo 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 latter is constructed by using the bitwise OR operator with any of any of the values listed in the following table.
|MSG_DONTROUTE||Specifies that the data should not be subject to routing. A Windows Socket service provider may choose to ignore this flag.|
|MSG_OOB||Send OOB data (stream-style socket such as SOCK_STREAM only).|
|MSG_PARTIAL||Specifies that lpBuffers only contains a partial message. Be aware that the error code WSAEOPNOTSUPP will be returned by transports that do not support partial message transmissions.|
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 WSAWaitForMultipleEvents or 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-NULLlpCompletionRoutine 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.
The completion routine follows the same rules as stipulated for Windows file I/O completion routines. The completion routine will not be invoked until the thread is in an alertable wait state such as can occur when the function WSAWaitForMultipleEvents with the fAlertable parameter set to TRUE is invoked.
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 preemptive context.
The prototype of the completion routine is as follows.
void CALLBACK CompletionROUTINE( IN DWORD dwError, IN DWORD cbTransferred, IN LPWSAOVERLAPPED lpOverlapped, IN DWORD dwFlags );
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 specified.
Windows 8.1 and Windows Server 2012 R2: This function is supported for Windows Store apps on Windows 8.1, Windows Server 2012 R2, and later.
|Minimum supported client||Windows 8.1, Windows Vista [desktop apps | UWP apps]|
|Minimum supported server||Windows Server 2003 [desktop apps | UWP apps]|