The Windows Sockets WSASend function sends data on a connected socket.

int WSASend (


LPWSABUF lpBuffers,

DWORD dwBufferCount,

LPDWORD lpNumberOfBytesSent,

DWORD dwFlags,






[in] A descriptor identifying a connected socket.


[in] A 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.


[in] The number of WSABUF structures in the lpBuffers array.


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


[in] Specifies the way in which the call is made.


[in] A pointer to a WSAOVERLAPPED structure (ignored for nonoverlapped sockets).


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


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

  1. It can be used in conjunction with overlapped sockets to perform overlapped send operations.

  2. It allows multiple send buffers to be specified making it applicable to the scatter/gather type of I/O.

WSASend is used to write outgoing data from one or more buffers on a connection-oriented socket specified by s. It may also be used, however, on connectionless sockets which have a stipulated default peer address established through the connect or WSAConnect function.

For overlapped sockets (created using WSASocket with flag WSA_FLAG_OVERLAPPED) this will occur using 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 (invocation of the completion routine or setting of an event object) when the supplied 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 WSASend adopts the same blocking semantics as send. Data is copied from the supplied buffer(s) into the transport's buffer. If the socket is nonblocking and stream oriented, and there is not sufficient space in the transport's buffer, WSASend will return with only part of the application's buffers having been consumed. Given the same buffer situation and a blocking socket, WSASend will block until all of the application's buffer contents have been consumed.

The array of WSABUF structures pointed to 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 provider, 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.

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

dwFlags may 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 or-ing any of the following values:

Specifies that the data should not be subject to routing. A Windows Sockets service provider can choose to ignore this flag.
Send out-of-band data on a stream-style socket such as SOCK_STREAM only. (See section Out-Of-Band data for a discussion of this topic.)
Specifies that lpBuffers only contains a partial message. Note that the error code WSAEOPNOTSUPP will be returned by transports which do not support partial message transmissions.

Overlapped socket I/O:

If an overlapped operation completes immediately, WSASend 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, WSASend 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.

This function may 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 preemptive 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 overlapped structure. The WSAOVERLAPPED structure has the following form:

typedef struct _WSAOVERLAPPED {

DWORD Internal; // reserved

DWORD InternalHigh; // reserved

DWORD Offset; // reserved

DWORD OffsetHigh; // reserved



If the lpCompletionRoutine parameter is NULL, the hEvent field 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 field is ignored and can be used by the application to pass context information to the completion routine.

The completion routine follows the same rules as stipulated for Win32 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.

The 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:

  • oid CALLBACK CompletionROUTINE(

CompletionRoutine is a placeholder for an application-defined or library-defined function name. dwError specifies the completion status for the overlapped operation as indicated by lpOverlapped. cbTransferred 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 may be called in any order, not necessarily in the same order the overlapped operations are completed. However, the posted buffers are guaranteed to be sent in the same order they are supplied.

Return Values

If no error occurs and the send operation has completed immediately, WSASend returns zero. Note that in this case, the completion routine will have already been scheduled, and 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 may 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.

Error Codes

A successful WSAStartup must occur before using this function.
The network subsystem has failed.
The requested address is a broadcast address, but the appropriate flag was not set.
The (blocking) call was canceled through WSACancelBlockingCall.
A blocking Windows Sockets 1.1 call is in progress, or the service provider is still processing a callback function.
The lpBuffers argument is not totally contained in a valid part of the user address space.
The connection has been broken due to the remote host resetting.
The Windows Sockets provider reports a buffer deadlock.
The socket is not connected.
The descriptor is not a socket.
MSG_OOB was specified, but the socket is not stream style such as type SOCK_STREAM, out-of-band 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.
The socket has been shut down; it is not possible to WSASend 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.
The socket is message oriented, and the message is larger than the maximum supported by the underlying transport.
The socket has not been bound with bind, or the socket is not created with the overlapped flag.
The virtual circuit was terminated due to a time-out or other failure.
The virtual circuit was reset by the remote side.
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.

See Also

, WSACreateEvent, WSAGetOverlappedResult, WSASocket, WSAWaitForMultipleEvents

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