WSASend function (winsock2.h)

The WSASend function sends data on a connected socket.


  [in]  SOCKET                             s,
  [in]  LPWSABUF                           lpBuffers,
  [in]  DWORD                              dwBufferCount,
  [out] LPDWORD                            lpNumberOfBytesSent,
  [in]  DWORD                              dwFlags,
  [in]  LPWSAOVERLAPPED                    lpOverlapped,


[in] s

A descriptor that identifies a connected socket.

[in] lpBuffers

A pointer to an array of WSABUF structures. Each WSABUF structure contains a pointer to a buffer and the length, in bytes, of the buffer. For a Winsock application, once the WSASend 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.

[in] dwBufferCount

The number of WSABUF structures in the lpBuffers array.

[out] lpNumberOfBytesSent

A pointer to the number, in 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.

[in] dwFlags

The flags used to modify the behavior of the WSASend function call. For more information, see Using dwFlags in the Remarks section.

[in] lpOverlapped

A pointer to a WSAOVERLAPPED structure. This parameter is ignored for nonoverlapped sockets.

[in] lpCompletionRoutine


A pointer to the completion routine called when the send operation has been completed. This parameter is ignored for nonoverlapped sockets.

Return value

If no error occurs and the send operation has completed immediately, WSASend 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.

Error code Meaning
The virtual circuit was terminated due to a time-out or other failure.
For a stream socket, the virtual circuit was reset by the remote side. The application should close the socket as it is no longer usable. For a UDP datagram socket, this error would indicate that a previous send operation resulted in an ICMP "Port Unreachable" message.
The lpBuffers, lpNumberOfBytesSent, lpOverlapped, lpCompletionRoutine parameter is not totally contained in a valid part of the user address space.
A blocking Windows Socket 1.1 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 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 stream socket, the connection has been broken due to keep-alive activity detecting a failure while the operation was in progress. For a datagram socket, this error indicates that the time to live has expired.
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, 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.
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.
Windows NT:  

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, the execution of the "SIO_FLUSH" command in WSAIoctl, or the thread that initiated the overlapped request exited before the operation completed. For more information, see the Remarks section.


The WSASend function provides functionality over and above the standard send 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.
The WSASend function is used to write outgoing data from one or more buffers on a connection-oriented socket specified by s. It can also be used, however, on connectionless sockets that have a stipulated default peer address established through the connect or WSAConnect function.

A socket created by the socket function will have the overlapped attribute as the default. A socket created by the WSASocket function with the dwFlags parameter passed to WSASocket with the WSA_FLAG_OVERLAPPED bit set will have the overlapped attribute. For sockets with the overlapped attribute, WSASend uses overlapped I/O unless both the lpOverlapped and lpCompletionRoutine parameters are NULL. In that case, the socket is treated as a non-overlapped socket. A completion indication will occur, invoking the completion of a 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.

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

For non-overlapped sockets, the last two parameters (lpOverlapped, lpCompletionRoutine) are ignored and WSASend adopts the same blocking semantics as send. Data is copied from the buffer(s) into the transport's buffer. If the socket is non-blocking 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 buffer contents have been consumed.

Note  The socket options SO_RCVTIMEO and SO_SNDTIMEO apply only to blocking sockets.
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, do not 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.

Windows Me/98/95:  The WSASend function does not support more than 16 buffers.

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

Using dwFlags

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.
Value Meaning
MSG_DONTROUTE Specifies that the data should not be subject to routing. A Windows Sockets service provider can choose to ignore this flag.
MSG_OOB Send OOB data on a 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.
Note  When issuing a blocking Winsock call such as WSASend with the lpOverlapped parameter set to NULL, Winsock may need to wait for a network event before the call can complete. Winsock performs an alertable wait in this situation, which can be interrupted by an asynchronous procedure call (APC) scheduled on the same thread. Issuing another blocking Winsock call inside an APC that interrupted an ongoing blocking Winsock call on the same thread will lead to undefined behavior, and must never be attempted by Winsock clients.

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.
Note  All I/O initiated by a given thread is canceled when that thread exits. For overlapped sockets, pending asynchronous operations can fail if the thread is closed before the operations complete. For more information, see ExitThread.
The WSASend function using overlapped I/O can be called from within the completion routine of a previous WSARecv, WSARecvFrom, WSASend, or WSASendTo function. This enables 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 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-NULL lpCompletionRoutine 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.

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 following C++ code example is a prototype of the completion routine.

void CALLBACK CompletionROUTINE(
  IN DWORD dwError,
  IN DWORD cbTransferred,
  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. 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 can 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 specified.

The order of calls made to WSASend is also the order in which the buffers are transmitted to the transport layer. WSASend should not be called on the same stream-oriented socket concurrently from different threads, because some Winsock providers may split a large send request into multiple transmissions, and this may lead to unintended data interleaving from multiple concurrent send requests on the same stream-oriented socket.

Example Code

The following code example shows how to use the WSASend function in overlapped I/O mode.

#include <Windows.h>

#include <winsock2.h>
#include <ws2tcpip.h>
#include <stdio.h>
#include <stdlib.h>

// Need to link with Ws2_32.lib
#pragma comment(lib, "ws2_32.lib")

#define DATA_BUFSIZE 4096
#define SEND_COUNT   10

int __cdecl main()
    WSADATA wsd;

    struct addrinfo *result = NULL;
    struct addrinfo hints;
    WSAOVERLAPPED SendOverlapped;


    WSABUF DataBuf;
    DWORD SendBytes;
    DWORD Flags;

    char buffer[DATA_BUFSIZE];

    int err = 0;
    int rc, i;

    // Load Winsock
    rc = WSAStartup(MAKEWORD(2, 2), &wsd);
    if (rc != 0) {
        printf("Unable to load Winsock: %d\n", rc);
        return 1;

    // Make sure the hints struct is zeroed out
    SecureZeroMemory((PVOID) & hints, sizeof(struct addrinfo));

    // Initialize the hints to obtain the 
    // wildcard bind address for IPv4
    hints.ai_family = AF_INET;
    hints.ai_socktype = SOCK_STREAM;
    hints.ai_protocol = IPPROTO_TCP;
    hints.ai_flags = AI_PASSIVE;

    rc = getaddrinfo(NULL, "27015", &hints, &result);
    if (rc != 0) {
        printf("getaddrinfo failed with error: %d\n", rc);
        return 1;

    ListenSocket = socket(result->ai_family,
                          result->ai_socktype, result->ai_protocol);
    if (ListenSocket == INVALID_SOCKET) {
        printf("socket failed with error: %d\n", WSAGetLastError());
        return 1;

    rc = bind(ListenSocket, result->ai_addr, (int) result->ai_addrlen);
    if (rc == SOCKET_ERROR) {
        printf("bind failed with error: %d\n", WSAGetLastError());
        return 1;

    rc = listen(ListenSocket, 1);
    if (rc == SOCKET_ERROR) {
        printf("listen failed with error: %d\n", WSAGetLastError());
        return 1;
    // Accept an incoming connection request
    AcceptSocket = accept(ListenSocket, NULL, NULL);
    if (AcceptSocket == INVALID_SOCKET) {
        printf("accept failed with error: %d\n", WSAGetLastError());
        return 1;

    printf("Client Accepted...\n");

    // Make sure the SendOverlapped struct is zeroed out
    SecureZeroMemory((PVOID) & SendOverlapped, sizeof (WSAOVERLAPPED));

    // Create an event handle and setup the overlapped structure.
    SendOverlapped.hEvent = WSACreateEvent();
    if (SendOverlapped.hEvent == NULL) {
        printf("WSACreateEvent failed with error: %d\n", WSAGetLastError());
        return 1;

    DataBuf.len = DATA_BUFSIZE;
    DataBuf.buf = buffer;

    for (i = 0; i < SEND_COUNT; i++) {

        rc = WSASend(AcceptSocket, &DataBuf, 1,
                     &SendBytes, 0, &SendOverlapped, NULL);
        if ((rc == SOCKET_ERROR) &&
            (WSA_IO_PENDING != (err = WSAGetLastError()))) {
            printf("WSASend failed with error: %d\n", err);

        rc = WSAWaitForMultipleEvents(1, &SendOverlapped.hEvent, TRUE, INFINITE,
        if (rc == WSA_WAIT_FAILED) {
            printf("WSAWaitForMultipleEvents failed with error: %d\n",

        rc = WSAGetOverlappedResult(AcceptSocket, &SendOverlapped, &SendBytes,
                                    FALSE, &Flags);
        if (rc == FALSE) {
            printf("WSASend failed with error: %d\n", WSAGetLastError());

        printf("Wrote %d bytes\n", SendBytes);





    return 0;

Windows Phone 8: This function is supported for Windows Phone Store apps on Windows Phone 8 and later.

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.


Requirement Value
Minimum supported client Windows 8.1, Windows Vista [desktop apps | UWP apps]
Minimum supported server Windows Server 2003 [desktop apps | UWP apps]
Target Platform Windows
Header winsock2.h
Library Ws2_32.lib
DLL Ws2_32.dll

See also









Winsock Functions

Winsock Reference