The WSASend function sends data on a connected socket.

Syntax

int WSASend(
  __in   SOCKET s,
  __in   LPWSABUF lpBuffers,
  __in   DWORD dwBufferCount,
  __out  LPDWORD lpNumberOfBytesSent,
  __in   DWORD dwFlags,
  __in   LPWSAOVERLAPPED lpOverlapped,
  __in   LPWSAOVERLAPPED_COMPLETION_ROUTINE lpCompletionRoutine
);

Parameters

s [in]

A descriptor that identifies a connected socket.

lpBuffers [in]

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.

dwBufferCount [in]

The number of WSABUF structures in the lpBuffers array.

lpNumberOfBytesSent [out]

A pointer to the number, in bytes, sent by this call if the I/O operation completes immediately. If the lpOverlapped parameter is non-NULL, this parameter is optional and can be set to NULL.

dwFlags [in]

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

lpOverlapped [in]

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

lpCompletionRoutine [in]

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.

Remarks

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.

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 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 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. 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.

If you are using I/O completion ports, be aware that the order of calls made to WSASend is also the order in which the buffers are populated. WSASend should not be called on the same socket simultaneously from different threads, because it can result in an unpredictable buffer order.

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>

#define DATA_BUFSIZE 4096
#define SEND_COUNT   10

int __cdecl main()
{
    WSADATA wsd;
    struct addrinfo *result = NULL;
    struct addrinfo hints = { 0 };
    WSAOVERLAPPED SendOverlapped = { 0 };
    SOCKET ListenSocket = INVALID_SOCKET;
    SOCKET AcceptSocket = INVALID_SOCKET;
    WSABUF DataBuf;
    DWORD SendBytes;
    DWORD Flags;
    char buffer[DATA_BUFSIZE];
    int err, rc, i;

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

    ListenSocket = socket(result->ai_family,
                          result->ai_socktype, result->ai_protocol);
    if (ListenSocket == INVALID_SOCKET) {
        fprintf(stderr, "socket failed: %d\n", WSAGetLastError());
        freeaddrinfo(result);
        return 1;
    }

    rc = bind(ListenSocket, result->ai_addr, (int) result->ai_addrlen);
    if (rc == SOCKET_ERROR) {
        fprintf(stderr, "bind failed: %d\n", WSAGetLastError());
        freeaddrinfo(result);
        closesocket(ListenSocket);
        return 1;
    }

    rc = listen(ListenSocket, 1);
    if (rc == SOCKET_ERROR) {
        fprintf(stderr, "listen failed: %d\n", WSAGetLastError());
        freeaddrinfo(result);
        closesocket(ListenSocket);
        return 1;
    }
    // Accept an incoming connection request
    AcceptSocket = accept(ListenSocket, NULL, NULL);
    if (AcceptSocket == INVALID_SOCKET) {
        fprintf(stderr, "accept failed: %d\n", WSAGetLastError());
        freeaddrinfo(result);
        closesocket(ListenSocket);
        return 1;
    }

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

    // Create an event handle and setup an overlapped structure.
    SendOverlapped.hEvent = WSACreateEvent();
    if (SendOverlapped.hEvent == NULL) {
        fprintf(stderr, "WSACreateEvent failed: %d\n", WSAGetLastError());
        freeaddrinfo(result);
        closesocket(ListenSocket);
        closesocket(AcceptSocket);
        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()))) {
            fprintf(stderr, "WSASend failed: %d\n", err);
            break;
        }

        rc = WSAWaitForMultipleEvents(1, &SendOverlapped.hEvent, TRUE, INFINITE,
                                      TRUE);
        if (rc == WSA_WAIT_FAILED) {
            fprintf(stderr, "WSAWaitForMultipleEvents failed: %d\n",
                    WSAGetLastError());
            break;
        }

        rc = WSAGetOverlappedResult(AcceptSocket, &SendOverlapped, &SendBytes,
                                    FALSE, &Flags);
        if (rc == FALSE) {
            fprintf(stderr, "WSASend operation failed: %d\n",
                    WSAGetLastError());
            break;
        }

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

        WSAResetEvent(SendOverlapped.hEvent);

    }

    WSACloseEvent(SendOverlapped.hEvent);
    closesocket(AcceptSocket);
    closesocket(ListenSocket);
    freeaddrinfo(result);

    WSACleanup();

    return 0;
}

Requirements

See Also

connect

send

socket

Winsock Reference

Winsock Functions

WSABUF

WSACloseEvent

WSAConnect

WSACreateEvent

WSAGetOverlappedResult

WSAOVERLAPPED

WSASocket

WSAWaitForMultipleEvents

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Build date: 10/29/2009