The WSARecv function receives data from a connected socket or a bound connectionless socket.
WSARecv函數從一個socket接收數據

Syntax

int WSARecv(
  __in     SOCKET s,
  __inout  LPWSABUF lpBuffers,
  __in     DWORD dwBufferCount,
  __out    LPDWORD lpNumberOfBytesRecvd,
  __inout  LPDWORD lpFlags,
  __in     LPWSAOVERLAPPED lpOverlapped,
  __in     LPWSAOVERLAPPED_COMPLETION_ROUTINE lpCompletionRoutine
);

Parameters

s [in]
A descriptor identifying a connected socket.
一個已經連接的 socket

lpBuffers [in, out]

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.
一個指向WSABUF結構數組的指針。每一個WSABUF結構包含一個緩沖區的指針和緩沖區的長度。

dwBufferCount [in]
The number of WSABUF structures in the lpBuffers array.
lpBuffers數組中WSABUF結構的大小。

lpNumberOfBytesRecvd [out]
????????? A pointer to the number, in bytes, of data received by this call if the receive operation completes immediately. If the lpOverlapped parameter is non-NULL, this parameter is optional and can be set to NULL.
????????? 如果接收操作立即結束，一個指向本調用所接收的字節數的指針。

lpFlags [in, out]
????????? A pointer to flags used to modify the behavior of the WSARecv function call. For more information, see the Remarks section.
??????????一個指向標志位的指針。
?

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

????????? 一個指向WSAOVERLAPPED結構的指針（對于非重疊套接口則忽略）。??????

lpCompletionRoutine [in]
?????????? A pointer to the completion routine called when the receive operation has been completed (ignored for nonoverlapped sockets).
?????????? 一個指向接收操作結束后調用的例程的指針（對于非重疊套接口則忽略）。

Return Value
返回值

If no error occurs and the receive operation has completed immediately, WSARecv 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.

若無錯誤發生且接收操作立即完成，則WSARecv()函數返回所接收的字節數。如果連接結束，則返回0。請注意在這種情況下完成指示（啟動指定的完成例程或設置一個事件對象）將早已發生。否則的話，將返回SOCKET_ERROR錯誤，應用程序可通過WSAGetLastError()來獲取相應的錯誤代碼。錯誤代碼WSA_IO_PENDING表示重疊操作成功啟動，過后將有完成指示。任何其他的錯誤表示重疊操作未能成功地啟動，以后也不會有完成指示。

　　如果設置了MSG_INTERRUPT標志，則返回值的含義變化。零表示成功，具體含義同上。否則的話，返回值直接包含如下所示的錯誤代碼。由于中斷環境中無法調用WSAGetLastError()，故是必需的。請注意僅適用于Win16環境，僅適用于PROTOCOL_INFO結構中設置了XP1_INTERRUPT位的協議。

Remarks

The WSARecv function provides some additional features compared with the standard recv function in three important areas:

• It can be used in conjunction with overlapped sockets to perform overlapped recv operations.
• It allows multiple receive buffers to be specified making it applicable to the scatter/gather type of I/O.
• The lpFlags parameter is used both on input and returned on output, allowing applications to sense the output state of the MSG_PARTIAL flag bit. However, the MSG_PARTIAL flag bit is not supported by all protocols.

The WSARecv function is used on connected sockets or bound connectionless sockets specified by the s parameter and is used to read incoming data. The socket's local address must be known. For server applications, this is usually done explicitly through bind or implicitly through accept or WSAAccept. Explicit binding is discouraged for client applications. For client applications the socket can become bound implicitly to a local address through connect, WSAConnect, sendto, WSASendTo, or WSAJoinLeaf.

For connected, connectionless sockets, this function restricts the addresses from which received messages are accepted. The function only returns messages from the remote address specified in the connection. Messages from other addresses are (silently) discarded.

For overlapped sockets, WSARecv is used to post one or more buffers into which incoming data will be placed as it becomes available, after which the application-specified completion indication (invocation of the completion routine or setting of an event object) occurs. If the operation does not complete immediately, the final completion status is retrieved through the completion routine or 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. See ExitThread for more information.

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

For nonoverlapped sockets, the blocking semantics are identical to that of the standard recv function and the lpOverlapped and lpCompletionRoutine parameters are ignored. Any data that has already been received and buffered by the transport will be copied into the specified user buffers. In the case of a blocking socket with no data currently having been received and buffered by the transport, the call will block until data is received. Windows Sockets 2 does not define any standard blocking time-out mechanism for this function. For protocols acting as byte-stream protocols the stack tries to return as much data as possible subject to the available buffer space and amount of received data available. However, receipt of a single byte is sufficient to unblock the caller. There is no guarantee that more than a single byte will be returned. For protocols acting as message-oriented, a full message is required to unblock the caller.

Note??The socket options SO_RCVTIMEO and SO_SNDTIMEO apply only to blocking sockets.

Whether or not a protocol is acting as byte stream is determined by the setting of XP1_MESSAGE_ORIENTED and XP1_PSEUDO_STREAM in its WSAPROTOCOL_INFO structure and the setting of the MSG_PARTIAL flag passed in to this function (for protocols that support it). The following table lists relevant combinations, (an asterisk (*) indicates that the setting of this bit does not matter in this case).

The buffers are filled in the order in which they appear in the array pointed to by lpBuffers, and the buffers are packed so that no holes are created.

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 byte stream-style sockets (for example, type SOCK_STREAM), incoming data is placed into the buffers until the buffers are filled, the connection is closed, or the internally buffered data is exhausted. Regardless of whether or not the incoming data fills all the buffers, the completion indication occurs for overlapped sockets.

For message-oriented sockets (for example, type SOCK_DGRAM), an incoming message is placed into the buffers up to the total size of the buffers, and the completion indication occurs for overlapped sockets. If the message is larger than the buffers, the buffers are filled with the first part of the message. If the MSG_PARTIAL feature is supported by the underlying service provider, the MSG_PARTIAL flag is set in lpFlags and subsequent receive operations will retrieve the rest of the message. If MSG_PARTIAL is not supported but the protocol is reliable, WSARecv generates the error WSAEMSGSIZE and a subsequent receive operation with a larger buffer can be used to retrieve the entire message. Otherwise, (that is, the protocol is unreliable and does not support MSG_PARTIAL), the excess data is lost, and WSARecv generates the error WSAEMSGSIZE.

For connection-oriented sockets, WSARecv can indicate the graceful termination of the virtual circuit in one of two ways that depend on whether the socket is byte stream or message oriented. For byte streams, zero bytes having been read (as indicated by a zero return value to indicate success, and lpNumberOfBytesRecvd value of zero) indicates graceful closure and that no more bytes will ever be read. For message-oriented sockets, where a zero byte message is often allowable, a failure with an error code of WSAEDISCON is used to indicate graceful closure. In any case a return error code of WSAECONNRESET indicates an abortive close has occurred.

The lpFlags 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 lpFlags parameter. The latter is constructed by using the bitwise OR operator with any of the values listed in the following table.

For message-oriented sockets, the MSG_PARTIAL bit is set in the lpFlags parameter if a partial message is received. If a complete message is received, MSG_PARTIAL is cleared in lpFlags. In the case of delayed completion, the value pointed to by lpFlags is not updated. When completion has been indicated, the application should call WSAGetOverlappedResult and examine the flags indicated by the lpdwFlags parameter.

Overlapped Socket I/O

If an overlapped operation completes immediately, WSARecv returns a value of zero and the lpNumberOfBytesRecvd parameter is updated with the number of bytes received and the flag bits indicated by the lpFlags parameter are also updated. If the overlapped operation is successfully initiated and will complete later, WSARecv returns SOCKET_ERROR and indicates error code WSA_IO_PENDING. In this case, lpNumberOfBytesRecvd and lpFlags are 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. Flag values are obtained by examining the lpdwFlags parameter of WSAGetOverlappedResult.

The WSARecv function can be called from within the completion routine of a previous WSARecv, WSARecvFrom, WSASend or WSASendTo function. 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 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.

The prototype of the completion routine is as follows:

Copy Code

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

CompletionRoutine is a placeholder for an application-defined or library-defined function name. The dwError specifies the completion status for the overlapped operation as indicated by lpOverlapped. The cbTransferred parameter specifies the number of bytes received. The dwFlags parameter contains information that would have appeared in lpFlags if the receive operation had completed immediately. This function does not return a value.

Returning from this function allows invocation of another pending completion routine for this socket. When using WSAWaitForMultipleEvents, 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 filled in the same order in which they are specified.

If you are using I/O completion ports, be aware that the order of calls made to WSARecv is also the order in which the buffers are populated. WSARecv 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 example shows how to use the WSARecv function in overlapped I/O mode.

Copy Code

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

#pragma warning(disable: 4127)      // Conditional expression is a constant

#define DATA_BUFSIZE 4096

void __cdecl main(int argc, char **argv) 
{
    WSADATA wsd;
    struct addrinfo *result = NULL,
        *ptr = NULL,
        hints = {0};
    WSAOVERLAPPED RecvOverlapped = {0};
    SOCKET ConnSocket = INVALID_SOCKET;
    WSABUF DataBuf;
    DWORD RecvBytes, Flags;
    char buffer[DATA_BUFSIZE];
    int err, rc;

    if (argc != 2) {
        fprintf(stderr, "usage: %s server-name\n", argv[0]);
        return;
    }

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

    // Initialize the hints to retrieve the server address for IPv4
    hints.ai_family = AF_INET;
    hints.ai_socktype = SOCK_STREAM;
    hints.ai_protocol = IPPROTO_TCP;

    rc = getaddrinfo(argv[1], "27015", &hints, &result);
    if (rc != 0) {
        fprintf(stderr, "getaddrinfo failed: %d\n", rc );
        return;
    }

    for(ptr=result; ptr != NULL ;ptr=ptr->ai_next) {

        ConnSocket = socket(ptr->ai_family, ptr->ai_socktype, ptr->ai_protocol);
        if (ConnSocket == INVALID_SOCKET) {
            fprintf(stderr, "socket failed: %d\n", WSAGetLastError());
            freeaddrinfo(result);
            return;
        }
    
        rc = connect(ConnSocket, ptr->ai_addr, (int)ptr->ai_addrlen);
        if (rc == SOCKET_ERROR) {

            if (WSAECONNREFUSED == (err = WSAGetLastError())) {
                closesocket(ConnSocket);
                ConnSocket = INVALID_SOCKET;
                continue;
            }
            fprintf(stderr, "connect failed: %d\n", err);
            freeaddrinfo(result);
            closesocket(ConnSocket);
            return;
        }
        break;
    }
   	if ( ConnSocket == INVALID_SOCKET ) {
		      fprintf(stderr, "Unable to establish connection with the server!\n");
		      freeaddrinfo(result);
		      return;
	   }

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

    // Create an event handle and setup an overlapped structure.
    RecvOverlapped.hEvent = WSACreateEvent();
    if (RecvOverlapped.hEvent  == NULL) {
        fprintf(stderr, "WSACreateEvent failed: %d\n", WSAGetLastError());
        freeaddrinfo(result);
        closesocket(ConnSocket);
        return;
    }

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

    // Call WSARecv until the peer closes the connection
    // or until an error occurs
    while(1) {

        Flags = 0;
        rc = WSARecv(ConnSocket, &DataBuf, 1, &RecvBytes, &Flags, &RecvOverlapped, NULL);
        if ( (rc == SOCKET_ERROR) && (WSA_IO_PENDING != (err = WSAGetLastError()))) {
            fprintf(stderr, "WSARecv failed: %d\n", err);
            break;
        }

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

        rc = WSAGetOverlappedResult(ConnSocket, &RecvOverlapped, &RecvBytes, FALSE, &Flags);
        if (rc == FALSE) {
            fprintf(stderr, "WSARecv operation failed: %d\n", WSAGetLastError());
            break;
        }

        printf("Read %d bytes\n", RecvBytes);

        WSAResetEvent(RecvOverlapped.hEvent);

        // If 0 bytes are received, the connection was closed
        if (RecvBytes == 0 )
            break;
    }

    WSACloseEvent(RecvOverlapped.hEvent);
    closesocket(ConnSocket);
    freeaddrinfo(result);

    WSACleanup();

    return;
}

Requirements

See Also