WSARecv

The Windows Sockets WSARecv function receives data from a connected socket.

int WSARecv (
  SOCKET s,                                               
  LPWSABUF lpBuffers,                                     
  DWORD dwBufferCount,                                    
  LPDWORD lpNumberOfBytesRecvd,                           
  LPDWORD lpFlags,                                        
  LPWSAOVERLAPPED lpOverlapped,                           
  LPWSAOVERLAPPED_COMPLETION_ROUTINE lpCompletionROUTINE  
);
 

Parameters

s

[in] A descriptor identifying a connected socket.

lpBuffers

[in/out] A pointer to an array of WSABUF structures. Each WSABUF structure contains a pointer to a buffer and the length of the buffer.

dwBufferCount

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

lpNumberOfBytesRecvd

[out] A pointer to the number of bytes received by this call if the receive operation completes immediately.

lpFlags

[in/out] A pointer to flags.

lpOverlapped

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

lpCompletionRoutine

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

Remarks

The WSARecv function provides functionality over and above the standard recv function in three important areas:

1. It can be used in conjunction with overlapped sockets to perform overlapped receive operations.
2. It allows multiple receive buffers to be specified making it applicable to the scatter/gather type of I/O.
3. The lpFlags parameter is both an input and an output parameter, 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.

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 supplied 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 Socket 2 does not define any standard blocking timeout mechanism for this function. For protocols acting as byte-stream protocols the stack tries to return as much data as possible subject to the supplied 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.

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 relevant combinations are summarized in the following table (an asterisk (*) indicates that the setting of this bit does not matter in this case).

The supplied 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.

The array of WSABUF structures pointed to by the lpBuffers parameter is transient. If this operation completes 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 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 supplied buffers up to the total size of the buffers supplied, and the completion indication occurs for overlapped sockets. If the message is larger than the buffers supplied, 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 a byte stream or message oriented. For byte streams, zero bytes having been read (as indicated by 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 or-ing any of the following values:

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. 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 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. A caller that passes a non-NULL lpCompletionRoutine and later calls WSAGetOverlappedResult for the same overlapped IO request may not set the fWait parameter for that invocation of WSAGetOverlappedResult to TRUE. In this case the usage of the hEvent field is undefined, and attempting to wait on the hEvent field would produce unpredictable results.

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:

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 they are supplied.

Return Values

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.

Error Codes

QuickInfo

  Windows NT: Yes
  Windows: Yes
  Windows CE: Unsupported.
  Header: Declared in winsock2.h.
  Import Library: Link with ws2_32.lib.

See Also

WSACloseEvent, WSACreateEvent, WSAGetOverlappedResult, WSASocket, WSAWaitForMultipleEvents