If no error occurs and the receive operation has completed immediately, this function returns zero. The overlapped structures are updated with the receive results, and the associated event is signalled.

In Windows Embedded CE, the completion object will not be signalled. If an error occurs, a value of SOCKET_ERROR is returned, and a specific error code can be retrieved by calling the WSAGetLastErrorfunction. 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.

The following table shows a list of possible error codes.

This function provides functionality over and above the standard recvfunction in the following three important areas:

• It can be used in conjunction with overlapped sockets to perform overlapped receive operations.
  
  
• It allows multiple receive buffers to be specified, enabling a scatter/gather type of I/O.
  
  
• The lpFlagsparameter is both an input and 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.
  
  

This function is used on connected sockets or bound connectionless sockets specified by the sparameter 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 (Windows Sockets)or implicitly through the accept (Windows Sockets)or WSAAcceptfunction. Explicit binding is discouraged for client applications. For client applications, the socket can become bound implicitly to a local address through the connect (Windows Sockets), WSAConnect, or sendto, WSASendTofunction.

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, WSARecvis 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 the WSAGetOverlappedResultfunction.

If both lpOverlappedand lpCompletionRoutineare 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 recvfunction and the lpOverlappedand lpCompletionRoutineparameters 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. Winsock 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 supplied buffer space and the 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_INFOstructure and the setting of the MSG_PARTIAL flag passed in to this function (for protocols that support it). The following table summarizes the relevant combinations, (an asterisk (*) indicates that the setting of this bit does not matter in this case).

The supplied buffers are completed 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 WSABUFstructures pointed to by the lpBuffersparameter is transient. If this operation completes in an overlapped manner, it is the service provider's responsibility to capture these WSABUFstructures before returning from this call. This enables applications to build stack-based WSABUFarrays.

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 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 MSG_PARTIAL is supported by the underlying service provider, the MSG_PARTIAL flag is set in lpFlagsand subsequent receive operations will retrieve the rest of the message. If MSG_PARTIAL is not supported but the protocol is reliable, WSARecvgenerates 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 WSARecvgenerates the error WSAEMSGSIZE.

For connection-oriented sockets, WSARecvcan 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 an lpNumberOfBytesRecvdvalue 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 lpFlagsparameter 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 lpFlagsparameter. The following table shows the values used with the bitwise OR operator to construct the lpFlagsparameter.

The MSG_PARTIAL flag is not supported by the default Windows Embedded CE provider. For message-oriented sockets, the MSG_PARTIAL bit is set in the lpFlagsparameter 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 lpFlagsis not updated. When completion has been indicated, the application should call WSAGetOverlappedResultand examine the flags indicated by the lpdwFlagsparameter.