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 recvfromfunction 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 an output parameter, allowing applications to sense the output state of the MSG_PARTIAL flag bit. Note, however, that the MSG_PARTIAL flag bit is not supported by all protocols.
  
  

This function is used primarily on a connectionless socket specified by socket s. The socket's local address must be known. For server applications, this is usually done explicitly through bind. Explicit binding is discouraged for client applications. For client applications using this function, the socket can become bound implicitly to a local address through the sendtoor WSASendTofunction.

For overlapped sockets, this function is used to post one or more buffers into which incoming data will be placed as it becomes available on a possibly connected socket, 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. Also, the values indicated by lpFromand lpFromlenare not updated until completion is itself indicated. Applications must not use or disturb these values until they have been updated, therefore the application must not use automatic (that is, stack-based) variables for these parameters.

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 WSARecvfunction 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. For 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.

The supplied buffers are completed in the order in which they appear in the array indicated 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 connectionless socket types, the address from which the data originated is copied to the buffer indicated by lpFrom. The value pointed to by lpFromlenis initialized to the size of this buffer and is modified on completion to indicate the actual size of the address stored there. As noted previously for overlapped sockets, the lpFromand lpFromlenparameters are not updated until after the overlapped I/O has completed. The memory pointed to by these parameters must, therefore, remain available to the service provider and cannot be allocated on the application's stack frame. The lpFromand lpFromlenparameters are ignored for connection-oriented sockets.

For byte stream style sockets (for example, type SOCK_STREAM), incoming data is placed into the buffers until certain events occur. The following list shows these events:

• The buffers are filled.
  
  
• The connection is closed.
  
  
• 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, 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, WSARecvFromgenerates 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 WSARecvFromgenerates the error WSAEMSGSIZE.

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 that are used with the bitwise OR operator to construct the lpFlagsparameter.

As an input parameter, this flag indicates that the receive operation should complete even if only part of a message has been received by the service 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 pointed to by the lpdwFlagsparameter.