This function is a Microsoft-specific extension to the Windows Sockets specification. For more information, see Microsoft Extensions and Windows Sockets 2.

The Windows Sockets WSARecvEx function is identical to the recv function, except the flags parameter is an in-out parameter. When a partial message is received while using datagram protocol, the MSG_PARTIAL bit is set in the flags parameter on return from the function.



char FAR * buf,

int len,

int *flags




[in] A descriptor identifying a connected socket.


[out] A buffer for the incoming data.


[in] The length of buf.


[in/out] Specifies whether the message is fully or partiall received for datagram sockets.


By default, for message-oriented transport protocols the Windows Sockets recv function receives a single message in each call to the function. This works fine for most cases, but there are two situations in which this is insufficient: when the application's data buffer is smaller than the message, and when the message is large and arrives in several pieces.

When the buffer is smaller than the data, as much of the message as will fit is copied into the user's buffer and recv returns with the error code WSAEMSGSIZE. A subsequent call to recv will get the next part of the message. Applications written for message-oriented transport protocols should be coded for this possibility if message sizing is not guaranteed by the application's data transfer protocol. If an application gets this error code, then the entire data buffer is filled with data.

It is more complicated when a very large message arrives a little at a time. For example, if an application sends a 1-megabyte message, the transport protocol must break up the message in order to send it over the physical network. It is theoretically possible for the transport protocol on the receiving side to buffer all the data in the message, but this would be quite expensive in terms of resources.

It would be better to allow a recv call to complete with only a partial message and some indication to the application that the data is only a partial message. However, the Windows Sockets recv function has only one output parameter: a pointer to the buffer for the incoming data. Therefore, Windows NT supplies the WSARecvEx function, which is identical to the recv function except that the flags parameter is an in-out parameter:

Return Values

On return from WSARecvEx, if a partial message was received, the MSG_PARTIAL bit is set in the flags parameter. If a complete message was received, MSG_PARTIAL is not set in flags. For a stream-oriented transport protocol, MSG_PARTIAL is never set on return from WSARecvEx; for stream transport protocols, this function behaves identically to the Windows Sockets recv function.

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