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.
int PASCAL FAR WSARecvEx (
| SOCKET s,
| 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
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
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:
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
; for stream transport protocols, this function behaves identically to the
Windows Sockets recv
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