Overlapped I/O and Event Objects
Windows Sockets 2 introduces overlapped I/O and requires that all transport
providers support this capability. Overlapped I/O follows the model established
in Win32 and can be performed only on sockets that were created through the WSASocket
function with the WSA_FLAG_OVERLAPPED flag set.
Creating a socket with the overlapped attribute has no impact on whether a
socket is currently in the blocking or nonblocking mode. Sockets created with
the overlapped attribute can be used to perform overlapped I/O
doing so does not change the blocking mode of a socket. Since overlapped I/O
operations do not block, the blocking mode of a socket is irrelevant for these
For receiving, applications use the WSARecv
functions to supply buffers into which data is to be received. If one or more
buffers are posted prior to the time when data has been received by the
network, that data could be placed in the user's buffers immediately as it arrives.
Thus, it can avoid the copy operation that would otherwise occur at the time the recv
function is invoked. If data is already present when receive buffers are
posted, it is copied immediately into the user's buffers.
If data arrives when no receive buffers have been posted by the application,
the network resorts to the familiar synchronous style of operation. That is, the
incoming data is buffered internally until the application issues a receive
call and thereby supplies a buffer into which the data can be copied. An
exception to this is when the appliation uses setsockopt
to set the size of the receive buffer to zero. In this instance, reliable
protocols would only allow data to be received when application buffers had been
posted and data on unreliable protocols would be lost.
On the sending side, applications use WSASend
to supply pointers to filled buffers and then agree to not disturb the
buffers in any way until the network has consumed the buffer's contents.
Overlapped send and receive calls return immediately. A return value of zero
indicates that the I/O operation was completed immediately and that the
corresponding completion indication already occurred. That is, the associated event
object has been signaled, or a completion routine has been queued and will be
executed when the calling thread gets into the alertable wait state.
A return value of SOCKET_ERROR coupled with an error code of WSA_IO_PENDING
indicates that the overlapped operation has been successfully initiated and
that a subsequent indication will be provided when send buffers have been
consumed or when a receive operation has been completed. However, for sockets that
are byte-stream style, the completion indication occurs whenever the incoming
data is exhausted, regardless of whether the buffers are full. Any other error
code indicates that the overlapped operation was not successfully initiated and
that no completion indication will be forthcoming.
Both send and receive operations can be overlapped. The receive functions can
be invoked several times to post receive buffers in preparation for incoming
data, and the send functions can be invoked several times to queue multiple
buffers to send. While the application can rely upon a series of overlapped send
buffers being sent in the order supplied, the corresponding completion indications
might occur in a different order. Likewise, on the receiving side, buffers
will be filled in the order they are supplied, but the completion indications
might occur in a different order.
The deferred completion feature of overlapped I/O is also available for WSAIoctl
, which is an enhanced version of ioctlsocket
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