MORE INFORMATION
When a socket is created, by default it is a blocking socket. You can use
the FIONBIO command in the ioctlsocket API call, WSAEventSelect, or
WSAAysncSelect to change the socket mode from blocking to nonblocking. If
a Winsock call cannot complete immediately, the call fails and
WSAGetLastError returns a WSAEWOULDBLOCK error if it's a nonblocking
socket, or the call blocks until the operation completes if it's a blocking
socket.
The socket overlapped I/O attribute is different from the socket's blocking
or nonblocking mode. Although the current Winsock implementation requires
overlapped I/O attribute for nonblocking socket mode, they are
conceptually independent and their programming model is different too. To
create a socket with the overlapped I/O attribute, you can either use the
socket API or the WSASocket API with the WSA_FLAG_OVERLAPPED flag set. If
an overlapped I/O operation can not complete immediately, the call fails
and WSAGetLastError or GetLastError return WSA_IO_PENDING or
ERROR_IO_PENDING, which is actually the same define as WSA_IO_PENDING. For
additional information, please see the following article in the
Microsoft Knowledge Base:
179942 INFO: WSA_FLAG_OVERLAPPED Is Needed For Non-Blocking Sockets
Please note that once a socket is created, there is no way to change the
socket overlapped attribute. However, you can call the setsockopt API with
SO_OPENTYPE option on any socket handles including an INVALID_SOCKET to
change the overlapped attributes for all successive socket calls in the
same thread. The default SO_OPENTYPE option value is 0, which sets the
overlapped attribute. All nonzero option values make the socket
synchronous and make it so that you cannot use a completion function.
By setting a socket's overlapped I/O attribute it doesn't mean that the
socket will perform an overlapped I/O operation. For example, if you
specify NULL for both the completion function and the overlapped structure
in WSARecv and WSASend, or you simply call recv or send functions, they
will complete in a blocking fashion. To make sure the I/O is performed in
an overlapped fashion you need to provide an overlapped structure in your
I/O function, depending on the function you use.
Overlapped I/O
In Winsock 1, you create an overlapped socket using the socket API, and use
Win32 file I/O API ReadFile, ReadFileEx, WriteFile, WriteFileEx to perform
overlapped I/O on the socket handle. In Winsock 2, you create an overlapped
socket using WSASocket with the WSA_FLAG_OVERLAPPED flag, or simply using
the socket API. You can use the above Win32 file I/O APIs or Winsock 2
WSASend, WSASendTo, WSARecv, and WSARecvFrom to initiate an overlapped I/O
operation.
If you use the SO_RCVBUF and SO_SNDBUF option to set zero TCP stack receive
and send buffer, you basically instruct the TCP stack to directly perform
I/O using the buffer provided in your I/O call. Therefore, in addition to
the nonblocking advantage of the overlapped socket I/O, the other
advantage is better performance because you save a buffer copy between the
TCP stack buffer and the user buffer for each I/O call. But you have to
make sure you don't access the user buffer once it's submitted for
overlapped operation and before the overlapped operation completes.
To determine whether the overlapped I/O operation is completed, you can use
one of the following options:
- You can provide an event handle in an overlapped structure used
in the I/O call and wait on the event handle to signal.
- Use GetOverlappedResult or WSAGetOverlappedResult to poll the status of the overlapped I/O operation. On Windows NT, you can specify NULL as the event handle in the overlapped structure. However, on Windows 95 the overlapped structure needs to contain a manual reset event handle. WSAGetOverlappedResult on Windows 98 and Windows Millennium Edition (ME) is modified such that it will behave like Windows NT and can also poll the completion status with a NULL event handle in the overlapped structure.
- Use ReadFileEx, WriteFileEx, WSARecv, WSARecvFrom, WSASend, or WSASendTo
and choose to provide a completion function to be called when the
overlapped I/O operation has completed.
If you use the completion function approach, at some point after you
have issued the overlapped I/O operation you need to issue a Win32 wait
function or a WSA version of wait function to wait on a nonsignaled
handle to put your current thread into an alertable wait state. When the
overlapped I/O operation completes, your completion function is called,
your wait function is going to return WAIT_IO_COMPLETION, and your
alertable wait thread wakes up.
- Use GetQueuedCompletionStatus and associate a socket, along with the
overlapped I/O attribute set, with a Windows NT I/O Completion
Port(IOCP).
With IOCP, you don't need to supply a completion function, wait on an
event handle to signal, or poll the status of the overlapped operation.
Once you create the IOCP and add your overlapped socket handle to the
IOCP, you can start the overlapped operation by using any of the I/O
APIs mentioned above (except recv, recvfrom, send, or sendto). You will
have your worker thread block on GetQueuedCompletionStatus API waiting
for an I/O completion packet. When an overlapped I/O completes, an I/O
completion packet arrives at the IOCP and GetQueuedCompletionStatus
returns.
IOCP is the Windows NT Operating System support for writing a scalable,
high throughput server using very simple threading and blocking code on
overlapped I/O operations. Thus there can be a significant performance
advantage of using overlapped socket I/O with Windows NT IOCPs.
Blocking and nonblocking mode
When a socket is created, by default it is a blocking socket. Under
blocking mode socket I/O operations, connect and accept operations all
block until the operation in question is completed. To change the socket
operation mode from blocking mode to nonblocking mode, you can either use
WSAAsyncSelect, WSAEventSelect, or the FIONBIO command in the ioctlsocket
API call.
WSAAsyncSelect maps socket notifications to Windows messages and is the
best model for a single threaded GUI application.
WSAEventSelect uses WSAEnumNetworkEvents to determine the nature of the
socket notification on the signaling event and maps socket notifications by
signaling an event. This is a useful model for non-GUI applications that
lack a message pump, such as a Windows NT service application.
The FIONBIO command in the ioctlsocket API call puts the socket into nonblocking mode as well. But you need to poll the status of the socket by
using the select API.
Update: Setting SO_RCVBUF to zero with overlapped recv is not necessary for Windows 2000, which can directly copy the data to application's recv buffer. Setting SO_SNDBUF to zero with overlapped send on Windows 2000 still has the same benefit of saving one memory copy as on Windows NT 4.0. It is also important that the application issues several outstanding overlapped send's or recv's if the application sets the corresponding stack buffer to 0.