Receiving Completion Indications
Several options are available for receiving completion indications, thus
providing applications with appropriate levels of flexibility. These include:
waiting (or blocking) on event objects, polling event objects, and socket I/O
Blocking and Waiting for Completion Indication
Applications can block while waiting for one or more event objects to become
set using the WSAWaitForMultipleEvents
function. In Win32 implementations, the process or thread will truly block.
Since Windows Sockets 2 event objects are implemented as Win32 events, the
native Win32 function, WaitForMultipleObjects
can also be used for this purpose. This is especially useful if the thread
needs to wait on both socket and nonsocket events.
Polling for Completion Indication
Applications that prefer not to block can use the WSAGetOverlappedResult
function to poll for the completion status associated with any particular
event object. This function indicates whether or not the overlapped operation has
completed, and if completed, arranges for the WSAGetLastError
function to retrieve the error status of the overlapped operation.
Using socket I/O completion routines
The functions used to initiate overlapped I/O (WSASend
) all take lpCompletionRoutine
as an optional input parameter. This is a pointer to an application-specific
function that will be called after a successfully initiated overlapped I/O
operation was completed (successfully or otherwise). The completion routine follows
the same rules as stipulated for Win32 file I/O completion routines. That is,
the completion routine will not be invoked until the thread is in an alertable
wait state, such as when the function WSAWaitForMultipleEvents
is invoked with the fAlertable
The transports allow an application to invoke send and receive operations from
within the context of the socket I/O completion routine and guarantee that,
for a given socket, I/O completion routines will not be nested. This permits
time-sensitive data transmissions to occur entirely within a preemptive context.
Summary of overlapped completion indication mechanisms
The particular overlapped I/O completion indication to be used for a given
overlapped operation is determined by whether the application supplies a pointer
to a completion function, whether a WSAOVERLAPPED
structure is referenced, and by the value of the hEvent member within the
WSAOVERLAPPED structure (if supplied). The following table summarizes the
completion semantics for an overlapped socket and shows the various combinations of lpOverlapped
, and lpCompletionRoutine:
||Operation completes synchronously. It behaves as if it were a nonoverlapped
||Operation completes overlapped, but there is no Windows Sockets 2-supported
completion mechanism. The completion port mechanism (if supported) can be used in
this case. Otherwise, there will be no completion notification.
||Operation completes overlapped, notification by signaling event object.
||Operation completes overlapped, notification by scheduling completion routine.
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