WPUQueueApc Function

Artikel
07/27/2011

The WPUQueueApc function queues a user mode–asynchronous procedure call (APC) to the specified thread in order to facilitate invocation of overlapped I/O completion routines.

Syntax

int WPUQueueApc(
  __in   LPWSATHREADID lpThreadId,
  __in   LPWSAUSERAPC lpfnUserApc,
  __in   DWORD dwContext,
  __out  LPINT lpErrno
);

Parameter

lpThreadId [in]
Pointer to a WSATHREADID structure that identifies the thread context. A pointer to this structure is supplied to the service provider by the Ws2_32.dll as an input parameter to an overlapped operation. The provider should store the WSATHREADID structure locally and provide a pointer to this local store. The local copy of WSATHREADID is no longer needed once WPUQueueApc returns.
lpfnUserApc [in]
Pointer to the APC function to be called.
dwContext [in]
32-bit context value that is subsequently supplied as an input parameter to the APC function.
lpErrno [out]
Pointer to the error code.

Rückgabewert

If no error occurs, WPUQueueApc returns zero and queues the completion routine for the specified thread. Otherwise, it returns SOCKET_ERROR, and a specific error code is available in lpErrno.

Error code	Meaning
WSAEFAULT	The dwThreadId parameter does not specify a valid thread.

Hinweise

This function queues an APC function against the specified thread. Under Windows, this will be done using a user mode–asynchronous procedure call (APC). The APC will only execute when the specified thread is blocked in an alertable wait and a callback will be made directly. This call is safe for use within an interrupt context.

LPWSAUSERAPC is defined as follows:

typedef void ( CALLBACK FAR * LPWSAUSERAPC )( DWORD dwContext );

Because the APC mechanism supports only a single context value, lpfnUserApc itself cannot be the client specified–completion routine, which involves more parameters. The service provider must instead supply a pointer to its own APC function that uses the supplied dwContext value to access the needed result information for the overlapped operation, and then invokes the client specified–completion routine.

For service providers where a user-mode component implements overlapped I/O, a typical usage of the APC mechanism is as follows.

When the I/O operation completes, the provider allocates a small buffer and packs it with a pointer to the client-supplied completion procedure and parameter values to pass to the procedure.
It queues an APC, specifying the pointer to the buffer as the dwContext value and its own intermediate procedure as the target procedure lpfnUserApc.
When the target thread eventually enters alertable wait state, the service provider's intermediate procedure is called in the proper thread context.
The intermediate procedure simply unpacks parameters, deallocates the buffer, and calls the client-supplied completion procedure.