WPUQueueApc function (ws2spi.h)

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.


int WPUQueueApc(
  [in]  LPWSATHREADID lpThreadId,
  [in]  LPWSAUSERAPC  lpfnUserApc,
  [in]  DWORD_PTR     dwContext,
  [out] LPINT         lpErrno


[in] lpThreadId

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.

[in] lpfnUserApc

Pointer to the APC function to be called.

[in] dwContext

32-bit context value that is subsequently supplied as an input parameter to the APC function.

[out] lpErrno

Pointer to the error code.

Return value

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
The dwThreadId parameter does not specify a valid thread.


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.


Minimum supported client Windows 2000 Professional [desktop apps only]
Minimum supported server Windows 2000 Server [desktop apps only]
Target Platform Windows
Header ws2spi.h

See also