Adds a user-mode asynchronous procedure call (APC) object to the APC queue of the specified thread.

Syntax

DWORD WINAPI QueueUserAPC(
  __in  PAPCFUNC pfnAPC,
  __in  HANDLE hThread,
  __in  ULONG_PTR dwData
);

Parameters

pfnAPC [in]

A pointer to the application-supplied APC function to be called when the specified thread performs an alertable wait operation. For more information, see APCProc.

hThread [in]

A handle to the thread. The handle must have the THREAD_SET_CONTEXT access right. For more information, see Synchronization Object Security and Access Rights.

dwData [in]

A single value that is passed to the APC function pointed to by the pfnAPC parameter.

Return Value

If the function succeeds, the return value is nonzero.

If the function fails, the return value is zero. There are no error values defined for this function that can be retrieved by calling GetLastError.

Remarks

The APC support provided in the operating system allows an application to queue an APC object to a thread. An APC can be queued from a 64-bit process to another 64-bit process or from a 32-bit process to another 32-bit process. You should not queue an APC from a 64-bit process to a 32-bit process or vice versa, because the address will be incorrect and the application will crash.

Each thread has its own APC queue. The queuing of an APC is a request for the thread to call the APC function. The operating system issues a software interrupt to direct the thread to call the APC function.

When a user-mode APC is queued, the thread is not directed to call the APC function unless it is in an alertable state. After the thread is in an alertable state, the thread handles all pending APCs in first in, first out (FIFO) order, and the wait operation returns WAIT_IO_COMPLETION. A thread enters an alertable state by using SleepEx, SignalObjectAndWait, WaitForSingleObjectEx, WaitForMultipleObjectsEx, or MsgWaitForMultipleObjectsEx to perform an alertable wait operation.

If an application queues an APC before the thread begins running, the thread begins by calling the APC function. After the thread calls an APC function, it calls the APC functions for all APCs in its APC queue.

It is possible to sleep or wait for an object within the APC. If you perform an alertable wait inside an APC, it will recursively dispatch the APCs. This can cause a stack overflow.

When the thread is terminated using the ExitThread or TerminateThread function, the APCs in its APC queue are lost. The APC functions are not called.

Note that the ReadFileEx, SetWaitableTimer, and WriteFileEx functions are implemented using an APC as the completion notification callback mechanism.

To compile an application that uses this function, define _WIN32_WINNT as 0x0400 or later. For more information, see Using the Windows Headers.

Requirements

See Also

APCProc

Asynchronous Procedure Calls

Synchronization Functions

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Build date: 10/8/2009