Queues a work item to a worker thread in the thread pool.

Syntax

BOOL WINAPI QueueUserWorkItem(
  __in      LPTHREAD_START_ROUTINE Function,
  __in_opt  PVOID Context,
  __in      ULONG Flags
);

Parameters

Function [in]

A pointer to the application-defined callback function of type LPTHREAD_START_ROUTINE to be executed by the thread in the thread pool. This value represents the starting address of the thread. This callback function must not call the TerminateThread function.

For more information, see ThreadProc.

Context [in, optional]

A single parameter value to be passed to the thread function.

Flags [in]

The flags that control execution. This parameter can be one or more of the following values.

Value	Meaning
WT_EXECUTEDEFAULT 0x00000000	By default, the callback function is queued to a non-I/O worker thread. The callback function is queued to a thread that uses I/O completion ports, which means they cannot perform an alertable wait. Therefore, if I/O completes and generates an APC, the APC might wait indefinitely because there is no guarantee that the thread will enter an alertable wait state after the callback completes.
WT_EXECUTEINIOTHREAD 0x00000001	The callback function is queued to an I/O worker thread. This thread performs an alertable wait. This is less efficient, so this flag should be used only if the callback generates APCs to the current thread and the APC should be executed after the thread returns to the thread pool. The callback function is queued as an APC. Be sure to address reentrancy issues if the function performs an alertable wait operation.
WT_EXECUTEINPERSISTENTTHREAD 0x00000080	The callback function is queued to a thread that never terminates. It does not guarantee that the same thread is used each time. This flag should be used only for short tasks or it could affect other timer operations. Note that currently no worker thread is truly persistent, although worker threads do not terminate if there are any pending I/O requests.
WT_EXECUTELONGFUNCTION 0x00000010	The callback function can perform a long wait. This flag helps the system to decide if it should create a new thread.
WT_TRANSFER_IMPERSONATION 0x00000100	Callback functions will use the current access token, whether it is a process or impersonation token. If this flag is not specified, callback functions execute only with the process token. Windows XP/2000:  This flag is not supported until Windows XP SP2 and Windows Server 2003.

Return Value

If the function succeeds, the return value is nonzero.

If the function fails, the return value is zero. To get extended error information, call GetLastError.

Remarks

If a function in a DLL is queued to a worker thread, be sure that the function has completed execution before the DLL is unloaded.

By default, the thread pool has a maximum of 512 threads per process. To raise the queue limit, use the WT_SET_MAX_THREADPOOL_THREAD macro defined in Winnt.h.

#define WT_SET_MAX_THREADPOOL_THREADS(Flags,Limit) \
    ((Flags)|=(Limit)<<16)

Use this macro in the call to QueueUserWorkItem to specify the Flags parameter. The macro parameters are the desired flags and the new limit, up to (2<<16)-1 threads. However, the size of the queue is limited by the size of the kernel nonpaged pool. Note that your application can improve its performance by keeping the number of worker threads low.

To compile an application that uses this function, define _WIN32_WINNT as 0x0500 or later. For more information, see Using the Windows Headers.

Requirements

See Also

Process and Thread Functions
Thread Pooling
ThreadProc

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Build date: 8/7/2008