KeWaitForSingleObject function (wdm.h)

The KeWaitForSingleObject routine puts the current thread into a wait state until the given dispatcher object is set to a signaled state or (optionally) until the wait times out.


KeWaitForSingleObject (
    PVOID Object,
    KWAIT_REASON WaitReason,
    BOOLEAN Alertable,


[in] Object

Pointer to an initialized dispatcher object (event, mutex, semaphore, thread, or timer) for which the caller supplies the storage. The dispatcher object must reside in nonpaged system memory. For more information, see Remarks.

[in] WaitReason

Specifies the reason for the wait. A driver should set this value to Executive, unless it is doing work on behalf of a user and is running in the context of a user thread, in which case it should set this value to UserRequest.

[in] WaitMode

Specifies whether the caller waits in KernelMode or UserMode. Lowest-level and intermediate drivers should specify KernelMode. If the given Object is a mutex, the caller must specify KernelMode.

[in] Alertable

Specifies a Boolean value that is TRUE if the wait is alertable and FALSE otherwise.

[in, optional] Timeout

Pointer to a time-out value that specifies the absolute or relative time, in 100-nanosecond units, at which the wait is to be completed.

A positive value specifies an absolute time, relative to January 1, 1601. A negative value specifies an interval relative to the current time. Absolute expiration times track any changes in the system time; relative expiration times are not affected by system time changes.

If Timeout = 0, the routine returns without waiting. If the caller supplies a NULL pointer, the routine waits indefinitely until the dispatcher object is set to the signaled state. For more information, see the following Remarks section.

Return value

KeWaitForSingleObject can return one of the following.

The NT_SUCCESS macro recognizes all of these status values as "success" values.

Return code Description
STATUS_SUCCESS The dispatcher object specified by the Object parameter satisfied the wait.
STATUS_ALERTED The wait was interrupted to deliver an alert to the calling thread.
STATUS_USER_APC The wait was interrupted to deliver a user asynchronous procedure call (APC) to the calling thread.
STATUS_TIMEOUT A time-out occurred before the object was set to a signaled state. This value can be returned when the specified set of wait conditions cannot be immediately met and Timeout is set to zero.


The current state of the specified Object is examined to determine whether the wait can be satisfied immediately. If so, the necessary side effects are performed on the object. Otherwise, the current thread is put in a waiting state and a new thread is selected for execution on the current processor.

The Alertable parameter determines when the thread can be alerted and its wait state consequently aborted. For additional information, see Waits and APCs.

A special consideration applies when the Object parameter passed to KeWaitForSingleObject is a mutex. If the dispatcher object waited on is a mutex, APC delivery is the same as for all other dispatcher objects during the wait. However, after KeWaitForSingleObject returns with STATUS_SUCCESS and the thread actually holds the mutex, only special kernel-mode APCs are delivered. Delivery of all other APCs, both kernel-mode and user-mode, is disabled. This restriction on the delivery of APCs persists until the mutex is released.

The dispatcher object pointed to by the Object parameter must reside in nonpaged system memory.

If the WaitMode parameter is UserMode, the kernel stack can be swapped out during the wait. Consequently, a caller must never attempt to pass parameters on the stack when calling KeWaitForSingleObject using the UserMode argument. If you allocate the event on the stack, you must set the WaitMode parameter to KernelMode.

It is especially important to check the return value of KeWaitForSingleObject when the WaitMode parameter is UserMode or Alertable is TRUE, because KeWaitForSingleObject might return early with a status of STATUS_USER_APC or STATUS_ALERTED.

All long-term waits that can be aborted by a user should be UserMode waits and Alertable should be set to FALSE.

Where possible, Alertable should be set to FALSE and WaitMode should be set to KernelMode, in order to reduce driver complexity. The principal exception to this is when the wait is a long-term wait.

If a NULL pointer is supplied for Timeout, the calling thread remains in a wait state until the Object is signaled.

A time-out value of zero allows the testing of a set of wait conditions and for the conditional performance of any side effects if the wait can be immediately satisfied, as in the acquisition of a mutex.

Time-out intervals are measured relative to the system clock, and the accuracy with which the operating system can detect the end of a time-out interval is limited by the granularity of the system clock. For more information, see Timer Accuracy.

A mutex can be recursively acquired only MINLONG times. If this limit is exceeded, the routine raises a STATUS_MUTANT_LIMIT_EXCEEDED exception.

Callers of KeWaitForSingleObject must be running at IRQL <= DISPATCH_LEVEL. However, if Timeout = NULL or Timeout != 0, the caller must be running at IRQL <= APC_LEVEL and in a nonarbitrary thread context. If Timeout != NULL and Timeout = 0, the caller must be running at IRQL <= DISPATCH_LEVEL.

KeWaitForMutexObject is a macro that converts to KeWaitForSingleObject, which can be used instead.

For better performance, use fast mutexes or guarded mutexes. For more information, see Alternatives to Mutex Objects.

For more information about mutex objects, see Mutex Objects.


Target Platform Universal
Header wdm.h (include Wdm.h, Ntddk.h, Ntifs.h)
Library NtosKrnl.lib
DLL NtosKrnl.exe
IRQL See Remarks section.
DDI compliance rules CompleteRequestStatusCheck(wdm), HwStorPortProhibitedDDIs(storport), IoAllocateIrpSignalEventInCompletionTimeout(wdm), IoBuildDeviceControlWait(wdm), IoBuildDeviceControlWaitTimeout(wdm), IoBuildFsdIrpSignalEventInCompletionTimeout(wdm), IoBuildSynchronousFsdRequestWait(wdm), IoBuildSynchronousFsdRequestWaitTimeout(wdm), IrpProcessingComplete(wdm), IrqlKeWaitForMutexObject(wdm), LowerDriverReturn(wdm), MarkIrpPending2(wdm), PendedCompletedRequest(wdm), PendedCompletedRequest2(wdm), PendedCompletedRequest3(wdm), PendedCompletedRequestEx(wdm), RemoveLockForwardDeviceControl(wdm), RemoveLockForwardDeviceControlInternal(wdm), RemoveLockForwardRead(wdm), RemoveLockForwardWrite(wdm), SpNoWait(storport), StartDeviceWait(wdm), StartDeviceWait2(wdm), StartDeviceWait3(wdm), StartDeviceWait4(wdm)

See also