KeReleaseSemaphore function (wdm.h)

The KeReleaseSemaphore routine releases the specified semaphore object.


LONG KeReleaseSemaphore(
  [in, out] PRKSEMAPHORE Semaphore,
  [in]      KPRIORITY    Increment,
  [in]      LONG         Adjustment,
  [in]      BOOLEAN      Wait


[in, out] Semaphore

A pointer to an initialized semaphore object for which the caller provides the storage.

[in] Increment

Specifies the priority increment to be applied if releasing the semaphore causes a wait to be satisfied.

[in] Adjustment

Specifies a value to be added to the current semaphore count. This value must be positive.

[in] Wait

Specifies whether the call to KeReleaseSemaphore is to be followed immediately by a call to one of the KeWaitXxx routines. If TRUE, the KeReleaseSemaphore call must be followed by a call to KeWaitForMultipleObjects, KeWaitForMutexObject, or KeWaitForSingleObject. For more information, see the following Remarks section.

Return value

If the return value is zero, the previous state of the semaphore object is not-signaled.


KeReleaseSemaphore supplies a run-time priority boost for waiting threads. If this call sets the semaphore to the signaled state, the semaphore count is augmented by the specified value. The caller can also specify whether it will call one of the KeWaitXxx routines as soon as KeReleaseSemaphore returns control.

Releasing a semaphore object causes the semaphore count to be augmented by the value of the Adjustment parameter. If the resulting value is greater than the limit of the semaphore object, the count is not adjusted and an exception, STATUS_SEMAPHORE_LIMIT_EXCEEDED, is raised.

Augmenting the semaphore object count causes the semaphore to attain a signaled state, and an attempt is made to satisfy as many waits as possible on the semaphore object.

The KeReleaseSemaphore routine might temporarily raise the IRQL. If the Wait parameter is FALSE, the routine, before it returns, restores the IRQL to the original value that it had at the start of the call.

If Wait = TRUE, the routine returns without lowering the IRQL. In this case, the KeReleaseSemaphore call must be immediately followed by a KeWaitXxx call. By setting Wait = TRUE, the caller can prevent an unnecessary context switch from occurring between the KeReleaseSemaphore call and the KeWaitXxx call. The KeWaitXxx routine, before it returns, restores the IRQL to its original value at the start of the KeReleaseSemaphore call. Although the IRQL disables context switches between the two calls, these calls cannot reliably be used as the start and end of an atomic operation. For example, between these two calls, a thread that is running at the same time on another processor might change the state of the semaphore object or of the target of the wait.


A pageable thread or pageable driver routine that runs at IRQL = PASSIVE_LEVEL should never call KeReleaseSemaphore with the Wait parameter set to TRUE. Such a call causes a fatal page fault if the calling function happens to be paged out between the calls to KeReleaseSemaphore and KeWaitXxx.

For more information about semaphore objects, see Semaphore Objects.

Callers of KeReleaseSemaphore must be running at IRQL <= DISPATCH_LEVEL provided that Wait is set to FALSE. Otherwise, the caller must be running at IRQL = PASSIVE_LEVEL.


Requirement Value
Minimum supported client Available starting with Windows 2000.
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 HwStorPortProhibitedDDIs(storport)

See also