ReleaseSemaphore function (synchapi.h)

Increases the count of the specified semaphore object by a specified amount.


BOOL ReleaseSemaphore(
  [in]            HANDLE hSemaphore,
  [in]            LONG   lReleaseCount,
  [out, optional] LPLONG lpPreviousCount


[in] hSemaphore

A handle to the semaphore object. The CreateSemaphore or OpenSemaphore function returns this handle.

This handle must have the SEMAPHORE_MODIFY_STATE access right. For more information, see Synchronization Object Security and Access Rights.

[in] lReleaseCount

The amount by which the semaphore object's current count is to be increased. The value must be greater than zero. If the specified amount would cause the semaphore's count to exceed the maximum count that was specified when the semaphore was created, the count is not changed and the function returns FALSE.

[out, optional] lpPreviousCount

A pointer to a variable to receive the previous count for the semaphore. This parameter can be NULL if the previous count is not required.

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.


The state of a semaphore object is signaled when its count is greater than zero and nonsignaled when its count is equal to zero. The process that calls the CreateSemaphore function specifies the semaphore's initial count. Each time a waiting thread is released because of the semaphore's signaled state, the count of the semaphore is decreased by one.

Typically, an application uses a semaphore to limit the number of threads using a resource. Before a thread uses the resource, it specifies the semaphore handle in a call to one of the wait functions. When the wait function returns, it decreases the semaphore's count by one. When the thread has finished using the resource, it calls ReleaseSemaphore to increase the semaphore's count by one.

Another use of ReleaseSemaphore is during an application's initialization. The application can create a semaphore with an initial count of zero. This sets the semaphore's state to nonsignaled and blocks all threads from accessing the protected resource. When the application finishes its initialization, it uses ReleaseSemaphore to increase the count to its maximum value, to permit normal access to the protected resource.

It is not possible to reduce the semaphore object count using ReleaseSemaphore, because lReleaseCount cannot be a negative number. To temporarily restrict or reduce access, create a loop in which you call the WaitForSingleObject function with a time-out interval of zero until the semaphore count has been reduced sufficiently. (Note that other threads can reduce the count while this loop is being executed.) To restore access, call ReleaseSemaphore with the release count equal to the number of times WaitForSingleObject was called in the loop.


For an example that uses ReleaseSemaphore, see Using Semaphore Objects.


Requirement Value
Minimum supported client Windows XP [desktop apps | UWP apps]
Minimum supported server Windows Server 2003 [desktop apps | UWP apps]
Target Platform Windows
Header synchapi.h (include Windows.h on Windows Server 2003, Windows Vista, Windows 7, Windows Server 2008 Windows Server 2008 R2)
Library Kernel32.lib
DLL Kernel32.dll

See also



Semaphore Objects

Synchronization Functions