ReleaseSemaphore

The ReleaseSemaphore function increases the count of the specified semaphore object by a specified amount.

BOOL ReleaseSemaphore(
  HANDLE hSemaphore,       // handle to semaphore
  LONG lReleaseCount,      // count increment amount
  LPLONG lpPreviousCount   // previous count
);

Parameters

hSemaphore

[in] Handle to the semaphore object. The CreateSemaphore or OpenSemaphore function returns this handle.

Windows NT/2000: This handle must have SEMAPHORE_MODIFY_STATE access. For more information, see Synchronization Object Security and Access Rights.

lReleaseCount

[in] Specifies 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.

lpPreviousCount

[out] 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 Values

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

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.

Requirements

  Windows NT/2000: Requires Windows NT 3.1 or later.
  Windows 95/98: Requires Windows 95 or later.
  Header: Declared in Winbase.h; include Windows.h.
  Library: Use Kernel32.lib.

See Also

Synchronization Overview, Synchronization Functions, CreateSemaphore, OpenSemaphore