CreateEvent

The CreateEvent function creates or opens a named or unnamed event object.

HANDLE CreateEvent(
  LPSECURITY_ATTRIBUTES lpEventAttributes, // SD
  BOOL bManualReset,                       // reset type
  BOOL bInitialState,                      // initial state
  LPCTSTR lpName                           // object name
);

Parameters

lpEventAttributes

[in] Pointer to a SECURITY_ATTRIBUTES structure that determines whether the returned handle can be inherited by child processes. If lpEventAttributes is NULL, the handle cannot be inherited.

Windows NT/2000: The lpSecurityDescriptor member of the structure specifies a security descriptor for the new event. If lpEventAttributes is NULL, the event gets a default security descriptor.

bManualReset

[in] Specifies whether a manual-reset or auto-reset event object is created. If TRUE, then you must use the ResetEvent function to manually reset the state to nonsignaled. If FALSE, the system automatically resets the state to nonsignaled after a single waiting thread has been released.

bInitialState

[in] Specifies the initial state of the event object. If TRUE, the initial state is signaled; otherwise, it is nonsignaled.

lpName

[in] Pointer to a null-terminated string specifying the name of the event object. The name is limited to MAX_PATH characters. Name comparison is case sensitive.

If lpName matches the name of an existing named event object, this function requests EVENT_ALL_ACCESS access to the existing object. In this case, the bManualReset and bInitialState parameters are ignored because they have already been set by the creating process. If the lpEventAttributes parameter is not NULL, it determines whether the handle can be inherited, but its security-descriptor member is ignored.

If lpName is NULL, the event object is created without a name.

If lpName matches the name of an existing semaphore, mutex, waitable timer, job, or file-mapping object, the function fails and the GetLastError function returns ERROR_INVALID_HANDLE. This occurs because these objects share the same name space.

Terminal Services: The name can have a "Global\" or "Local\" prefix to explicitly create the object in the global or session name space. The remainder of the name can contain any character except the backslash character (\). For more information, see Kernel Object Name Spaces.

Windows 2000: On Windows 2000 systems without Terminal Services running, the "Global\" and "Local\" prefixes are ignored. The remainder of the name can contain any character except the backslash character.

Windows NT 4.0 and earlier, Windows 95/98: The name can contain any character except the backslash character.

Return Values

If the function succeeds, the return value is a handle to the event object. If the named event object existed before the function call, the function returns a handle to the existing object and GetLastError returns ERROR_ALREADY_EXISTS.

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

Remarks

The handle returned by CreateEvent has EVENT_ALL_ACCESS access to the new event object and can be used in any function that requires a handle to an event object.

Any thread of the calling process can specify the event-object handle in a call to one of the wait functions. The single-object wait functions return when the state of the specified object is signaled. The multiple-object wait functions can be instructed to return either when any one or when all of the specified objects are signaled. When a wait function returns, the waiting thread is released to continue its execution.

The initial state of the event object is specified by the bInitialState parameter. Use the SetEvent function to set the state of an event object to signaled. Use the ResetEvent function to reset the state of an event object to nonsignaled.

When the state of a manual-reset event object is signaled, it remains signaled until it is explicitly reset to nonsignaled by the ResetEvent function. Any number of waiting threads, or threads that subsequently begin wait operations for the specified event object, can be released while the object's state is signaled.

When the state of an auto-reset event object is signaled, it remains signaled until a single waiting thread is released; the system then automatically resets the state to nonsignaled. If no threads are waiting, the event object's state remains signaled.

Multiple processes can have handles of the same event object, enabling use of the object for interprocess synchronization. The following object-sharing mechanisms are available:

• A child process created by the CreateProcess function can inherit a handle to an event object if the lpEventAttributes parameter of CreateEvent enabled inheritance.
• A process can specify the event-object handle in a call to the DuplicateHandle function to create a duplicate handle that can be used by another process.
• A process can specify the name of an event object in a call to the OpenEvent or CreateEvent function.

Use the CloseHandle function to close the handle. The system closes the handle automatically when the process terminates. The event object is destroyed when its last handle has been closed.

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.
  Unicode: Implemented as Unicode and ANSI versions on Windows NT/2000.

See Also

Synchronization Overview, Synchronization Functions, CloseHandle, CreateProcess, DuplicateHandle, OpenEvent, ResetEvent, SECURITY_ATTRIBUTES, SetEvent, Object Names