CreateEventExA function (synchapi.h)

Creates or opens a named or unnamed event object and returns a handle to the object.


HANDLE CreateEventExA(
  [in, optional] LPSECURITY_ATTRIBUTES lpEventAttributes,
  [in, optional] LPCSTR                lpName,
  [in]           DWORD                 dwFlags,
  [in]           DWORD                 dwDesiredAccess


[in, optional] lpEventAttributes

A pointer to a SECURITY_ATTRIBUTES structure. If lpEventAttributes is NULL, the event handle cannot be inherited by child processes.

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. The ACLs in the default security descriptor for an event come from the primary or impersonation token of the creator.

[in, optional] lpName

The name of the event object. The name is limited to MAX_PATH characters. Name comparison is case sensitive.

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

If lpName matches the name of another kind of object in the same namespace (such as 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 namespace.

The name can have a "Global" or "Local" prefix to explicitly create the object in the global or session namespace. The remainder of the name can contain any character except the backslash character (\). For more information, see Kernel Object Namespaces. Fast user switching is implemented using Terminal Services sessions. Kernel object names must follow the guidelines outlined for Terminal Services so that applications can support multiple users.

The object can be created in a private namespace. For more information, see Object Namespaces.

[in] dwFlags

This parameter can be one or more of the following values.

Value Meaning
The initial state of the event object is signaled; otherwise, it is nonsignaled.
The event must be manually reset using 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.

If this flag is not specified, the system automatically resets the event after releasing a single waiting thread.

[in] dwDesiredAccess

The access mask for the event object. For a list of access rights, see Synchronization Object Security and Access Rights.

Return value

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.


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 dwFlags 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.

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.


The synchapi.h header defines CreateEventEx as an alias which automatically selects the ANSI or Unicode version of this function based on the definition of the UNICODE preprocessor constant. Mixing usage of the encoding-neutral alias with code that not encoding-neutral can lead to mismatches that result in compilation or runtime errors. For more information, see Conventions for Function Prototypes.


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

See also


Event Objects

Object Names

Synchronization Functions