ZwCreateEvent function (ntifs.h)

Article
07/06/2023

The ZwCreateEvent routine creates an event object, sets the initial state of the event to the specified value, and opens a handle to the object with the specified desired access.

Syntax

NTSYSAPI NTSTATUS ZwCreateEvent(
  [out]          PHANDLE            EventHandle,
  [in]           ACCESS_MASK        DesiredAccess,
  [in, optional] POBJECT_ATTRIBUTES ObjectAttributes,
  [in]           EVENT_TYPE         EventType,
  [in]           BOOLEAN            InitialState
);

Parameters

[out] EventHandle

A pointer to a variable that will receive the event object handle. The handle includes bookkeeping information, such as a reference count and security context.

[in] DesiredAccess

The ACCESS_MASK value that represents the desired types of access for the event object. The following table contains the event-specific ACCESS_MASK values.

Value	Desired access
EVENT_QUERY_STATE	Query the state of the event object.
EVENT_MODIFY_STATE	Modify the state of the event object.
EVENT_ALL_ACCESS	All possible access rights to the event object.

[in, optional] ObjectAttributes

A pointer to the object attributes structure supplied by the caller to be used for the specified object. These attributes would include the ObjectName and the SECURITY_DESCRIPTOR, for example. This parameter is initialized by calling the InitializeObjectAttributes macro.

[in] EventType

The type of the event, which can be SynchronizationEvent or a NotificationEvent. These values belong to the EVENT_TYPE enumeration, which is defined in the ntdef.h header file.

[in] InitialState

The initial state of the event object. Set to TRUE to initialize the event object to the Signaled state. Set to FALSE to initialize the event object to the not-Signaled state.

Return value

ZwCreateEvent returns STATUS_SUCCESS or an appropriate error status. Possible error status codes include the following:

Return code	Description
STATUS_INSUFFICIENT_RESOURCES	Resources required by this function could not be allocated.
STATUS_INVALID_PARAMETER	The supplied ObjectAttributes structure contained an invalid parameter value.
STATUS_INVALID_PARAMETER_4	The specified EventType parameter was invalid.
STATUS_OBJECT_NAME_INVALID	The ObjectAttributes parameter contained an ObjectName in the OBJECT_ATTRIBUTES structure that was invalid.
STATUS_OBJECT_PATH_SYNTAX_BAD	The ObjectAttributes parameter did not contain a RootDirectory member, but the ObjectName member in the OBJECT_ATTRIBUTES structure was an empty string or did not contain an OBJECT_NAME_PATH_SEPARATOR character. This indicates incorrect syntax for the object path.
STATUS_PRIVILEGE_NOT_HELD	The caller did not have the required privilege to create a handle with the access specified in the DesiredAccess parameter.

Remarks

ZwCreateEvent creates an event object, sets its initial state to the specified value, and opens a handle to the object with the specified desired access.

Events are used to coordinate execution. Events can be used by file system drivers to allow a caller to wait for completion of the requested operation until the given event is set to the Signaled state.

ZwCreateEvent can create either notification or synchronization events:

Notification events can be used to notify one or more threads of execution that an event has occurred.
Synchronization events can be used in the serialization of access to hardware between two otherwise unrelated drivers.

A synchronization event is auto-resetting. When a synchronization event is set to the Signaled state, a single thread of execution that was waiting for the event to be signaled is released, and the event is automatically reset to the Not-Signaled state.

Unlike a synchronization event, a notification event is not auto-resetting. Once a notification event is in the Signaled state, it remains in that state until it is explicitly reset.

To synchronize on a notification event:

Create the notification event with ZwCreateEvent with the EventType parameter set to NotificationEvent.
Wait for the event to be signaled by calling ZwWaitForSingleObject with the EventHandle returned by ZwCreateEvent. More than one thread of execution can wait for a given notification event to be signaled. To poll instead of stall, specify a Timeout of zero to ZwWaitForSingleObject.
Close the handle to the notification event with ZwClose when access to the event is no longer needed.

The ZwCreateEvent function is called after the InitializeObjectAttributes macro is used to set attributes in the OBJECT_ATTRIBUTES structure for the object.

There are two alternate ways to specify the name of the object passed to ZwCreateEvent:

As a fully qualified pathname, supplied in the ObjectName member of the input ObjectAttributes.
As pathname relative to the directory represented by the handle in the RootDirectory member of the input ObjectAttributes.

To release the event, a driver calls ZwClose with the event handle.

For more information about events, see Event Objects.

Note

If the call to the ZwCreateEvent routine occurs in user mode, you should use the name "NtCreateEvent" instead of "ZwCreateEvent".

For calls from kernel-mode drivers, the NtXxx and ZwXxx versions of a Windows Native System Services routine can behave differently in the way that they handle and interpret input parameters. For more information about the relationship between the NtXxx and ZwXxx versions of a routine, see Using Nt and Zw Versions of the Native System Services Routines.

Requirements

Requirement	Value
Minimum supported client	Windows XP.
Target Platform	Universal
Header	ntifs.h (include Ntifs.h)
Library	NtosKrnl.lib
DLL	NtosKrnl.exe
IRQL	PASSIVE_LEVEL
DDI compliance rules	HwStorPortProhibitedDDIs(storport), PowerIrpDDis(wdm)