EventWrite function (evntprov.h)

Writes an ETW event that uses the current thread's activity ID.

Syntax

ULONG EVNTAPI EventWrite(
  REGHANDLE              RegHandle,
  PCEVENT_DESCRIPTOR     EventDescriptor,
  ULONG                  UserDataCount,
  PEVENT_DATA_DESCRIPTOR UserData
);

Parameters

RegHandle

Registration handle of the provider. The handle comes from EventRegister. The generated event will use the ProviderId associated with the handle.

EventDescriptor

EVENT_DESCRIPTOR with event information (metadata) including ID, Version, Level, Keyword, Channel, Opcode, and Task.

Important: ProviderId, Level and Keyword are the primary means for filtering events. Other kinds of filtering are possible but have much higher overhead. Always assign a nonzero level and keyword to every event.

UserDataCount

Number of EVENT_DATA_DESCRIPTOR structures in UserData. The maximum number is 128.

UserData

An array of UserDataCount EVENT_DATA_DESCRIPTOR structures that describe the data to be included in the event. UserData may be NULL if UserDataCount is zero.

Each EVENT_DATA_DESCRIPTOR describes one block of memory to be included in the event. The specified blocks will be concatenated in order with no padding or alignment to form the event content. If using manifest-based decoding, the event content must match the layout specified in the template associated with the event in the manifest.

Return value

Returns ERROR_SUCCESS if successful or an error code. Possible error codes include the following:

  • ERROR_INVALID_PARAMETER: One or more of the parameters is not valid.
  • ERROR_INVALID_HANDLE: The registration handle of the provider is not valid.
  • ERROR_ARITHMETIC_OVERFLOW: The event size is larger than the allowed maximum (64KB - header).
  • ERROR_MORE_DATA: The session buffer size is too small for the event.
  • ERROR_NOT_ENOUGH_MEMORY: Occurs when filled buffers are trying to flush to disk, but disk IOs are not happening fast enough. This happens when the disk is slow and event traffic is heavy. Eventually, there are no more free (empty) buffers and the event is dropped.
  • STATUS_LOG_FILE_FULL: The real-time playback file is full. Events are not logged to the session until a real-time consumer consumes the events from the playback file.

The error code is primarily intended for use in debugging and diagnostic scenarios. Most production code should continue to run and continue to report events even if an ETW event could not be written, so release builds should usually ignore the error code.

Remarks

Most event providers will not call EventWrite directly. Instead, most event providers are implemented using an ETW framework that wraps the calls to EventRegister, EventWrite, and EventUnregister. For example, you might write an event manifest and then use the Message Compiler to generate C/C++ code for the events, or you might use TraceLogging to avoid the need for a manifest.

EventWrite will route the event to the appropriate trace sessions based on the ProviderId (determined from the RegHandle), Level, Keyword, and other event characteristics. If no trace sessions are recording this event, this function will do nothing and return ERROR_SUCCESS.

To reduce the performance impact of events that are not recorded by any trace session, you can call EventEnabled to determine whether any trace session is recording your event before preparing the data and calling EventWrite.

EventWrite sets the event's activity ID to the current thread's activity ID. EventWrite does not include a related activity ID in the event. To specify a different activity ID or to add a related activity ID, use EventWriteTransfer.

EventWrite is equivalent to EventWriteEx with 0 for Filter, 0 for Flags, NULL for ActivityId, and NULL for RelatedActivityId.

Requirements

   
Minimum supported client Windows Vista [desktop apps | UWP apps]
Minimum supported server Windows Server 2008 [desktop apps | UWP apps]
Target Platform Windows
Header evntprov.h
Library Advapi32.lib
DLL Advapi32.dll

See also

EventActivityIdControl

EventRegister

EventWriteTransfer

EventWriteEx

Writing Manifest-based Events.