Windows ETW: Kernel consumer receives no EventCallback or BufferCallback events

i am trying to listen to ETW kernel events.

Step 1: Call OpenTrace, specifying the EventCallback and optional BufferCallback functions that will be called during my call to ProcessTrace:

var
    logFile: EVENT_TRACE_LOGFILE;
    currentTrace: TRACEHANDLE;
begin
    ZeroMemory(@logFile, sizeof(logFile));

    logFile.LoggerName := KERNEL_LOGGER_NAME;
    logFile.LogFileName := 'C:\Users\Ian\foo.etl';
    logFile.ProcessTraceMode := 0;
    logFile.EventCallback := RealtimeEventCallback;
    logFile.BufferCallback := BufferCallback; //optional

    currentTrace := OpenTrace(@logFile);
    if (currentTrace = INVALID_PROCESSTRACE_HANDLE) or (currentTrace = -1) then
           RaiseLastWin32Error();

Step 2: Enable kernel events. This is done by calling StartTrace. In my case i want to trace kernel interrupts (EVENT_TRACE_FLAG_INTERRUPT) and deferred procedure calls (EVENT_TRACE_FLAG_DPC):

var
    sessionProperties: PEVENT_TRACE_PROPERTIES;
    bufferSize: Int64;
    th: TRACEHANDLE;
    loggerName: string;
    logFilePath: string;
begin
    loggerName := KERNEL_LOGGER_NAME;
    logFilePath := 'C:\Users\Ian\foo.etl';

    bufferSize := sizeof(EVENT_TRACE_PROPERTIES)
        + 1024 //maximum session name is 1024 characters
        + 1024; //maximum log file name is 1024 characters

    sessionProperties := AllocMem(bufferSize);
    ZeroMemory(sessionProperties, bufferSize);

    sessionProperties.Wnode.BufferSize := bufferSize;
    sessionProperties.Wnode.ClientContext := 1; //QPC clock resolution
    sessionProperties.Wnode.Flags := WNODE_FLAG_TRACED_GUID;
    sessionProperties.Wnode.Guid := SystemTraceControlGuid;
    sessionProperties.EnableFlags := EVENT_TRACE_FLAG_INTERRUPT or EVENT_TRACE_FLAG_DPC;
    sessionProperties.LogFileMode := EVENT_TRACE_FILE_MODE_CIRCULAR;
    sessionProperties.MaximumFileSize := 5;  // 5 MB
    sessionProperties.LoggerNameOffset := sizeof(EVENT_TRACE_PROPERTIES);
    sessionProperties.LogFileNameOffset := sizeof(EVENT_TRACE_PROPERTIES)+1024;

    //Copy LoggerName to the offset address
    MoveMemory(Pointer(Cardinal(sessionProperties)+sessionProperties.LoggerNameOffset), PChar(loggerName), Length(loggerName)+1);

    //Copy LogFilePath to the offset address
    MoveMemory(Pointer(Cardinal(sessionProperties)+sessionProperties.LogFileNameOffset), PChar(logFilePath), Length(logFilePath)+1);

    hr := StartTrace({var}th, PChar(loggerName), sessionProperties);
    if (hr <> ERROR_SUCCESS) then
          raise EWin32Error.Create(SysErrorMessage(hr));

And the log is started sucessfully (i can see foo.etl begin to grow to its 5 MB circuluar limit).

Step 3: Call ProcessTrace, which blocks until it has delivered all pending events to the EventCallback handler specified in Step 1:

var
   res: LongWord;
begin
   res := EventTrace.ProcessTrace(@currentTrace, 1, nil, nil);
   if (res <> ERROR_SUCCESS) then
      raise EWin32Error.Create(SysErrorMessage(res));

Except that ProcessTrace repeatedly returns immediately, and no callback is called - even though the etl file is present and growing.

If i change the logging from File Based to Realtime logging:

Step 1 - OpenTrace changes to support realtime:

logFile.ProcessTraceMode := PROCESS_TRACE_MODE_REAL_TIME;

Step 2 - StartTrace changes to support realtime:

sessionProperties.LogFileMode := EVENT_TRACE_REAL_TIME_MODE;

In this case ProcessTrace never returns, but neither EventCallback or BufferCallback are ever called.

What am i doing wrong?

Update: My callback functions:

function BufferCallback(Logfile: PEVENT_TRACE_LOGFILE): LongWord; stdcall;
begin
   ShowMessage('BufferCallback');
   Result := 1; //return true to keep processing rows
end;

procedure RealtimeEventCallback(pEvent: PEVENT_TRACE); stdcall;
begin
   ShowMessage('EventCallback');
   nEvents := nEvents+1;
end;

Solution

i found the problem.

The headers i was using were not quad-word aligned.

In Delphi parlance, the keyword packed was being used.