about timesetevent

I'm using DAQ card to get data,I need a timer to do it
I code a timesetevent and a callback function
but when I executes the program
I got a error message " cannot written "
so...wanna asking about the timesetevent statement has any error?

#include <mmsystem.h>

/*callback function*/

void CALLBACK CAI_TIMEDlg::TimerProc(UINT uTimerID, UINT uMsg, DWORD
dwUser, DWORD dw1, DWORD dw)
{
CAI_TIMEDlg* User;
//warning C4101: 'User' : unreferenced local variable

User->OnTimer;
//warning C4551: function call missing argument list
}

/*a button to start timer*/

void CAI_TIMEDlg::OnBNStart()
{
    UINT i_Scantime;
    CString cs_Scantime;
    m_OK.EnableWindow( FALSE );
    m_Read.EnableWindow( TRUE );
    m_Start.EnableWindow( FALSE );
    m_Stop.EnableWindow( TRUE );
    m_DaqAi.SetOverallInputRange( m_InputRange.GetCurSel());
    m_EScanTime.GetWindowText( cs_Scantime );
    i_Scantime = atoi( cs_Scantime );

wTimerID=timeSetEvent(i_Scantime,1,TimerProc,(DWORD)User,TIME_PERIODIC);

}

/*a button to close timer*/

void CAI_TIMEDlg::OnBNStop()
{
    m_DaqAi.CloseDevice();
    m_OK.EnableWindow( TRUE );
    m_Read.EnableWindow( TRUE );
    m_Start.EnableWindow( TRUE );
    m_Stop.EnableWindow( FALSE );

    timeKillEvent(wTimerID); // cancel the event
    wTimerID = 0;

}

void CAI_TIMEDlg::OnTimer()
{
   //TO CODE FOR TIMER ROUTINE
   int i_Channel;
   float vReading;
   char buffer[10] ;
   FILE *out;
   out=fopen("output.xls","a");
   vReading = m_DaqAi.RealInput( i_Channel );
// warning C4700: local variable 'i_Channel' used without having been
initialized
   gcvt(vReading,7,buffer);
   fprintf(out,"%s\n",buffer);
   fclose(out);
   m_ReturnData.SetWindowText(buffer);
}