Cstring WriteString not working also write not working

 Hi I am trying to cahnge one line in a TXT file (log file of my application),
the TXT file already include many line and i try to recognize the line i
need and then update it with the string i need,
But WriteString looks like it finish fine no fail,
But the TXT file doesn't change .
Here is the code:

CString readLine,CompareResultFile="c:\a.txt";//the file path is just an
example
CStdioFile LogFile_CStdioFile;
char * full_Log_file_path= (char*)malloc (_MAX_PATH);
int numLines=0;
TCHAR PASS_test[] = _T("Result: PASS");
TCHAR FAIL_test[] = _T("Result: FAIL");

if (!LogFile_CStdioFile.Open(CompareResultFile, CFile::modeReadWrite ))
{
     return FALSE;
}
while(LogFile_CStdioFile.ReadString(readLine) )
{
                sprintf(full_Log_file_path,"%S",readLine.GetBuffer(0));
    if(strncmp(full_Log_file_path,"Result:",7))
    {
        numLines++;
        continue;
    }
    else
    {
          if(Status)
          {
            LogFile_CStdioFile.WriteString(PASS_test);
          }
          else
         {
        LogFile_CStdioFile.WriteString(FAIL_test);
         }
         break;
    }
}
LogFile_CStdioFile.Close();

I need suggestion what is the best way to update one line?