Fprintf does not write to the pipe

I successfully open the handset in the gnuplot window using _popen. But failed to write stream using fprintf. I checked the value of the file pointer and it is not zero. I searched a lot of sources and used fflush and it does not work. I could not find a solution.

Actually I asked a similar question before this gnuplot C ++ interface via the -cannot open wgnuplot channels with some changes.

Any suggestions would be helpful.

FILE* gp;
  string command = "set style data lines\n" ;
  char *path = "\"C:\\Program Files\\gnuplot\\bin\\wgnuplot\" -persist";

  gp = _popen(path , "wt");

  if (gp == NULL)
    return -1;

 fprintf(gp,command );
 fflush(gp);
 _pclose(gp);

I used this code without using pipes and it uses createprocess. The same situation is here, Gnuplot.exe opens, but without output.

int _tmain (int argc, LPTSTR argv [])

{
    DWORD i;
    HANDLE hReadPipe, hWritePipe;

    SECURITY_ATTRIBUTES PipeSA = {sizeof (SECURITY_ATTRIBUTES), NULL, TRUE};
            /* Init for inheritable handles. */
    TCHAR outBuf[ ] = TEXT("a=2; plot sin(a*x)/x; pause mouse; plot exp(-a*x); pause mouse") ;  
    TCHAR inBuf[80];
    DWORD dwWritten, dwRead ;
    BOOL  bSuccess = FALSE;
    PROCESS_INFORMATION  ProcInfo2;
    STARTUPINFO StartInfoCh2;


    /* Startup info for the Gnuplot process. */

    GetStartupInfo (&StartInfoCh2);

    /* Create an anonymous pipe with default size.
        The handles are inheritable. */

    bSuccess = CreatePipe (&hReadPipe, &hWritePipe, &PipeSA, 0);
    if (bSuccess == TRUE) printf("pipe created\n");

    WriteFile(hWritePipe, outBuf, sizeof(outBuf), &dwWritten, NULL) ;   
    printf("Wrote %d bytes to Gnuplot\n", dwWritten) ;

    CloseHandle (hWritePipe);

    /* Repeat (symmetrically) for the child process. */

    StartInfoCh2.hStdInput  = hReadPipe;
    StartInfoCh2.hStdError  = GetStdHandle (STD_ERROR_HANDLE);
    StartInfoCh2.hStdOutput = GetStdHandle (STD_OUTPUT_HANDLE);
    StartInfoCh2.dwFlags = STARTF_USESTDHANDLES;
    bSuccess = FALSE ;
    bSuccess = CreateProcess ("C:\\Program Files\\gnuplot\\bin\\wgnuplot.exe", NULL, NULL, NULL,
            TRUE,0, NULL, NULL, &StartInfoCh2, &ProcInfo2);
    if (bSuccess == TRUE)
      printf("Created Gnuplot Process\n" ) ;

    WaitForSingleObject (ProcInfo2.hProcess, INFINITE);
    CloseHandle (ProcInfo2.hThread); 
    CloseHandle (hReadPipe);

    /* Wait for Gnuplot process to complete.*/

    CloseHandle (ProcInfo2.hProcess);
    return 0;
}