change function "gtk_text_buffer_select_range" to "gtk_text_buffer_apply_tag", then doesn't work!

[gepolv <sssssiqn gmail com>]

when i run the ATTACHED SOURCE CODE,it works perfectly.But when i change the
function search_button_clicked as follow:
void search_button_clicked (GtkWidget *search_button, App *app)
{
  const gchar *text;
  GtkTextBuffer *buffer;
  GtkTextIter iter;
  GtkTextIter mstart, mend;
  gboolean found;

  /*my own*/
   GtkTextTag *tag;
  /*my own end*/

  text = gtk_entry_get_text (GTK_ENTRY (app->search_entry));

  /* Get the buffer associated with the text view widget. */
  buffer = gtk_text_view_get_buffer (GTK_TEXT_VIEW (app->text_view));
  /* Search from the start from buffer for text. */
  gtk_text_buffer_get_start_iter (buffer, &iter);
  found = gtk_text_iter_forward_search (&iter, text, 0, &mstart, &mend,
NULL);

  /*my own*/
  tag = gtk_text_buffer_create_tag (buffer, NULL,"foreground", "red",
NULL);
  /*my own end */

  if (found)
    {
      /* If found, hilight the text. */
     /*my own*/
      gtk_text_buffer_apply_tag (buffer, tag, &mstart, &mend);
     /*my own end*/
    }
}
After changing,when i press the button one time,it does not work at all,but
when i press again,it works!
Namely,i press two times ,then it works.But the original program requires
only time press,then it works.
I am confused,please relax me!

ANY HELP WILL BE APPRECITAED!

THE ATTACHED SOURCE CODE:
#include <gtk/gtk.h>
typedef struct _App
{
  GtkWidget *text_view;
  GtkWidget *search_entry;
} App;
int i=0;
/* Called when main window is destroyed. */
void
win_destroy (void)
{
  gtk_main_quit();
}

/* Called when search button is clicked. */
void
search_button_clicked (GtkWidget *search_button, App *app)
{
  const gchar *text;
  GtkTextBuffer *buffer;
  GtkTextIter iter;
  GtkTextIter mstart, mend;
  gboolean found;
    //GtkTextTag *tag;
  text = gtk_entry_get_text (GTK_ENTRY (app->search_entry));

  /* Get the buffer associated with the text view widget. */
  buffer = gtk_text_view_get_buffer (GTK_TEXT_VIEW (app->text_view));
  /* Search from the start from buffer for text. */
  gtk_text_buffer_get_start_iter (buffer, &iter);
  found = gtk_text_iter_forward_search (&iter, text, 0, &mstart, &mend,
NULL);
  //tag = gtk_text_buffer_create_tag (buffer, NULL,
                          "foreground", "red", NULL);
  if (found)
    {
      /* If found, hilight the text. */
          gtk_text_buffer_select_range (buffer, &mstart, &mend);

      //gtk_text_buffer_apply_tag (buffer, tag, &mstart, &mend);
    }
}

int
main (int argc, char *argv[])
{
  GtkWidget *win;
  GtkWidget *vbox;
  GtkWidget *hbox;
  GtkWidget *search_button;
  GtkWidget *swindow;

  App app;

  gtk_init (&argc, &argv);

  /* Create a window with a search entry, search button and a text
     area. */
  win = gtk_window_new (GTK_WINDOW_TOPLEVEL);
  g_signal_connect (G_OBJECT (win), "destroy", win_destroy, NULL);

  vbox = gtk_vbox_new (FALSE, 2);
  gtk_container_add (GTK_CONTAINER (win), vbox);

  hbox = gtk_hbox_new (FALSE, 2);
  gtk_box_pack_start (GTK_BOX (vbox), hbox, FALSE, FALSE, 0);

  app.search_entry = gtk_entry_new ();
  gtk_box_pack_start (GTK_BOX (hbox), app.search_entry, TRUE, TRUE, 0);

  search_button = gtk_button_new_with_label ("Search");
  gtk_box_pack_start (GTK_BOX (hbox), search_button, FALSE, FALSE, 0);
  g_signal_connect (G_OBJECT (search_button), "clicked",
                    G_CALLBACK (search_button_clicked), &app);

  /* A scrolled window which automatically displays horizontal and
     vertical scrollbars when the text exceeds the text view's size. */
  swindow = gtk_scrolled_window_new (NULL, NULL);
  gtk_scrolled_window_set_policy (GTK_SCROLLED_WINDOW (swindow),
                                  GTK_POLICY_AUTOMATIC,
                                  GTK_POLICY_AUTOMATIC);
  gtk_box_pack_start (GTK_BOX (vbox), swindow, TRUE, TRUE, 0);
  app.text_view = gtk_text_view_new ();
  gtk_container_add (GTK_CONTAINER (swindow), app.text_view);
  gtk_widget_show_all (win);
  gtk_main();
}