problem on treeview

[Hariyanto <xhonghar telkom net>]

I have treeview with 4 column.
1. First Problem :
   Can I freeze cell B  when I have fill cell A? 
   And freeze cell A when cell B has been fill.
   How I can make this freeze and not editable?
--------------------------------------------------------------
        A        |        B        |       C      |     D
--------------------------------------------------------------
          123    |   freeze cell   |      50      |    A + C
   freeze cell   |          100    |      35      |    B + C

2. Second Problem :
   How I can fill cell D with automatic calc A + C on start editing cell D ?
   I try with this one but still can't fill cell D.

void cell_d_editing_start(.....) 
{
....
  dbl_d = atof(a) + atof(c);
  str_d = format_num_to_string(dbl_d,3);
  g_print("Str d is %s \n", str_d);        <<< OK - I see correct Value.

  g_object_set(cell, "text", str_d, NULL);             <<< 1st try (Cell D still Empty)

/* gtk_list_store_set(GTK_LIST_STORE (model), &iter,   <<< 2nd try (Cell D still Empty)
                         3, str_d,
 		        -1);
*/
}

void cell_d_edited(.....) 
{
....> same with code on gtk-demo
  switch (column)
    {
    case COLUMN_d:
      {
  	g_array_index (data_tw, JPrefs, i).col4 = g_strdup (new_text);    
        g_print("Str d is %s \n", g_strdup (new_text));     <<< Still Empty  ???.    
      }
}

thx.
Hariyanto