diff -uNr chemtool-1.6.11/main.c chemtool-1.6.11-new/main.c
--- chemtool-1.6.11/main.c	2007-08-26 14:49:26 +0300
+++ chemtool-1.6.11-new/main.c	2008-05-16 00:42:45 +0300
@@ -2938,7 +2938,7 @@
    or save operations as defined by the current i/o mode and displays error
    message dialog if necessary */
 {
-  int error;
+  int error, length;
   char errtext[255];
   char oldname[512];
   char *tempstr;
@@ -2950,6 +2950,23 @@
   strcpy (oldname, filename);
   strcpy (filename,
 	  gtk_file_selection_get_filename (GTK_FILE_SELECTION (fs)));
+
+  while ((length = strlen(filename)) > 507 )
+  {
+    snprintf (errtext, 255, "Input filename %s contains %d\r characters and this too long.\n", filename, strlen(filename));
+    gtk_label_set_text (GTK_LABEL (message), errtext);
+    gtk_widget_show (messagew);
+    gtk_grab_add (messagew);
+    strcpy (filename, oldname);
+    snprintf (errtext,255, "Chemtool 1.6.11");
+    gtk_window_set_title (GTK_WINDOW (window), errtext);
+	
+    return;
+  }
+    
+    if (strstr(filename, ".cht") == NULL)    
+	strcat(filename, ".cht");
+	  
   if (datadir == NULL || (int)strlen (datadir) == 0)
     {
       tempstr = strrchr (filename, '/');