gdb/

Make some lval_funcs methods to default on NULL.
	* valops.c (value_fetch_lazy): Check if lval_computed read method is
	NULL.
	(value_assign): Check if lval_computed write method is NULL.
	* value.h (struct lval_funcs): Comment NULL values for read and write
	methods.

gdb/ChangeLog

@@ -1,3 +1,12 @@
+2011-10-09  Jan Kratochvil  <jan.kratochvil@redhat.com>
+
+	Make some lval_funcs methods to default on NULL.
+	* valops.c (value_fetch_lazy): Check if lval_computed read method is
+	NULL.
+	(value_assign): Check if lval_computed write method is NULL.
+	* value.h (struct lval_funcs): Comment NULL values for read and write
+	methods.
+
 2011-10-09  Jan Kratochvil  <jan.kratochvil@redhat.com>
 
 	Display @entry parameter values (without references).

gdb/valops.c

@@ -1110,7 +1110,8 @@ value_fetch_lazy (struct value *val)
 	 watchpoints from trying to watch the saved frame pointer.  */
       value_free_to_mark (mark);
     }
-  else if (VALUE_LVAL (val) == lval_computed)
+  else if (VALUE_LVAL (val) == lval_computed
+	   && value_computed_funcs (val)->read != NULL)
     value_computed_funcs (val)->read (val);
   else if (value_optimized_out (val))
     /* Keep it optimized out.  */;
@@ -1381,9 +1382,13 @@ value_assign (struct value *toval, struct value *fromval)
       {
 	const struct lval_funcs *funcs = value_computed_funcs (toval);
 
+	if (funcs->write != NULL)
+	  {
 	    funcs->write (toval, fromval);
-      }
 	    break;
+	  }
+      }
+      /* Fall through.  */
 
     default:
       error (_("Left operand of assignment is not an lvalue."));

gdb/value.h

@@ -156,13 +156,15 @@ struct lval_funcs
 {
   /* Fill in VALUE's contents.  This is used to "un-lazy" values.  If
      a problem arises in obtaining VALUE's bits, this function should
-     call 'error'.  */
+     call 'error'.  If it is NULL value_fetch_lazy on "un-lazy"
+     non-optimized-out value is an internal error.  */
   void (*read) (struct value *v);
 
   /* Handle an assignment TOVAL = FROMVAL by writing the value of
      FROMVAL to TOVAL's location.  The contents of TOVAL have not yet
      been updated.  If a problem arises in doing so, this function
-     should call 'error'.  */
+     should call 'error'.  If it is NULL such TOVAL assignment is an error as
+     TOVAL is not considered as an lvalue.  */
   void (*write) (struct value *toval, struct value *fromval);
 
   /* Check the validity of some bits in VALUE.  This should return 1