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<H2><A NAME="sec:12.1"><SPAN class="sec-nr">12.1</SPAN> <SPAN class="sec-title">Object-base 
consistency</SPAN></A></H2>

<P>Unlike Prolog, <font size=-1>XPCE</font> is not <EM>secure</EM>: if a 
Prolog environment traps a fatal error there is almost always a bug in 
the Prolog system. Except for violating system limits there is no Prolog 
program that can make the Prolog environment crash. For <font size=-1>XPCE</font> 
this is different. Consider the following example:

<PRE class="code">
1 ?- new(@p, picture),
     send(@p, display, new(B, box(100,100))),
     get(B, area, Area),
     free(Area).

Area = @ 803438, B = @803419/box
</PRE>

<P>After these calls, the <B><CODE>&lt;-</CODE>area</B> attribute of the 
box has been destroyed, but the box is not aware of this fact. The 
utility predicate
<A NAME="idx:checkpce0:576"></A><A class="pred" href="sec-D.2.html#checkpce/0">checkpce/0</A> 
scans the <font size=-1>XPCE</font> object-base for various 
inconsistencies and will report that the box contains a slot referring 
to a freed object.

<PRE class="code">
2 ?- checkpce.

[WARNING: Freed object in slot area of @803419/box: @803438/area]
[PCE: Checked 13173 objects]
</PRE>

<P><font size=-1>XPCE</font> uses heuristics trying to avoid that such 
problems actually crash the system (in the example above execution 
continues normally).

<P>We advice using <A NAME="idx:checkpce0:577"></A><A class="pred" href="sec-D.2.html#checkpce/0">checkpce/0</A> 
regularly during program development to verify your application is not 
violating object consistency. Please see <A class="sec" href="whoiswho.html">section 
10.4</A> and <A class="sec" href="hyper.html">section 10.11</A> for 
techniques to avoid `dangling' object references.

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