<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" "http://www.w3.org/TR/html4/strict.dtd"> <HTML> <HEAD> <TITLE>Programming in XPCE/Prolog: Section 12.1</TITLE><LINK REL=home HREF="index.html"> <LINK REL=contents HREF="Contents.html"> <LINK REL=index HREF="DocIndex.html"> <LINK REL=summary HREF="summary.html"> <LINK REL=previous HREF="debugging.html"> <LINK REL=next HREF="sec-12.2.html"> <STYLE type="text/css"> /* Style sheet for SWI-Prolog latex2html */ dd.defbody { margin-bottom: 1em; } dt.pubdef { background-color: #c5e1ff; } pre.code { margin-left: 1.5em; margin-right: 1.5em; border: 1px dotted; padding-top: 5px; padding-left: 5px; padding-bottom: 5px; background-color: #f8f8f8; } div.navigate { text-align: center; background-color: #f0f0f0; border: 1px dotted; padding: 5px; } div.title { text-align: center; padding-bottom: 1em; font-size: 200%; font-weight: bold; } div.author { text-align: center; font-style: italic; } div.abstract { margin-top: 2em; background-color: #f0f0f0; border: 1px dotted; padding: 5px; margin-left: 10%; margin-right:10%; } div.abstract-title { text-align: center; padding: 5px; font-size: 120%; font-weight: bold; } div.toc-h1 { font-size: 200%; font-weight: bold; } div.toc-h2 { font-size: 120%; font-weight: bold; margin-left: 2em; } div.toc-h3 { font-size: 100%; font-weight: bold; margin-left: 4em; } div.toc-h4 { font-size: 100%; margin-left: 6em; } span.sec-nr { } span.sec-title { } span.pred-ext { font-weight: bold; } /* Footnotes */ sup.fn { color: blue; text-decoration: underline; } span.fn-text: { display: none; } sup.fn span {display: none;} sup:hover span { display: block !important; position: absolute; top: auto; left: auto; width: 80%; color: #000; background: white; border: 2px solid; padding: 5px; margin: 10px; z-index: 100; font-size: smaller; } </STYLE> </HEAD> <BODY BGCOLOR="white"> <DIV class="navigate"><A class="nav" href="index.html"><IMG SRC="home.gif" BORDER=0 ALT="Home"></A> <A class="nav" href="Contents.html"><IMG SRC="index.gif" BORDER=0 ALT="Contents"></A> <A class="nav" href="DocIndex.html"><IMG SRC="yellow_pages.gif" BORDER=0 ALT="Index"></A> <A class="nav" href="summary.html"><IMG SRC="info.gif" BORDER=0 ALT="Summary"></A> <A class="nav" href="debugging.html"><IMG SRC="prev.gif" BORDER=0 ALT="Previous"></A> <A class="nav" href="sec-12.2.html"><IMG SRC="next.gif" BORDER=0 ALT="Next"></A> </DIV> <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><-</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. <P></BODY></HTML>