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<H2><A NAME="sec:3.5"><SPAN class="sec-nr">3.5</SPAN> <SPAN class="sec-title">The 
Graphical Debugger</SPAN></A></H2>

<A NAME="sec:guitracer"></A>

<P>SWI-Prolog offers two debuggers. One is the traditional text-console 
based 4-port Prolog tracer and the other is a window-based source-level 
debugger. The window-based debugger requires XPCE installed. It operates 
based on the <A NAME="idx:prologtraceinterception4:292"></A><A class="pred" href="tracehook.html#prolog_trace_interception/4">prolog_trace_interception/4</A> 
hook and other low-level functionality described in <A class="sec" href="hack.html">chapter 
B</A>.

<P>Window-based tracing provides much better overview due to the eminent 
relation to your source-code, a clear list of named variables and their 
bindings as well as a graphical overview of the call and choice-point 
stack. There are some drawbacks though. Using a textual trace on the 
console one can scroll back and examine the past, while the graphical 
debugger just presents a (much better) overview of the current state.

<H3><A NAME="sec:3.5.1"><SPAN class="sec-nr">3.5.1</SPAN> <SPAN class="sec-title">Invoking 
the window-based debugger</SPAN></A></H3>

<P>Whether the text-based or window-based debugger is used is controlled 
using the predicates <A NAME="idx:guitracer0:293"></A><A class="pred" href="guitracer.html#guitracer/0">guitracer/0</A> 
and <A NAME="idx:noguitracer0:294"></A><A class="pred" href="guitracer.html#noguitracer/0">noguitracer/0</A>. 
Entering debug mode is controlled using the normal predicates for this: <A NAME="idx:trace0:295"></A><A class="pred" href="debugger.html#trace/0">trace/0</A> 
and
<A NAME="idx:spy1:296"></A><A class="pred" href="debugger.html#spy/1">spy/1</A>. 
In addition, PceEmacs prolog mode provides the command
<STRONG>Prolog/Break at</STRONG> (<CODE>Control-c b</CODE>) to insert a 
break-point at a specific location in the source-code.

<P>The graphical tracer is particulary useful for debugging threads. The 
tracer must be loaded from the <CODE>main</CODE> thread before it can be 
used from a background thread.

<DL>
<DT class="pubdef"><A NAME="guitracer/0"><STRONG>guitracer</STRONG></A></DT>
<DD class="defbody">
This predicate installs the above-mentioned hooks that redirect tracing 
to the window-based environment. No window appears. The debugger window 
appears as actual tracing is started through <A NAME="idx:trace0:297"></A><A class="pred" href="debugger.html#trace/0">trace/0</A>, 
by hitting a spy-point defined by <A NAME="idx:spy1:298"></A><A class="pred" href="debugger.html#spy/1">spy/1</A> 
or a break-point defined using PceEmacs command <STRONG>Prolog/Break at</STRONG> (<CODE>Control-c 
b</CODE>).</DD>
<DT class="pubdef"><A NAME="noguitracer/0"><STRONG>noguitracer</STRONG></A></DT>
<DD class="defbody">
Disable the hooks installed by <A NAME="idx:guitracer0:299"></A><A class="pred" href="guitracer.html#guitracer/0">guitracer/0</A>, 
reverting to normal text-console based tracing.</DD>
<DT class="pubdef"><A NAME="gtrace/0"><STRONG>gtrace</STRONG></A></DT>
<DD class="defbody">
Utility defined as <CODE>guitracer,trace</CODE>.</DD>
<DT class="pubdef"><A NAME="gdebug/0"><STRONG>gdebug</STRONG></A></DT>
<DD class="defbody">
Utility defined as <CODE>guitracer,debug</CODE>.</DD>
<DT class="pubdef"><A NAME="gspy/1"><STRONG>gspy</STRONG>(<VAR>+Predicate</VAR>)</A></DT>
<DD class="defbody">
Utility defined as <CODE>guitracer,spy(Predicate)</CODE>.
</DD>
</DL>

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