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<H1><A NAME="sec:F"><SPAN class="sec-nr">F</SPAN> <SPAN class="sec-title">Commonly
encountered problems</SPAN></A></H1>
<A NAME="sec:trouble"></A>
<P>In this chapter we list a number of commonly encountered problems in
using PCE/Prolog.
<DL>
<LI><I>Cannot open display</I><BR>
PCE tries to open the display from the address specified by the DISPLAY
environment variable. It ignores the `--display' command line option.
The display might also be specified explicitly using <A NAME="idx:displaybothaddress:729"></A>`<B>display<CODE><-></CODE>address</B>'.
PCE will open the display as soon as it needs X-resource values or it
needs graphical operations. This will fail if the specified address is
not legal, there is no X-server at that address or the X-server denies
the access. Examine the error message carefully. Make sure X-windows is
running at the specified address. Make sure you have access to this
server. See <A NAME="idx:xauth:730">xauth</A> (when running <A NAME="idx:MITMAGICCOOKIE:731">MIT_MAGIC_COOKIE</A>)
and <A NAME="idx:xhost:732">xhost</A>. If PCE still complains, validate
the access rights by starting a normal X-application (e.g. xterm) in the
same context. Always restart PCE after a fatal or system error as the
system might be corrupted. This problem is not possible in the Win32
implementation.
<LI><I>Bad integer reference</I><BR>
This is a PCE/Prolog interface warning. It implies the integer object
reference given to <A NAME="idx:send212:733"></A><A class="pred" href="sec-7.1.html#send/[2-12]">send/[2-12]</A>,
etc. is not valid. The most common reason is that the object has already
be freed, either explicitly or by PCE's incremental garbage collector.
See <A class="sec" href="memory.html">section E</A>.
<LI><I>Unknown class</I><BR>
Attempt to create an instance of a non-existing class. Apart from the
common mistakes like mistyped class-names, etc. this might be caused by
1) giving a list argument to a send- or get- operation (class `.') or 2)
trying to pass a term through <A NAME="idx:send212:734"></A><A class="pred" href="sec-7.1.html#send/[2-12]">send/[2-12]</A>
or <A NAME="idx:get313:735"></A><A class="pred" href="sec-D.1.html#get/3">get/[3-13]</A>.
See
<A class="sec" href="notprolog.html">section 6.1</A>.
<LI><I>Illegal PCE object description</I><BR>
This implies a non-translatable Prolog datum was passed to the
interface. Normally this will be a non-ground<SUP class="fn">20<SPAN class="fn-text">A
`ground' term is a Prolog term that has no unbound variables.</SPAN></SUP>
argument to <A NAME="idx:new2:736"></A><A class="pred" href="sec-D.1.html#new/2">new/2</A>, <A NAME="idx:send212:737"></A><A class="pred" href="sec-7.1.html#send/[2-12]">send/[2-12]</A>
or <A NAME="idx:get313:738"></A><A class="pred" href="sec-D.1.html#get/3">get/[3-13]</A>.
</DL>
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