\chapter{Commonly encountered problems}		\label{sec:trouble}

In this chapter we list a number of commonly encountered problems
in using PCE/Prolog.  

\begin{description}
    \tick{Cannot open display}
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 `display <->address'.
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 \idx{xauth} (when running \idx{MIT_MAGIC_COOKIE}) and \idx
{xhost}.  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.

    \tick{Bad integer reference}
This is a PCE/Prolog interface warning.  It implies the integer
object reference given to send/[2-12], 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 \secref{memory}.

    \tick{Unknown class}
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 send/[2-12] or get/[3-13].  See
\secref{notprolog}.   

    \tick{Illegal PCE object description}
This implies a non-translatable Prolog datum was passed to the interface.
Normally this will be a non-ground%
    \footnote{A `ground' term is a Prolog term that has no
	      unbound variables.}
argument to new/2, send/[2-12] or get/[3-13].
\end{description}