F Commonly encountered problems

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

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 xauth (when running MIT_MAGIC_COOKIE) and 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.

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 section E.

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 section 6.1.

Illegal PCE object description
This implies a non-translatable Prolog datum was passed to the interface. Normally this will be a non-ground^{20A `ground' term is a Prolog term that has no unbound variables.} argument to new/2, send/[2-12] or get/[3-13].