finding source of segfault

Adam Jackson ajax at
Wed Jun 14 08:02:27 PDT 2006

On Wednesday 14 June 2006 01:34, Drew Parsons wrote:

> I'm writing now on the off-chance that someone might recognise the
> signature of the segfault and be able to pinpoint  which lib needs to be
> upgraded to X11R7.1 (or beyond).  The backtrace looks like:
> #0  0x00000000 in ?? ()
> #1  0x080bc2ee in compCreateWindow (pWin=0x875e6e8) at compwindow.c:600

This sort of signature typically means you attempted to call through a 
function pointer that was set to NULL.  compCreateWindow calls through three 
of the screen's function pointer chains: CreateWindow, GetWindowPixmap, and 

Line 600 of compwindow.c calls both the GetWindowPixmap and SetWindowPixmap 
chains.  These are normally always provided by fb, but while Xprint's raster 
and pcl backends wrap fb, the postscript backend does not, and does not 
itself provide implementations for those chains.  Woo!

Right now those chains are only ever called from the Damage and Composite 
extensions, so the minimal hack might be to just always disable those in 
Xprt; I had thought this was done already though.  The better solution might 
be to add stub implementations to the postscript backend and see what breaks.  
Since the ps backend clearly believes it can implement the world without 
using fb, it might be fine to just have those calls no-op.

- ajax
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