Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!tut.cis.ohio-state.edu!ucbvax!hplabs!hpfcso!stroyan
From: stroyan@hpfcso.HP.COM (Mike Stroyan)
Newsgroups: comp.sys.hp
Subject: Re: bug? -- XDrawLine can freeze HP-UX 6.5 X server
Message-ID: <7370085@hpfcso.HP.COM>
Date: 18 Feb 90 22:23:35 GMT
References: <17056@boulder.Colorado.EDU>
Organization: Hewlett-Packard, Fort Collins, CO, USA
Lines: 27

>   I'm writing a program that does graphics in an X window,
> and at a certain point, during an XDrawLine call, my
> display freezes and I have no choice except to kill the
> X server.  This is under HP-UX 6.5 -- we don't have 7.0
> here yet; and it's on an HP 370.

>   The arguments to XDrawLine (corresponding to
> XDrawLine(display, d, gc, x1, y1, x2, y2)) are x1 = 19642,
> y1 = 25985, x2 = -9529, y2 = -20184.  These numbers are very
> big, of course, but I don't want to fool with them, because
> with clipping it is possible that part of the line will pass
> through my window.  ...
  
>   Is this a server bug?  Can anyone from HP comment?
> 	-- Bill Skaggs

Yep, it's a server bug.  The Bresenham vector generator is overflowing
when computing the error term for very long lines.  The magnitude of the
error term exceeds 2^31, and sign errors can result in an infinite loop.
The problem was found after 7.0 went out.  It will be fixed in the next
release.  For now you can either-

1) Clip your lines to the window boundaries.
2) Use width 1 wide lines instead of width 0 lines.  (This will be slower.)
3) Talk to your HP service representative about getting a patched X server.

Mike Stroyan, stroyan@hpfcla.hp.com