[Lazarus] GTK2/X Appcrash when TPen.Pattern has invalid value

Alexander Hofmann alexander.hofmann at new-h.de
Sat Jan 6 20:14:30 CET 2018

Dear all,

I spend some time debugging a programming error (read: misinterpretation
of available documentation) and though I should share my findings...

I wanted to have a TChart series drawn with a dashed line, the pen-style
"psDashed" had two short dashes for my eyes, so I used "psPattern" and
set the Pen-Pattern as such:

  Pattern:array [0..7] of longint=(1, 1, 1, 1, 0, 0, 0, 0)

This is wrong of course; however reading some google I thought that each
array element in the TPenPattern array describes one pixel. The problem
now was, that Qt produces quite reasonable results: a 4-pixel dashed
line. Also GTK2 did the job - if the line was drawn _outside_ any

In that case (e.g. when actually setting this as a Pen-Pattern for a
Chart series), the X-Function 58 (SetDashes) produced the Errorcode 2
(Out of range) and the Program ended directly at the 1st Paint-Event -
leaving Lazarus and GDB in the wild with no backtrace or alike.

Of course it works now with the Pattern

  Pattern:array [0..1] of longint=(4, 4)

But: I think that, if this is the correct way to do it, and the value
"0" for any portion of the pattern is indeed not allowed (as stated in
the X-Documentation e.g. here:
shouldn't TPen.SetPattern raise an exception when an invalid pattern is

I don't know about other OSes / Widgestsets, if there are the same "rules".

I could try to file a bugreport / try to provide a patch; but maybe it's
enough to cite the X-Docs somewhere in the Lazarus-Documentation? What
do you think?



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