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

Marc Weustink marc at dommelstein.nl
Sun Jan 7 11:39:11 CET 2018

On January 6, 2018 8:14:30 PM GMT+01:00, Alexander Hofmann via Lazarus <lazarus at lists.lazarus-ide.org> wrote:
>Dear all,
>I spend some time debugging a programming error (read:
>of available documentation) and though I should share my findings...
>I wanted to have a TChart series drawn with a dashed line, the
>"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
>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

Don't think so. The gtk implementation should have checked this and probably skip the 0

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

Imo the QT implementation should be adapted to behave the same.
Wat does Delphi in this case?

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

We shouldn't "copy"the x behaviour, but behave the same as Delphi.


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