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IDProjectCategoryView StatusLast Update
0020710LazarusDebuggerpublic2013-01-13 14:19
ReporterDenis Kozlov Assigned ToMartin Friebe  
Status closedResolutionno change required 
Summary0020710: Variable inspection uses wrong index for reading characters from string, showing S[I+1] instead of S[I]
DescriptionDebugger features use a wrong index when inspecting the character value of a string, e.g. S[I]. In fact, you are going to see S[I+1] instead of S[I].

A simple demonstration is attached. This affect all of the debugger features, e.g. watch, inspect, mouse over in IDE.

Check the "Steps To Reproduce" for more info.
Steps To Reproduce1) Use the code below as a sample program.
2) Set breakpoint on a first WriteLn.
3) Run the program and wait for the breakpoint to be triggered.
4) Once breakpoint is triggered, inspect values of I and S[I].

Upon the first breakpoint inspection, you will get:
>> S='12345'; I=1; S[I]='2'; S[I+1]='3';

In reality, it should be:
>> S='12345'; I=1; S[I]='1'; S[I+1]='2';

program Project1;

{$mode objfpc}{$H+}


  S: string = '12345';
  A: array [1..5] of Char = ('1', '2', '3', '4', '5');

  I: Integer;

  for I := 1 to Length(S) do
    WriteLn(I, '=', S[I], S[I+1]);
  for I := Low(A) to High(A) do
    WriteLn(I, '=', A[I], A[I+1]);

P.S. I've included an array of chars in the example to double check and demonstrate that inspection of array values does not get affected by this issue.
Additional InformationLazarus
FPC 2.4.4
TagsNo tags attached.
Fixed in Revision
Attached Files


related to 0017619 closedMartin Friebe Debugger handles strings as 0-based, while they are 1-based. Leading to wrong char being displayed 


Martin Friebe

2011-11-19 20:39

manager   ~0054267

Just a note:
You have filed this as "FPC IDE" issue, but you also mention Lazarus, which is a different IDE.

I do not know if the FPC IDE has this issue, but I do guess it does. As for Lazarus please see 0017619

Denis Kozlov

2011-11-19 22:37

reporter   ~0054272

I couldn't find where to specify the Lazarus bit, so I just added it to the additional information. I simply clicked on the "Report Issue" link. Now I see that I should have selected the Lazarus project first and then followed to the reporting.

Anyway, I'm guessing the debugger is the same for both IDE, so the issue would occur in both FPC IDE and Lazarus.

Martin Friebe

2011-11-19 23:05

manager   ~0054274

Indeed both use GDB as debugger. However please look at the related issue.

With current GDB it is not always possible to distinguish between pchar and string. Therefore both results are now shown.

This is fixed in Lazarus 0.9.31

Expressions with both pchar and string are not supported and will not be until GDB will handle dwarf 3.

please indicate if the issue can be closed?

Martin Friebe

2011-11-21 12:53

manager   ~0054337

Please read the comments here and on the related issue.

If you know a better solution, please reopen and specify.

Denis Kozlov

2013-01-10 18:00

reporter   ~0064801

Thanks for the fix! I have just noticed debugger displaying both pchar and string values while using Lazarus 1.0.4 :)

Issue History

Date Modified Username Field Change
2011-11-19 20:19 Denis Kozlov New Issue
2011-11-19 20:39 Martin Friebe Note Added: 0054267
2011-11-19 22:03 Marco van de Voort Project FPC => Lazarus
2011-11-19 22:23 Juha Manninen Relationship added related to 0017619
2011-11-19 22:37 Denis Kozlov Note Added: 0054272
2011-11-19 23:05 Martin Friebe Note Added: 0054274
2011-11-19 23:05 Martin Friebe LazTarget => -
2011-11-19 23:05 Martin Friebe Status new => feedback
2011-11-19 23:09 Martin Friebe Assigned To => Martin Friebe
2011-11-19 23:09 Martin Friebe Category IDE => Debugger
2011-11-19 23:09 Martin Friebe Product Version 2.4.4 =>
2011-11-21 12:53 Martin Friebe Status feedback => resolved
2011-11-21 12:53 Martin Friebe Resolution open => no change required
2011-11-21 12:53 Martin Friebe Note Added: 0054337
2013-01-10 18:00 Denis Kozlov Note Added: 0064801
2013-01-13 14:19 Bart Broersma Status resolved => closed