Free nonexistent stringlist error?
lazarus / 31-LCL

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Ruptor · 2013-02-07 00:49

It appears that if a stringlist that has not been loaded is freed the
system gives an exception error SIGSEGV. I read about debugger
problems but it happens using the exe file. Is this a compiler problem
or is to be expected and down to the user not to do it?
I am using windows XP, fpc 2.6.1, Lazarus 1.1 r40186

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taazz · 2013-02-07 00:53

show us some code to better understand your position "string list that
has not been loaded" has at least 2 different interpretations
1) the object has been created but no data has been inserted in it.
2) The object has not been created.

in the 1st case freeing the object should work in the second it should
not.

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Ruptor · 2013-02-07 01:33

Here is what is left of the procedure after I chopped everything out
of it to find the problem.
Code: [Select]procedure TForm1.Button2Click(Sender: TObject);
var
  Str: TStringList;
  Price: string;
  PriceStr: Packed Array[1..300] of char;
  cafl: array[0..1000] of Integer;
  ask,bid,freq,y: Real;
  code,ifreq,x: Integer;
  asc,i,j,timsec: LongInt;
  morech,opened:boolean;
begin
  i:=0;
  while i<1000 do
  begin
    cafl[i]:=0;
    i:=i+1;
  end;
  Str.Free;
end;
Commenting out the STR.Free statement stops the exception.

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typo · 2013-02-07 02:00

Quote from: Ruptor on February 07, 2013, 02:33:20 amCommenting out the
STR.Free statement stops the exception.

Of course. You can not Free a non-created object.

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Ruptor · 2013-02-07 02:40

Why doesn't the compiler say something? It warns about uninitialised
variables so why not objects?

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Martin_fr · 2013-02-07 02:55

I just copied your code. And I get all of the below messages.

Starts with a warning about Str.

Code: [Select]unit1.pas(48,3) Warning: Local variable "Str" does not
seem to be initialized
unit1.pas(34,3) Note: Local variable "Price" not used
unit1.pas(35,3) Note: Local variable "PriceStr" not used
unit1.pas(36,3) Note: Local variable "cafl" is assigned but never used
unit1.pas(37,3) Note: Local variable "ask" not used
unit1.pas(37,7) Note: Local variable "bid" not used
unit1.pas(37,11) Note: Local variable "freq" not used
unit1.pas(37,16) Note: Local variable "y" not used
unit1.pas(38,3) Note: Local variable "code" not used
unit1.pas(38,8) Note: Local variable "ifreq" not used
unit1.pas(38,14) Note: Local variable "x" not used
unit1.pas(39,3) Note: Local variable "asc" not used
unit1.pas(39,9) Note: Local variable "j" not used
unit1.pas(39,11) Note: Local variable "timsec" not used
unit1.pas(40,3) Note: Local variable "morech" not used
unit1.pas(40,10) Note: Local variable "opened" not used

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Ruptor · 2013-02-07 03:09

Yes you are right I missed the new message but with the whole
procedure in place the message doesn't occur. Here is the whole
routine that I am messing around with.
Code: [Select]procedure TForm1.Button2Click(Sender: TObject);
var
  Str: TStringList;
  Price: string;
  PriceStr: Packed Array[1..300] of char;
  cafl: array[0..1000] of Integer;
  ask,bid,freq,y: Real;
  code,ifreq,x: Integer;
  asc,i,j,timsec: LongInt;
  morech,opened:boolean;
begin
  i:=0;
//  while i<sizeof(cafl) do
  while i<1000 do
  begin
    cafl[i]:=0;
    i:=i+1;
  end;
  opened:=true;
  Str := TStringList.Create;
  try
    Str.LoadFromFile(FileListBox1.FileName);
//      Str.LoadFromFile(UTF8ToSys(FileListBox1.FileName));
  except
    opened:=false;
    Edit2.text:='Cannot open file';
  end;
  if opened then
  begin
    if FileListBox1.Items.Count-1<>-1 then
    begin
      ProgressBar1.Min:=0;
      ProgressBar1.Max:=Str.Count;
      ProgressBar1.Step:=1;
      ProgressBar1.StepBy(1);
      Edit2.text:=inttostr(Str.Count);
      BidLine.clear;
      i:=0;
      while i<Str.Count do
      begin
        PriceStr:=Str[i];
        morech:=true;
        Price:='';
        j:=1;
        while morech do
        begin
          asc:=ord(PriceStr[j]);
          if (asc >=$30) and (asc <=$39) or (asc =$2E) then
          begin
            Price:=Price+PriceStr[j];
            j:=j+1;
          end
          else
            begin
              if asc =$2C then
              begin
                val(Price,freq,code);
                ifreq:=round(freq);
                cafl[ifreq]:=cafl[ifreq]+1;
                Price:='';
                j:=j+1;
              end
              else morech:=false;
            end;
          end;
        end;
        ProgressBar1.StepIt;
        i:=i+1;
      end;
      ifreq:=0;
      while ifreq<sizeof(cafl) do
      begin
        x:=ifreq;
        y:=cafl[ifreq];
        BidLine.ADDXY(x,y);
        ifreq:=ifreq+1;
      end;
      Edit2.text:=inttostr(ifreq);
    end else
    begin
      Edit2.text:='File Empty';
    end;
  Str.Free;
end;I suppose the compiler doesn't know that the stringlist might not
be initialised but it should be since it is done after the first loop.

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Martin_fr · 2013-02-07 03:35

Well because in that code you do initialize it:
Code: [Select]  Str := TStringList.Create;

That is correct. The error must be in something else.

1) check the debugger is correctly setup (or rather your project):
  http://wiki.lazarus.freepascal.org/Debugger_Setup
- No optimization, except level 1
- no smart linking

If you do have optimization level 2 or 3 it may show you the wrong
line, or other wrong info.

2) Ensure you have "Range Check enabled.

I see no checks, that
Code: [Select]                cafl[ifreq]:=cafl[ifreq]+1;
ifreq stays within the 0..1000 of cafl.

If you write to an array member outside this range (and have no range
check enabled, then you destroy other data. That is like a time bomb.
So it is possible, this can cause the later crash.

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User137 · 2013-02-07 10:02

I don't know if you meant it, but sizeof(cafl), when cafl is
Code: [Select]cafl: array[0..1000] of Integer;will result in 1001*4 =
4004. So last loops ifreq draws from 0 to 4003.

Firstly, if you intend data to go from 0..999, then array is defined
0..999. Last number in array definition means the Max value, not
Count. Secondly, you can use High() to get index of last element
(999), and Length() to get index count (1000). Then there's also Low()
which tells the first index (0).

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Ruptor · 2013-02-07 12:29

Well I ran my unchanged code this morning and the exception has gone.
The code doesn't work of course but at least now I can test it. Thanks
for finding the sizeof() problem before it occurred that has saved me
a lot of time. It looks like once the error happened it was locked and
resetting the debugger or even starting lazarus was not enough to
clear it. A computer power off was required maybe because firefox was
running all the time, I don't know. It smacks of a pointer problem in
my code but strange that it hasn't happened after turning off the
computer. It must still be there but maybe the way I have started the
apps today the memory being screwed up is not being used.