TStream.Read doesn't work on array components.
lazarus / 12-Debugger

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masonwheeler · 2008-01-10 21:50

I'm using an object to read BER compressed integers from a file.  (I'm
aware that this implementation is a very kludgy way to do it.  It's
legacy code I wrote several years ago before I became aware of a
better way to do so, and I've just never gotten around to improving
it.)  Here's a partial class declaration and the read procedure:

Code: [Select]
   TBer = class (TObject)
   private
      value: array[1..10] of byte;
      len: byte;
   public
      procedure read (theFile: TStream);
      ...
   end;

procedure TBer.read(theFile: TStream);
var i, dummy: byte;
begin
   i := 0;
   repeat
   begin
      i := i + 1;
      theFile.Read(value[i], 1)
   end
[b]   until value[i] < $80;[/b]
   len := i;
   for i := 1 to len div 2 do
   begin
      dummy := value[len + 1 - i];
      value[len + 1 - i] := value[i];
      value[i] := dummy
   end
end;

This procedure works just fine on Delphi, but Lazarus doesn't seem to
like it.  The line I wrote in bold evaluates to TRUE every time it
runs. I don't have the same problem using read() with non-array
variables in other parts of the program, which makes me think that
either TStream.read (or TFileStream.read, as the stream in question is
a TFileStream) is passing corrupt data to the array element, or that
the element's memory address is being passed incorrectly.

Unfortunately, the problem is compounded by the debugger's inability
to look inside of objects and view their data members, so I can't
inspect self and see what's going on.

Any help here?

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Phil · 2008-01-13 04:22

Quote from: "masonwheeler"This procedure works just fine on Delphi,
but Lazarus doesn't seem to like it.  The line I wrote in bold
evaluates to TRUE every time it runs. I don't have the same problem
using read() with non-array variables in other parts of the program,
which makes me think that either TStream.read (or TFileStream.read, as
the stream in question is a TFileStream) is passing corrupt data to
the array element, or that the element's memory address is being
passed incorrectly.

So it reads a stream of bytes until it finds a byte with the high bit
set? And you're saying that you're hitting the high-bit byte before
you think you should be hitting it?

Maybe a simple test app that uses TFileStream would be the place to
start to determine if the problem is there. TStream and TFileStream
are used pretty extensively so it would be kind of surprising that
they're still buggy, but you never know.

Thanks.

-Phil

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masonwheeler · 2008-01-13 16:44

I'm not hitting the high-bit byte too early.  I wrote the app in
Delphi, where it runs smoothly.  (That particular piece of code dates
back about 4 years, and has never given me any trouble until now.)
 I've run it under both Delphi and Lazarus, parsing the same data
file.  This routine gets called about 1/4 of the way in.  I've
verified that the Lazarus version is at the same position in the file
that the Delphi version is at, at this point.  Then it tries to read
something directly to an array element and we begin to have divergent
behavior.

I'm somewhat inclined to agree with you--the problem's most likely not
in TStream.  More likely, it's got something to do with passing an
array element to a var parameter.  I suspect that the wrong memory
address is getting passed, producing corrupt data.  But I can't do too
much to verify this, as the debugger doesn't show memory addresses.
 Can anyone check up on it?

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Phil · 2008-01-13 18:04

Quote from: "masonwheeler"I'm somewhat inclined to agree with you--the
problem's most likely not in TStream.  More likely, it's got something
to do with passing an array element to a var parameter.  I suspect
that the wrong memory address is getting passed, producing corrupt
data.  But I can't do too much to verify this, as the debugger doesn't
show memory addresses.  Can anyone check up on it?

Maybe try passing in a non-array variable, either local or global, and
see if this makes any difference.

Thanks.

-Phil

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masonwheeler · 2008-01-13 18:06

It works just fine.  As I said, this is about 1/4 of the way through
the file and everything's gone fine up to this point.  Only when I
begin to pass array elements in does it start to cause trouble.

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Phil · 2008-01-13 19:32

Quote from: "masonwheeler"It works just fine.  As I said, this is
about 1/4 of the way through the file and everything's gone fine up to
this point.  Only when I begin to pass array elements in does it start
to cause trouble.

Sounds like you've found a workaround?

To get this fixed, create a small example program and post it on the
FPC bug tracker. Remember, it has to be something people can compile,
not just look at.

Thanks.

-Phil