I am writing an application on iMac, and having problem dynamically creating objects and reading them into an array:

// I got the following object:
     PChunk = ^Chunk;
     Chunk = Object
              SubChunks: array of PChunk;
              Data: Pointer; .
              Function NewSubchunk: integer;
              Procedure Read(var Fl: TStream);
              Constructor Create;
              Destructor Destroy;
             end;
// creating and reading more data
SubChunks[NewSubChunk]^.read(Fl);
// NewSubChunk function
Function Chunk.NewSubchunk: integer;
begin
  setlength(SubChunks, High(SubChunks)+2);
  New(subchunks[High(SubChunks)], create);
  SubChunks[High(SubChunks)]^.Data := Self.Data;
  Result := High(SubChunks);
end;
// ---------

The procedure fails when running the:

SubChunks[NewSubChunk]^.read(Fl);

All the procedure works fine on Delphi, but on fpc under iMac it fails.

Why?

Regards,

Amit

What is the error message you are having ?

Ido
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The general error it raises is: "Access Violation"

Are you compiling in Delphi mode? Please *always* post compilable source code that demonstrates the problem when asking for help, not uncompilable snippets. Don't make people guess about what you might have written or about how you are compiling it.

Jonas

I am compiling in a delphi mode.
Sorry for not posting a runnable source code,.
The code portion was taken from a huge source code, which is probably hard to detach from the rest of the code.
It is reading a TStream object, which is an input 3ds file given to the app.

---
Amit

Can you write a small proof of concept that does exactly the same thing with the problem ?

Ido
LINESIP - Opening the source for communication
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http://www.linesip.co.il

Have you got the prof.rar?

Amit

Amit Bueno wrote

Have you got the prof.rar?

No. I've just looked at the posts held for moderation and your mail is over 5MB. That is way too large to send to a mailing list. First of all, make sure to only include source code, not compiled units, programs or resources. Secondly, if it's that big, put it online on a website and only send the link to the mailing list.

Jonas

Seems that the ios doesn't allocated properly my objects,. after allocation when trying to run a function from the object it fails. I tried replacing the New(subchunks[X]) to SubChunks[X] := AllocMem(sizeof(Chunk));
Again this works on the windows, it doesn't on the ios.

Have you got any ideas?
---
Amit

Does "X" actually exists as an index (I mean the value of it) ?

Ido
LINESIP - Opening the source for communication
http://www.linesip.com
http://www.linesip.co.il

The SubChunks are -- 'var subchunks array of pchunk;' contained in chunk before allocating the subchunks a setlength(subchunks, High(subchunks)+2) is called.
---
Amit

Amit Bueno wrote

The SubChunks are -- 'var subchunks array of pchunk;' contained in chunk before allocating the subchunks a setlength(subchunks, High(subchunks)+2) is called.

But does the X variable point to existed index in the array ? if not, then
it explains the "access violation".

Ido
LINESIP - Opening the source for communication
http://www.linesip.com
http://www.linesip.co.il

Of course it does -- otherwise it won't have worked on delphi under windows. The X = High(SubChunks);
---
Amit

I have created a proof of concept from your code:

----------------------------
{$mode delphi}
program test;
type
  PChunk = ^Chunk;
  Chunk = Object
       SubChunks : array of PChunk;
       data : Pointer;
       Function NewSubchunk: integer;
  end;
function Chunk.NewSubchunk :integer;
var
 x : integer;
begin
  x := High(SubChunks)+2;
  setlength(SubChunks, x);
  New(subchunks[x]);
  SubChunks[x]^.Data := Self.Data;
  Result := x;
end;
var
 ac : Chunk;
begin
 ac.data := PChar('Hello world');
 writeln(ac.NewSubchunk);
end.

-----------------------

Does it work for you (I do not have any apple to test it on, but it works on my Linux) ?

And yes it leaks memory :)

Ido
LINESIP - Opening the source for communication
http://www.linesip.com
http://www.linesip.co.il

Seems that I found a workaround.

When calling the NewSubChunk in the following syntax:

SubChunks[NewSubChunk]^.read(fl);  /// raise an exception.

While

MyIndex := NewSubChunk;
SubChunks[MyIndex]^.read(fl);  /// raise an exception.

Doesn't raise any excetion….

Maybe the experts could say why, I don't know why…

Amit

Amit Bueno wrote

Snipped

And you say it was on iOS? On ARM I suppose?

What sort of compiler version and options do you use?

I tested it both on ARM, and on i386 (iphonesim).

I used the latest fpc svn trunk. Options I can say it wasn't something exceptional, only –Mdelphi or something.

Amit

I just saw the new message in the moderation queue, but it's still over 600KB (email makes files about 30% bigger because of the inefficient encoding), which is too big for a mailing list (50-100KB is the maximum I'll let through, <=50KB is let through automatically). However, it's also available for download now:

***
Attached is the shrinked prof.rar file (481kb).
You can download it as well from:
http://bueno.co.il/files/prof.rar

---
Amit
***

Jonas

The reason is that SubChunks (a dynamic array) can be nil before NewSubChunk() (a function that allocates 2 extra elements for that array) is called. So if the compiler first loads SubChuncks (=nil) in a register, then calls NewSubChunk, and then indexes this nil pointer using that function result, you will get at an access violation.

The original code is wrong because Pascal does not define a particular order in which such expressions have to be evaluated by the compiler. In general, calling functions with side effects inside expressions that can be affected by those side effects is very bad practice (and dangerous, as you've noticed).

The fact that it doesn't crash in the Delphi version that you tested is pure coincidence, and it may very well crash is older or newer versions.

Jonas