[fpc-pascal] Newbie problems with memory leak and access violation/AV

[Michael Van Canneyt]

On Tue, 30 Mar 2010, Jim wrote:

> Hi all,
>
> Background: Hobby programmer; Microsoft Access VBA/Visual Basic 6/Visual
> Basic.Net background.
> I'm a FreePascal newbie but have fiddled with it some time ago. Was
> looking into learning C++ but that seems too curly, so started again.
> I've decided to write a file scanner program that indexes files and can
> function as a Unix locate or CD indexer program replacement.
>
> System: Windows Vista x64, (Lazarus and) Free Pascal Compiler version
> 2.4.1 [2010/03/13] for x86_64.
> Had FPC 2.4 on Ubuntu 9.10 x32 as well, but got rid of it as my disk
> space ran low. Have an OS X machine with Lazarus/FPC (don't remember
> which version.
>
> Problem: Probably due to my innocence, I seem to leak memory all over
> the place as attested by fpc -gl -gh.
> Also, after fiddling with swallowing exceptions and then deciding to
> allow them to see what code is broken and fix the code, I get an AV.
> To try and pinpoint the problem, I've slimmed down the program into a
> simple program which you can find at:
> http://bitbucket.org/jb/flocate/issue/1/access-violation-and-memory-leaks
> I've included the heaptrc trace as well as my debug output in the comments.

After allocating an object which you will free in the same routine, you 
should ALWAYS put a try..finally statement, and free the object in the
finally clause:
   Search := TFileSearch.Create;
   try
     // your code
   finally
     Search.free;
   end;

After that, the error still happens, but at least there is no memory leak.

The error is the following:

    FreeAndNil(Item); //Don't really now if we should do this but it can't hurt. Can it?

It CAN hurt: TSearchRec is a record, you don't need to free it.

Michael.
>
> What I'm trying to do: I'm adding details for each file I've found into
> a new record. I then try to add a pointer to this record to an entry in
> a TFPList. The code seems to bomb on the Res.Add line:
> Procedure TFileSearch.AddFileToResult(ThisDir: Ansistring; item:
> TSearchRec);
> var
>   APResultRecord : PResultRecord; // We use this pointer to store our
> result.
> begin
>   //{$IFDEF DEBUG}(stderr, 'Debug: ', DateTimeToStr(Now), ': File
> ',ThisDir,Item.Name,' is a symlink: ', (Item.Attr And
> faSymlink)=faSymLink);{$ENDIF}
>   New(APResultRecord); //Create and assign memory to a new
> TResultRecord record and store its pointer:
>   APResultRecord^.Directory:=ThisDir;
>   APResultRecord^.FileName:=Item.Name;
>   APResultRecord^.ModifyTime:=FileDateToDateTime(Item.Time);
>   APResultRecord^.Size:=Item.Size;
>   {$IFDEF CRAZYDEBUG}writeln(stderr, 'Debug: ', DateTimeToStr(Now), ':
> going to get hash for:', ThisDir, Item.Name);{$ENDIF}
>   APResultRecord^.MD5Hash:=GetMD5Hash(ThisDir + Item.Name);
>   {$IFDEF DEBUG}writeln(stderr, 'Debug: ', DateTimeToStr(Now), ': going
> to add data to list for:', ThisDir, Item.Name);{$ENDIF}
>   try
>   Res.Add(APResultRecord); //add the pointer to the TFPList
>   except
>   {$IFDEF DEBUG}
>   //doesn't seem to get called either.... just get av!!
>   writeln(stderr, 'Debug: ', DateTimeToStr(Now), ': Adding point to
> list went wrong. Info: ', ThisDir, Item.Name);
>   writeln(stderr, 'Debug: ', DateTimeToStr(Now), ': ',
> APResultRecord^.Directory);
>   writeln(stderr, 'Debug: ', DateTimeToStr(Now), ': ',
> APResultRecord^.FileName);
>   writeln(stderr, 'Debug: ', DateTimeToStr(Now), ': ',
> APResultRecord^.ModifyTime);
>   writeln(stderr, 'Debug: ', DateTimeToStr(Now), ': ',
> APResultRecord^.Size);
>   writeln(stderr, 'Debug: ', DateTimeToStr(Now), ': ',
> APResultRecord^.MD5Hash);
>   {$ENDIF}
>   end;
> end;
>
> Could somebody tell me:
> 1. what I've done wrong regarding the AV
> 2. how I can fix the memory leaks
> I probably don't grasp some basics on memory management...
>
> Of course, suggestions for improvement are also welcomed; if you want
> to, you can have a look at the rest of the program or get the latest
> version on:
> http://bitbucket.org/jb/flocate/get/tip.zip
>
> If you need more detail, I'd be happy to (try and ;)  give it...
>
> Thanks,
> jb
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