[DUG] Memcheck (an overview of features)

[Ben Taylor]

> FAstMM should be used for production work.  Memcheck is better at detecting leaks
> though so it is better when debugging.

I'd have to disagree there. fastmm has much better leak reporting. 

however, memcheck currently makes it much easier to locate a leak, due to it's
'integration' with the ide (when it raises exceptions on the lines causing the
leaks).

examples:

1)

  TmyRec = record
    myStr:string;
  end;
  PmyRec=^TmyRec;

var
 p:PmyRec;
begin
 p:=AllocMem(SizeOf(TmyRec));
 p.myStr:='sometext';
 FreeMem(p);

memcheck will report no leaks. fastmm will correctly report a leaked string. adding 
 p.myStr:='';
is required before freeing the record.

2) memcheck will also miss objects that are created in initialization sections. eg:

initialization

  TObject.Create;

fastmm also provides detection of many 'bad memory use' situations, including the
following common ones:

1) buffer overruns. (fastmm can't be perfect in this area, it will miss some.)

var
 s:string;
begin
 SetLength(s,10);
 s[15]:='x';

2) fastmm can detect you're calling a virtual method on an already-freed object. it
tells you the object that used to be there, and what method you called (in this
case, TWinControl and Destroy);

var
 o:TComponent;
begin
 o:=TWinControl.Create(nil);
 o.Free;
 //<snip> some other code in your app
 o.Free;

3) memory modified after being freed:

var
 p:PChar;
begin
 p:=AllocMem(10);
 FreeMem(p);
 p[5]:=#50;
 p:=AllocMem(10); //will raise exception here
 FreeMem(p);

good coding practices can prevent these situations happening in the first place, but
1) everyone makes mistakes, and 2) there's a good chance you're using other people's
code.

fastmm is also designed to resist memory fragmentation, is more consistent in the
time taken to allocate memory, and is generally significantly faster than the
standard borland mm. this is most useful for long running or memory allocation
intensive applications, eg applications running as a service. depending on the
application, you could expect a 20% increase in speed "for free".

fastmm is in dexter/delphi2006, but i'm half expecting all the useful debug code
will be removed (and i agree with that). you will still be able to replace it with
the 'full' fastmm (or even the old borland mm). due to fastmm's different handling
of memory, it is likely to expose existing (undiscovered) bugs in people's
applications. if you want to prepare for d2006, it would be a good idea to try
fastmm now, and make sure your app is behaving as it should be :-)

anyway, hope someone finds this "+1, interesting" ;-)

b

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