What is difference between FastMM and BorlndMM in Delphi 2009 - delphi

Question

I am having a memory problem that cannot explain root cause, please help me!
The problem is as follows:
My program throws exception when i exit.
I investigated and thought that the cause of the error was the use Copymemory function to copy the record containing the variable have string data type.
Below is my demo program by delphi 2009:
In *.dpr file, I added ShareMem unit for use BorlndMM.dll instead of using FastMM memory management.
I define a struct containing a String variable.
Allocate 1 array of 2048 PByte elements.
Use Memory Copy function to copy 2 struct.
finally, Free array 2048.
when i exit program, my program threw exception.
type
TMyStructure = record
F1: TMyStructure;
str1: string;
unit 1
procedure TForm9.FormCreate(Sender: TObject);
var
i: Integer;
begin
F1.str1 := 'This is a string to demo for copying the String data type
using the Copymemory method';
end;
unit 2
procedure TForm8.btn1Click(Sender: TObject);
var
Form9: TForm9;
i: Integer;
s1: array[0..2048-1] of PByte;
begin
// Alloc 1st 2048 segments, each segment gain 1KB
for i := 0 to Length(s1) - 1 do
begin
s1[i] := AllocMem(1024);
end;
// Create Form9
Form9 := TForm9.Create(Owner);
// -> ERROR
CopyMemory(#Self.F1, #Form9.F1, SizeOf(TMyStructure));
// -> OK
// Self.F1 := Form9.F1;
// Free memory s1 array
for i := 0 to Length(s1) - 1 do
begin
FreeMem(s1[i]);
end;
end;
Note:
I have known management in memory of string is reference counting.
I've been investigating because of the use of Memorycopy, so reference count of the string is not incremented although there are 2 references to it.
I investigated that when the string was free for the first time, its reference count dropped to zero, but that memory was not returned to the OS.However, it seems that when another free variable has a nearby address, that memory loses the reference, so when accessing the second free will cause an exception.
Since no source code can be found as well as documentation describing the operation mechanism of BorlndMM.dll should read the spec as well as GetMem.inc source code file to understand the mechanism of operation FastMM and speculate the root cause of the bug. I'm guessing when a free variable, BorlndMM find ahead and after that free space and the combination leads to the memory area that can not be referenced anymore.Of course, using Memorycopy to copy two string variables is a action wrong and must fix. However, I would like to understand how the Memory Management mechanism works to explain the root cause of the above phenomenon.
Expect for help!
If possible please explain the root cause of the above phenomenon for me. And, If there is a document / source explaining how the operation of Memory Management 2005-BorlndMM , please send it to me. Thanks you so much!

Answer 1

The problem with your code is that it bypasses string reference counting. By copying the structure using CopyMemory you end up with two variables, both referring to the same string object. But that object still has a reference count of one.
What happens after that is unpredictable. You may encounter runtime errors. Or you may not. The behaviour is not well defined.
From your question you appear to be trying to understand why different memory managers lead to different behaviours. The thing about undefined behaviour is that it is, well, not defined. There's no point trying to reason about the behaviour. It's not possible to reason about it.
You must fix your code by replacing the memory copy with simple assignment:
F1 := Form9.F1;