mem size

[Paolo]

Hello

with this code (win-64, FPC 3.2.2)

Code: Pascal  [Select][+][-]

1. type
2.   PntDouble = ^double;
3.  
4. procedure TForm1.Button3Click(Sender: TObject);
5. var
6.   P : PntDouble;
7.   V : QWord;
8.   S, i : integer;
9. begin
10.   S:=SizeOf(Double);
11.   for i:=1 to 16 do begin
12.     GetMem(P, i*S);
13.     V:=MemSize(P);
14.     Memo1.Lines.Add(V.ToString);
15.   end;
16. end;
17.  

I have this as output.
24
24
24
56
56
56
56
88
88
88
88
120
120
120
120
152

tha was for me unexpected result.

what I missed ?

I was expecting 8 byte more after any iteration....
8
16
32
etc...

[LV]

Look, something on this topic was discussed
https://forum.lazarus.freepascal.org/index.php/topic,68982
https://gitlab.com/freepascal.org/fpc/documentation/-/issues/39403

[Paolo]

@LV
I have just updated the multiplication matrix routines with dynamically allocate memory, everything seems working fine, but once I checked with memSize I was confused.
I'll look at the link you provided.

[LV]

I would like to add that the result depends on the selected memory manager, as you are currently using the default memory manager. Try it

uses
 cmem;

Code: Text  [Select][+][-]

1. 8
2. 16
3. 24
4. 32
5. 40
6. 48
7. 56
8. 64
9. 72
10. 80
11. 88
12. 96
13. 104
14. 112
15. 120
16. 128
17.  

uses
 heaptrc;

Code: Text  [Select][+][-]

1. 8
2. 16
3. 24
4. 32
5. 40
6. 48
7. 56
8. 64
9. 72
10. 80
11. 88
12. 96
13. 104
14. 112
15. 120
16. 128
17.  

[jamie]

Old problem. Don't use memsize, it's not exactly what u allocated at all times.
Its more for internal use.

[Paolo]

I saw...

thanks to all.

[PascalDragon]

Code: Pascal  [Select][+][-]

1. type
2.   PntDouble = ^double;
3.  
4. procedure TForm1.Button3Click(Sender: TObject);
5. var
6.   P : PntDouble;
7.   V : QWord;
8.   S, i : integer;
9. begin
10.   S:=SizeOf(Double);
11.   for i:=1 to 16 do begin
12.     GetMem(P, i*S);
13.     V:=MemSize(P);
14.     Memo1.Lines.Add(V.ToString);
15.   end;
16. end;
17.  

Please note that aside from the notes regarding MemSize this code will leak memory. You should better use ReallocMem as that will properly either grow the allocated memory if possible or safely move it to a new, larger location. And don't forget a FreeMem at the end.

[Paolo]

Sure Freemem is there in the real code,

thanks

[LV]

Please note that aside from the notes regarding MemSize this code will leak memory. You should better use ReallocMem as that will properly either grow the allocated memory if possible or safely move it to a new, larger location. And don't forget a FreeMem at the end.

Let's try using the default memory manager first.

Code: Pascal  [Select][+][-]

1. program Project1;
2.  
3. //uses
4.   //cmem;
5.   //heaptrc;
6.  
7. var
8.   P: Pointer;
9.   V, S, i: PtrUInt;
10.  
11. begin
12.  
13.   S := 8;
14.   GetMem(P, S);
15.   for i := 1 to 16 do
16.   begin
17.     ReallocMem(P, i * S);
18.     V := MemSize(P);
19.     writeln('i * S = ', i * S, '   V = ', V);
20.   end;
21.   Freemem(P, V);
22.  
23.   Readln;
24.  
25. end.  
26.  

The default memory manager:

Code: Text  [Select][+][-]

1. i * S = 8   V = 24
2. i * S = 16   V = 24
3. i * S = 24   V = 24
4. i * S = 32   V = 88
5. i * S = 40   V = 88
6. i * S = 48   V = 88
7. i * S = 56   V = 88
8. i * S = 64   V = 88
9. i * S = 72   V = 88
10. i * S = 80   V = 88
11. i * S = 88   V = 88
12. i * S = 96   V = 216
13. i * S = 104   V = 216
14. i * S = 112   V = 216
15. i * S = 120   V = 216
16. i * S = 128   V = 216
17.  

Then, we will use the cmem memory manager, noting that heaptrc is the same:

Code: Text  [Select][+][-]

1. i * S = 8   V = 8
2. i * S = 16   V = 16
3. i * S = 24   V = 24
4. i * S = 32   V = 32
5. i * S = 40   V = 40
6. i * S = 48   V = 48
7. i * S = 56   V = 56
8. i * S = 64   V = 64
9. i * S = 72   V = 72
10. i * S = 80   V = 80
11. i * S = 88   V = 88
12. i * S = 96   V = 96
13. i * S = 104   V = 104
14. i * S = 112   V = 112
15. i * S = 120   V = 120
16. i * S = 128   V = 128
17.  

P.S. Memsize is a documented function  https://www.freepascal.org/docs-html/rtl/system/memsize.html

[PascalDragon]

P.S. Memsize is a documented function  https://www.freepascal.org/docs-html/rtl/system/memsize.html

And did you read the documentation?

MemSize returns the size of a memory block on the heap.

It does not say that it will be the same size that you passed to the GetMem or ReallocMem function. If MemSize is larger than the allocated size then the heap manager will very likely use the remaining space for growing it when ReallocMem is used.

Though all of this highly depends on the used heap manager and thus should be considered an implementation detail of the heap manager.

[LV]

@PascalDragon, thanks for the clarification. I have read the documentation and the issue mentioned at https://gitlab.com/freepascal.org/fpc/documentation/-/issues/39403.
Latest reply:

"Michael Van Canneyt @mvancanneyt · 4 months ago Owner
I tested on linux. The numbers are indeed different, which is of course a bug...
Please file a bug report in the FPC source repository, so the behavior can be corrected.
Once corrected, I will adapt the documentation accordingly."

[marcov]

Pascaldragon's remarks is also what I remember. This is used to decide if a memblock can be enlarged without reallocating (e.g. lengthening a string).

The numbers are memory manager dependent.