What bookkeeping data does a Delphi dynamic array contain?

Question

Here's a simple program to check memory allocation. Checking before and after values with Task Manager suggests that each dynamic array takes up 20 bytes of memory at size = 1. The element size is 4, which means 16 bytes of overhead for bookkeeping data.

From looking through system.pas, I can find an array length field at -4 bytes, and a reference count at -8 bytes, but I can't seem to find any references to the other 8. Anyone know what they do?

Sample program:

program Project1;

{$APPTYPE CONSOLE}

type
   TDynArray = array of integer;
   TLotsOfArrays = array[1..1000000] of TDynArray;
   PLotsOfArrays = ^TLotsOfArrays;

procedure allocateArrays;
var
   arrays: PLotsOfArrays;
   i: integer;
begin
   new(arrays);
   for I := 1 to 1000000 do
      setLength(arrays^[i], 1);
end;

begin
  readln;
  allocateArrays;
  readln;
end.

Answer 1

I had a look into System.pas as well and noticed that the GetMem call in _DynArrayCopyRange supports your analyis:

allocated size = count * element size + 2 * Sizeof(Longint)

. So maybe the numbers you get from task manager aren't very accurate. You could try Pointer(someDynArray) := nil and check which memory leak size FastMM reports for more reliable numbers.

Edit: I did a little test program:

program DynArrayLeak;

{$APPTYPE CONSOLE}

uses
  SysUtils;

procedure Test;
var
  arr: array of Integer;
  i: Integer;
begin
  for i := 1 to 6 do
  begin
    SetLength(arr, i);
    Pointer(arr) := nil;
  end;
end;

begin
  ReportMemoryLeaksOnShutdown := True;
  Test;
end.

This yields

  An unexpected memory leak has occurred. The unexpected small block leaks are:

  1 - 12 bytes: Unknown x 1
  13 - 20 bytes: Unknown x 2
  21 - 28 bytes: Unknown x 2
  29 - 36 bytes: Unknown x 1

which supports the 8 byte overhead theory.