Size of memory

[Okoba]

Hi,

Can I ask why result of MemSize is different of what I asked from GetMem?
And is the memory of that size is safe to use?

Code: Pascal  [Select][+][-]

1. WriteLn(MemSize(GetMem(1024)));

This code returns 1032 on Windows instead of 1024. Why? And can I use those 1032 bytes safely?

Also, what is the difference between MemSize and SysMemSize? I can not find anything about that in the documentation?
I can not find anything about SysFreememSize too, what it does?

[jamie]

Its a managed memory block. The size you are getting is including the pointer size type that denotes the amount of memory that is allocated. IN the case of 64 bits, that would be an 8 byte pointer, so add that to the 1024+8 = 1032
and on 32 bit systems, it results in 1028.

Not sure if that is supposed to report that way or not, but it allows for FreeMem to free the block without knowing the size.

[jamie]

That of course would cause some issues, SysMemsize reports the same, so maybe we have a bug here?

[Okoba]

I do not know about the underlying of the default memory manager and the functions I mentioned are barley documented, so maybe we should wait for someone with more official knowledge.

[jamie]

Code: Pascal  [Select][+][-]

1. procedure TForm1.Button1Click(Sender: TObject);
2. begin
3.   Caption := (MemSize(Getmem(1024))-SizeOf(Pointer)).Tostring;
4. end;          
5.  

That would be the corrected value in either platform.

but still, this isn't Documented as such.

[Thaddy]

Getmem() and Allocmem() are not allocating managed memory at all, Jamie: it is strictly unmanaged heap memory, so it is strange. Here it says as on the tin: 1024.

Code: Pascal  [Select][+][-]

1. var
2. a,b:pointer;
3. begin
4.   a:=getmem(1024);
5.   writeln(memsize(a));//1024
6.   freemem(a);
7.   b:=allocmem(1024);
8.   writeln(memsize(b));//1024
9.   freemem(b);
10. end.

[Okoba]

Here on Windows 10 64bit and FPC 3.3.1 from 2 months ago shows 1032.
What's your OS?

[Thaddy]

Tested 3.3.1 64 bit Windows and Linux from today and 3.2.2 32 bit Windows.
memsize may be affected by a memorymanager, but will always return at least the size of the requested memory,
Running with heaptrc? or cmem?
The actual size is stored at a negative offset from the memory pointer returned, so technically speaking the allocation is bigger by at least the size of a pointer.

[Okoba]

No it is a clean simple project, on Release mode.

[Thaddy]

Can not reproduce. 1024 on all platforms. Both with heaptrc and the default memorymanager.

One remark: both of you lost your memory: impossible to free it.

[MarkMLl]

Getmem() and Allocmem() are not allocating managed memory at all, Jamie: it is strictly unmanaged heap memory, so it is strange. Here it says as on the tin: 1024.

I think you two are sparring over terminology: Jamie means that it includes the management block associated with the allocation.

Here it consistently reports 1032.

And filling in the important info: FPC 3.2.2 on x86_64 Debian 12.

MarkMLl

[jamie]

I did the test with 3.2.2-32 bit windows

[jamie]

I just fired up an OLD copy of laz on my Win2000PC, Version 2.0.0.4, I think that was 2.7.1 or something FPC>

Anyways, that also reports the same, it includes the place holder for the size of memory block.

Then on that PC I have OLD Delphi, that does not even have that function.

But it does have the SysmemSize funciton

This must be an added function of Free Pascal?

[ccrause]

Can not reproduce. 1024 on all platforms. Both with heaptrc and the default memorymanager.

With fpc 3.2.2 win32 for i386 I get 1028 bytes on Windows 10 Enterprise:

Code: [Select]

C:\OpenSourceProjects\tests>copy con mem.pp
program mem;
var a: pointer;
begin
  a := GetMem(1024);
  WriteLn(MemSize(a));
  FreeMem(a);
end.
^Z
        1 file(s) copied.

C:\OpenSourceProjects\tests>c:\OpenSourceProjects\lazarus-3.2\fpc\3.2.2\bin\i386-win32\fpc.exe -vhiwn -l mem.pp
Hint: Start of reading config file c:\OpenSourceProjects\lazarus-3.2\fpc\3.2.2\bin\i386-win32\fpc.cfg
Hint: End of reading config file c:\OpenSourceProjects\lazarus-3.2\fpc\3.2.2\bin\i386-win32\fpc.cfg
Free Pascal Compiler version 3.2.2 [2024/02/26] for i386
Copyright (c) 1993-2021 by Florian Klaempfl and others
Target OS: Win32 for i386
Compiling mem.pp
Linking mem.exe
7 lines compiled, 0.0 sec, 28896 bytes code, 1316 bytes data
2 hint(s) issued

C:\OpenSourceProjects\tests>mem.exe
1028

C:\OpenSourceProjects\tests>systeminfo

Host Name:                 xxxxx
OS Name:                   Microsoft Windows 10 Enterprise

[jamie]

sorry, I was mistaken, Delphi does not have the SysMemSize either