Zeromemory vs. Fillchar

[Ten_Mile_Hike]

Hello Everyone,

  Can anyone tell me why there is a "ZeroMemory(Destination: Pointer; Length: NativeUInt)" operation when we
  already have FillChar(Destination^, Length, 0)? When should I use one over the other, pros and cons etc....
 
  TIA As Always...

[dsiders]

Hello Everyone,

  Can anyone tell me why there is a "ZeroMemory(Destination: Pointer; Length: NativeUInt)" operation when we
  already have FillChar(Destination^, Length, 0)? When should I use one over the other, pros and cons etc....
 
  TIA As Always...

Did you not notice? ZeroMemory is declared only for Windows and that is is implemented as:

Code: [Select]

procedure ZeroMemory(Destination:PVOID; Length:DWORD);
begin
  FillChar(Destination^,Length,#0);
end;


[Thaddy]

Background is simply that ZeroMemory is a Windows API call.
https://learn.microsoft.com/en-us/previous-versions/windows/desktop/legacy/aa366920(v=vs.85)
Since it is a C macro, in FPC it is implemented as a function.
On Windows it is marked legacy and now has as alternative SecureZeroMemory.
In FreePascal we don't need that. because the only reason Microsoft introduced it, is to prevent overaggressive C(++) compilers to simply remove the call to zeromemory altogether.
FPC can't be that aggressive yet  It won't skip function calls.
So in FPC SecureZeroMemory would be the exact same as ZeroMemory.

[440bx]

Background is simply that ZeroMemory is a Windows API call.

RtlZeroMemory is a Windows function.  ZeroMemory is a macro.

https://learn.microsoft.com/en-us/previous-versions/windows/desktop/legacy/aa366920(v=vs.85)

As is rather usual, MS doesn't tell the whole story.

RtlZeroMemory is a function implemented in NTDLL, it's (32bit) code is:

Code: ASM  [Select][+][-]

1. .text:77062890 ; Exported entry 1304. RtlZeroMemory
2. .text:77062890
3. .text:77062890 ; =============== S U B R O U T I N E =======================================
4. .text:77062890
5. .text:77062890
6. .text:77062890 ; __stdcall RtlZeroMemory(x, x)
7. .text:77062890                 public _RtlZeroMemory@8
8. .text:77062890 _RtlZeroMemory@8 proc near              ; DATA XREF: .text:off_77020210↑o
9. .text:77062890
10. .text:77062890 arg_0           = dword ptr  4
11. .text:77062890 arg_4           = dword ptr  8
12. .text:77062890
13. .text:77062890                 push    edi
14. .text:77062891                 mov     edi, [esp+4+arg_0]
15. .text:77062895                 mov     ecx, [esp+4+arg_4]
16. .text:77062899                 xor     eax, eax
17. .text:7706289B                 cld
18. .text:7706289C                 mov     edx, ecx
19. .text:7706289E                 and     edx, 3
20. .text:770628A1                 shr     ecx, 2
21. .text:770628A4                 rep stosd
22. .text:770628A6                 or      ecx, edx
23. .text:770628A8                 jnz     short loc_770628AE
24. .text:770628AA                 pop     edi
25. .text:770628AB                 retn    8
26. .text:770628AE ; ---------------------------------------------------------------------------
27. .text:770628AE
28. .text:770628AE loc_770628AE:                           ; CODE XREF: RtlZeroMemory(x,x)+18↑j
29. .text:770628AE                 rep stosb
30. .text:770628B0                 pop     edi
31. .text:770628B1                 retn    8
32. .text:770628B1 _RtlZeroMemory@8 endp
33.  

One nice thing about it is that it tries to minimize the number of moves.

Since it is a C macro, in FPC it is implemented as a function.

It often is a macro that is a synonym of an inline implementation of memset.  BTW, it is definitely not a C macro when programming in Assembler.  In assembler it resolves to the ntdll function.

On Windows it is marked legacy and now has as alternative SecureZeroMemory.

It is questionable that the Secure version is more secure.  In both cases, sizeof is going to be used.  The "security" comes from the occasionally undesirable behavior from the C compiler.  IOW, as genuine functions (not macros), they are both as secure or insecure however you want to look at it.

In FreePascal we don't need that.

It's very useful in FPC, in C and in assembler.

FPC generates inline versions, e.g, character arrays and structures initialized with Default() but Default is simply too inconvenient to be used.

The inline capability is nice but, it can definitely grow the size of the executable (not that anyone who's using the LCL would ever notice.)
 

[Ten_Mile_Hike]

Zeromemory is a wrapper for fillchar... (I should have seen it)

[d2010]

Hello Everyone,

C:Q1=How to initialize my records  with string? E.g you search ???
   I need initialize with fillchar? Perhaps a combinatioan fillchar and directly zero?
 

Code: Pascal  [Select][+][-]

1.  
2. Typ
3.   NavTree=^TreeRec;
4.   TreeRec = record
5.               dxf5:integer;                        //rem:contine adresa curenta a variabile tr
6.               prefix,midifix,sufix:integer;
7.               typ: NodeType;
8.               left, right: NavTree;
9.               rl: double;
10.               s:string;tip:_Ttyps;StrL,StrR:word;
11.               lNode,RNode:NodeType;
12.               ls,rs:string;???
13.            end;
14.  

Code: Pascal  [Select][+][-]

1. Function makeNode(t: NodeType; l, r: NavTree): NavTree;
2. begin
3.     result:=nil;
4.     New(result); ???
5. //rem: if (tr<>nil) then fillchar(tr^,ksizeof_treerec,0);
6. fillchar(tr^,sizeof(tr^)-1,0);
7. ???
8.     with result^ do
9.        begin
10.          typ := t;
11.          dxf5:=getmypid[1].lo; //rem:dxf5:=integer(tr);
12.          rl:=0.0;
13.          s:='';
14.          prefix:=0;
15.          if (length(fmpstr.left)>0) and (pp_vars2scr_setq(prefix,fmpstr.left,nil))
16.                  then  fmpstr.left:='';
17.  
18.          sufix:=0;
19.          if (length(fmpstr.rigt)>0) and (pp_vars2scr_setq(sufix,fmpstr.rigt,nil))
20.                  then fmpstr.rigt:='';
21.  
22.          midifix:=0;
23.          if (length(fmpstr.midi)>0)  and (pp_vars2scr_setq(midifix,fmpstr.midi,nil))
24.                 then  fmpstr.midi:='';
25.          f113();
26.          left := l;
27.          right := r;
28.     end;
29. end;
30.  
31.   function makeConstNode(T:NodeType;v: double;a:str63;b:_Ttyps): Tree;
32.   var
33.     tr: Tree;
34.   begin
35.     New(tr);???
36.         //rem: if (tr<>nil) then fillchar(tr^,ksizeof_treerec,0);
37.      // I ASK here???
38.     with tr^ do begin
39.         typ := T; rl := v;s:=a;tip:=b;
40.         if s='$C[..]' then Begin strL:=laVal.strL;
41.                                  strR:=laVal.strR;
42.                            End;
43.     end;
44.     makeConstNode := tr;
45.   end;
46.  
47.  

[Thaddy]

Hello Everyone,

C:Q1=How to initialize my records  with string? E.g you search ???
   I need initialize with fillchar? Perhaps a combinatioan fillchar and directly zero?
 

Code: Pascal  [Select][+][-]

1.  
2. Typ
3.   NavTree=^TreeRec;
4.   TreeRec = record
5.               dxf5:integer;                        //rem:contine adresa curenta a variabile tr
6.               prefix,midifix,sufix:integer;
7.               typ: NodeType;
8.               left, right: NavTree;
9.               rl: double;
10.               s:string;tip:_Ttyps;StrL,StrR:word;
11.               lNode,RNode:NodeType;
12.               ls,rs:string;???
13.            end;
14.  

Code: Pascal  [Select][+][-]

1. Function makeNode(t: NodeType; l, r: NavTree): NavTree;
2. begin
3.     result:=nil;
4.     New(result); ???
5. //rem: if (tr<>nil) then fillchar(tr^,ksizeof_treerec,0);
6. fillchar(tr^,sizeof(tr^)-1,0);
7. ???
8.     with result^ do
9.        begin
10.          typ := t;
11.          dxf5:=getmypid[1].lo; //rem:dxf5:=integer(tr);
12.          rl:=0.0;
13.          s:='';
14.          prefix:=0;
15.          if (length(fmpstr.left)>0) and (pp_vars2scr_setq(prefix,fmpstr.left,nil))
16.                  then  fmpstr.left:='';
17.  
18.          sufix:=0;
19.          if (length(fmpstr.rigt)>0) and (pp_vars2scr_setq(sufix,fmpstr.rigt,nil))
20.                  then fmpstr.rigt:='';
21.  
22.          midifix:=0;
23.          if (length(fmpstr.midi)>0)  and (pp_vars2scr_setq(midifix,fmpstr.midi,nil))
24.                 then  fmpstr.midi:='';
25.          f113();
26.          left := l;
27.          right := r;
28.     end;
29. end;
30.  
31.   function makeConstNode(T:NodeType;v: double;a:str63;b:_Ttyps): Tree;
32.   var
33.     tr: Tree;
34.   begin
35.     New(tr);???
36.         //rem: if (tr<>nil) then fillchar(tr^,ksizeof_treerec,0);
37.      // I ASK here???
38.     with tr^ do begin
39.         typ := T; rl := v;s:=a;tip:=b;
40.         if s='$C[..]' then Begin strL:=laVal.strL;
41.                                  strR:=laVal.strR;
42.                            End;
43.     end;
44.     makeConstNode := tr;
45.   end;
46.  
47.  

Can you make that code generic? So it can fill with bytes, words, integers, qwords etc? 

[ASerge]

C:Q1=How to initialize my records  with string?

The same. The compiler implicitly initializes all strings in records with a zero (in New call). So FillChar won't make it worse.
Why do you use the value "SizeOf(...) - 1" in FillChar?

[Thaddy]

Instead of length()? 

[TBMan]

I would have used

Code: Pascal  [Select][+][-]

1. sizeof(TreeRec)
2.  

If you passed a nil pointer things could get wonky.

[d2010]

Instead of length()? 

I break my function MakeNode onto theee functions

Code: [Select]

   makeNode_idf("fillchar")
   makeNode_id2(",,,,:=0000 or :='' ");
   makeNode_idz("zeromemory")
I execute boca100=001 and I go\t   the results..All thre function all same results (fillchar isEqualTicks  zeromemory isEqual  d2)

Code: [Select]

.oNow_MakeIdNode.zer="fElapsed",Time="00"
oNow_MakeIdNode.zer="zElapsed",Time="00"
.oNow_MakeIdNode.zer="0Elapsed",Time="00"
But, when I reset boca100=100 at ??? isinside For loop:="; that
100xfillchar is more better that 100xZeroMemory..
100xfillchars got 124
100xZeroMemory got 186;

Other information/s about  "MakeIdNode.zer="0" you found  in attched.zip

Code: [Select]

oBod!t086good..oNow_MakeIdNode.zer="fElapsed",Time="124"
oBod!t086good..oNow_MakeIdNode.zer="zElapsed",Time="186"
oBod!t086good..oNow_MakeIdNode.zer="0Elapsed",Time="79"
The "bigger" is worst, the "small" is more better.

I run my

Code: Pascal  [Select][+][-]

1. Var boca100=001;???
2. Function makeNode_id0(rtype: NodeType;lef,rig: NavTree): NavTree;
3. Var dwg,loop:integer;
4. Begin dwg:=GetTickCount;
5.       New(result);
6.       for loop:=01 to boca100 do
7.        Case Now_MakeIdNode.zer of
8. 'f':  makeNode_idf(result,rtype,lef,rig);
9. '0':  makeNode_id2(result,rtype,lef,rig);
10. 'z':  makeNode_idz(result,rtype,lef,rig);
11.       End;
12.       Now_MakeIdNode.tim:=Now_MakeIdNode.tim+(GetTickCount-dwg);
13. End;
14.  

[ASerge]

But, when I reset boca100=100 at ??? isinside For loop:="; that
100xfillchar is more better that 100xZeroMemory..
100xfillchars got 124
100xZeroMemory got 186;

The functions differ in line 7:

Code: Pascal  [Select][+][-]

1. Procedure makeNode_idf(Var dest:NavTree;rtype: NodeType; lef, rig: NavTree);
2. begin
3.     fillchar(dest^,sizeof(TreeRec),0);
4.     with dest^ do
5.        begin
6.          typ := rtype;
7.          dxf5:=integer(dest);
8.          rle:=0.0;
9. ...

Code: Pascal  [Select][+][-]

1. Procedure makeNode_idz(Var dest:NavTree;rtype: NodeType; lef, rig: NavTree);
2. begin
3.     ZeroMemory(dest,sizeof(TreeRec));
4.     with dest^ do
5.        begin
6.          typ := rtype;
7.          dxf5:=getmypid[1].lo;
8.          rle:=0.0;
9. ...