Zeromemory vs. Fillchar
lazarus / 62-General

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Ten_Mile_Hike · 2025-07-03 22:08

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...

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dsiders · 2025-07-03 22:13

Quote from: Ten_Mile_Hike on July 04, 2025, 12:08:19 amHello 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;

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Thaddy · 2025-07-04 05:11

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.

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440bx · 2025-07-04 06:06

Quote from: Thaddy on July 04, 2025, 07:11:35 amBackground is simply
that ZeroMemory is a Windows API call.
RtlZeroMemory is a Windows function.  ZeroMemory is a macro.

Quote from: Thaddy on July 04, 2025, 07:11:35
amhttps://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][+][-]window.onload = function(){var x1 =
document.getElementById("main_content_section"); if (x1) { var x =
document.getElementsByClassName("geshi");for (var i = 0; i
.text:77062890 ; Exported entry 1304.
RtlZeroMemory.text:77062890.text:77062890 ; =============== S U B R O
U T I N E =======================================.text:77062890.text:7
7062890.text:77062890 ; __stdcall RtlZeroMemory(x, x).text:77062890  
             public _RtlZeroMemory@8.text:77062890 _RtlZeroMemory@8
proc near              ; DATA XREF:
.text:off_77020210↑o.text:77062890.text:77062890 arg_0           =
dword ptr  4.text:77062890 arg_4           = dword ptr
 8.text:77062890.text:77062890                 push  
 edi.text:77062891                 mov     edi,
[esp+4+arg_0].text:77062895                 mov     ecx,
[esp+4+arg_4].text:77062899                 xor     eax,
eax.text:7706289B                 cld.text:7706289C                
mov     edx, ecx.text:7706289E                 and     edx,
3.text:770628A1                 shr     ecx, 2.text:770628A4         
      rep stosd.text:770628A6                 or      ecx,
edx.text:770628A8                 jnz     short
loc_770628AE.text:770628AA                 pop     edi.text:770628AB 
              retn    8.text:770628AE ; ------------------------------
---------------------------------------------.text:770628AE.text:77062
8AE loc_770628AE:                           ; CODE XREF:
RtlZeroMemory(x,x)+18↑j.text:770628AE                 rep
stosb.text:770628B0                 pop     edi.text:770628B1        
       retn    8.text:770628B1 _RtlZeroMemory@8 endp One nice thing
about it is that it tries to minimize the number of moves.


Quote from: Thaddy on July 04, 2025, 07:11:35 amSince 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.

Quote from: Thaddy on July 04, 2025, 07:11:35 amOn 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.

Quote from: Thaddy on July 04, 2025, 07:11:35 amIn 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.)

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Ten_Mile_Hike · 2025-07-04 07:45

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

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d2010 · 2025-07-09 15:37

Quote from: Ten_Mile_Hike on July 04, 2025, 12:08:19 amHello 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][+][-]window.onload = function(){var x1 =
document.getElementById("main_content_section"); if (x1) { var x =
document.getElementsByClassName("geshi");for (var i = 0; i  Typ 
NavTree=^TreeRec;  TreeRec = record              dxf5:integer;       
               //rem:contine adresa curenta a variabile tr           
 prefix,midifix,sufix:integer;              typ: NodeType;           
 left, right: NavTree;              rl: double;             
s:string;tip:_Ttyps;StrL,StrR:word;             
lNode,RNode:NodeType;              ls,rs:string;???           end; 
Code: Pascal  [Select][+][-]window.onload = function(){var x1 =
document.getElementById("main_content_section"); if (x1) { var x =
document.getElementsByClassName("geshi");for (var i = 0; i Function
makeNode(t: NodeType; l, r: NavTree): NavTree;begin    result:=nil;  
New(result); ???//rem: if (tr<>nil) then
fillchar(tr^,ksizeof_treerec,0);fillchar(tr^,sizeof(tr^)-1,0);???   
with result^ do       begin         typ := t;       
 dxf5:=getmypid[1].lo; //rem:dxf5:=integer(tr);         rl:=0.0;     
  s:='';         prefix:=0;         if (length(fmpstr.left)>0) and
(pp_vars2scr_setq(prefix,fmpstr.left,nil))                 then
 fmpstr.left:='';          sufix:=0;         if
(length(fmpstr.rigt)>0) and (pp_vars2scr_setq(sufix,fmpstr.rigt,nil)) 
               then fmpstr.rigt:='';          midifix:=0;         if
(length(fmpstr.midi)>0)  and
(pp_vars2scr_setq(midifix,fmpstr.midi,nil))                then
 fmpstr.midi:='';         f113();         left := l;         right :=
r;    end;end;   function makeConstNode(T:NodeType;v:
double;a:str63;b:_Ttyps): Tree;  var    tr: Tree;  begin   
New(tr);???        //rem: if (tr<>nil) then
fillchar(tr^,ksizeof_treerec,0);     // I ASK here???    with tr^ do
begin        typ := T; rl := v;s:=a;tip:=b;         if s='$C[..]' then
Begin strL:=laVal.strL;                               
 strR:=laVal.strR;                           End;    end;   
makeConstNode := tr;  end;

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Thaddy · 2025-07-09 17:10

Quote from: d2010 on July 09, 2025, 05:37:57 pmQuote from:
Ten_Mile_Hike on July 04, 2025, 12:08:19 amHello 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][+][-]window.onload = function(){var x1 =
document.getElementById("main_content_section"); if (x1) { var x =
document.getElementsByClassName("geshi");for (var i = 0; i  Typ 
NavTree=^TreeRec;  TreeRec = record              dxf5:integer;       
               //rem:contine adresa curenta a variabile tr           
 prefix,midifix,sufix:integer;              typ: NodeType;           
 left, right: NavTree;              rl: double;             
s:string;tip:_Ttyps;StrL,StrR:word;             
lNode,RNode:NodeType;              ls,rs:string;???           end; 
Code: Pascal  [Select][+][-]window.onload = function(){var x1 =
document.getElementById("main_content_section"); if (x1) { var x =
document.getElementsByClassName("geshi");for (var i = 0; i Function
makeNode(t: NodeType; l, r: NavTree): NavTree;begin    result:=nil;  
New(result); ???//rem: if (tr<>nil) then
fillchar(tr^,ksizeof_treerec,0);fillchar(tr^,sizeof(tr^)-1,0);???   
with result^ do       begin         typ := t;       
 dxf5:=getmypid[1].lo; //rem:dxf5:=integer(tr);         rl:=0.0;     
  s:='';         prefix:=0;         if (length(fmpstr.left)>0) and
(pp_vars2scr_setq(prefix,fmpstr.left,nil))                 then
 fmpstr.left:='';          sufix:=0;         if
(length(fmpstr.rigt)>0) and (pp_vars2scr_setq(sufix,fmpstr.rigt,nil)) 
               then fmpstr.rigt:='';          midifix:=0;         if
(length(fmpstr.midi)>0)  and
(pp_vars2scr_setq(midifix,fmpstr.midi,nil))                then
 fmpstr.midi:='';         f113();         left := l;         right :=
r;    end;end;   function makeConstNode(T:NodeType;v:
double;a:str63;b:_Ttyps): Tree;  var    tr: Tree;  begin   
New(tr);???        //rem: if (tr<>nil) then
fillchar(tr^,ksizeof_treerec,0);     // I ASK here???    with tr^ do
begin        typ := T; rl := v;s:=a;tip:=b;         if s='$C[..]' then
Begin strL:=laVal.strL;                               
 strR:=laVal.strR;                           End;    end;   
makeConstNode := tr;  end;  Can you make that code generic? So it can
fill with bytes, words, integers, qwords etc?

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ASerge · 2025-07-09 18:35

Quote from: d2010 on July 09, 2025, 05:37:57 pmC: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?

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Thaddy · 2025-07-09 19:09

Instead of length()?

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TBMan · 2025-07-09 19:22

I would have used

Code: Pascal  [Select][+][-]window.onload = function(){var x1 =
document.getElementById("main_content_section"); if (x1) { var x =
document.getElementsByClassName("geshi");for (var i = 0; i
sizeof(TreeRec) 
If you passed a nil pointer things could get wonky.

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d2010 · 2025-07-10 07:36

Quote from: Thaddy on July 09, 2025, 09:09:22 pmInstead 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][+][-]window.onload = function(){var x1 =
document.getElementById("main_content_section"); if (x1) { var x =
document.getElementsByClassName("geshi");for (var i = 0; i Var
boca100=001;???Function makeNode_id0(rtype: NodeType;lef,rig:
NavTree): NavTree;Var dwg,loop:integer;Begin dwg:=GetTickCount;     
New(result);      for loop:=01 to boca100 do       Case
Now_MakeIdNode.zer of'f':  makeNode_idf(result,rtype,lef,rig);'0':
 makeNode_id2(result,rtype,lef,rig);'z':
 makeNode_idz(result,rtype,lef,rig);      End;     
Now_MakeIdNode.tim:=Now_MakeIdNode.tim+(GetTickCount-dwg);End;

----------------------------------------------------------------------
ASerge · 2025-07-10 18:25

Quote from: d2010 on July 10, 2025, 09:36:14 amBut, 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][+][-]window.onload = function(){var x1 =
document.getElementById("main_content_section"); if (x1) { var x =
document.getElementsByClassName("geshi");for (var i = 0; i Procedure
makeNode_idf(Var dest:NavTree;rtype: NodeType; lef, rig:
NavTree);begin    fillchar(dest^,sizeof(TreeRec),0);    with dest^ do 
     begin         typ := rtype;         dxf5:=integer(dest);       
 rle:=0.0;...Code: Pascal  [Select][+][-]window.onload =
function(){var x1 = document.getElementById("main_content_section");
if (x1) { var x = document.getElementsByClassName("geshi");for (var i
= 0; i Procedure makeNode_idz(Var dest:NavTree;rtype: NodeType; lef,
rig: NavTree);begin    ZeroMemory(dest,sizeof(TreeRec));    with dest^
do       begin         typ := rtype;         dxf5:=getmypid[1].lo;   
     rle:=0.0;...