[SOLVED] Function FillDWord() produces Fatal Segmentation Fault
lazarus / 64-FPC development

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domibay_hugo · 2020-11-09 12:03

In the intent to swiftly assigning a Value to an Array I tried to use
the FillDWord() Function.
When I initialize it with FillByte() it is correctly initialized. But
when I want to assign the actual Value with FillDWord() it produces an
unrecoverable Crash.

To demonstrate this Behaviour I extended the Example104 from the
Official Documentation at:
FillDWord() Documentation
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 Program
demofilldword; uses  SysUtils; { Program to demonstrate the FillDWord
function. } Const  ArraySize = 256; Var  S : Array [0..ArraySize - 1]
of Int64;  v: Cardinal;  I : longint; begin  v := 7;  S[0] := 0;  
try    WriteLn('Array S - FillByte(): do ...');    
FillByte(S,SizeOf(S),0);     For I:=1 to ArraySize - 1 do      If
S[i]<>0 then        WriteLn('Position ',i,' not zeroed out');    
WriteLn('Array S : Init done.');     WriteLn('Array S - FillDWord():
do ...');     FillDWord(S,SizeOf(S),v);     WriteLn('Array S -
FillDWord(): Execution done.');     For I:=1 to ArraySize do     
WriteLn('Position ',i,' has the value: '#39, S[I], #39);    
WriteLn('Array S - FillDWord(): Check done.');   except    on e:
Exception do WriteLn('Exception [', e.HelpContext, ']: '#39,
e.Message, #39);  end;   WriteLn('Demo App done.');end.              
               
This little Prototype produces this Output:

$ ./demofilldword
Array S - FillByte(): do ...
Array S : Init done.
Array S - FillDWord(): do ...
Segmentation fault (core dumped)

(I don't know where the Core Dump is written to but I can't find it)

The actual Value in this Use Case in v is actually "7" so I could use
FillByte(), too, but that wouldn't cover other Use Cases for Values >
255 .

Please, can anyone give Insight about this Behaviour ?

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marcov · 2020-11-09 12:09

You get the size of bytes, and then  use store that amount of
dwords?!?!

divide by count to fill by sizeof(dword)

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domibay_hugo · 2020-11-09 12:45

Yes, I was actually misusing SizeOf().
I replaced it by Length() or by the  ArraySize Constant.
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  
WriteLn('Array S - SizeOf(): '#39, SizeOf(S), #39);  WriteLn('Array S
- SizeOf(DWord): '#39, SizeOf(DWord), #39);  WriteLn('Array S -
SizeOf()/SizeOf(DWord): '#39, (SizeOf(S) / SizeOf(DWord)), #39); 
Which produces:

Array S - SizeOf(): '2048'
Array S - SizeOf(DWord): '4'
Array S - SizeOf()/SizeOf(DWord): ' 5.1200000000000000E+002'


I found the Value of 2048 very suspicious so I set ArraySize = 100
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 Const 
ArraySize = 100; Now I get:

Array S - SizeOf(): '800'
Array S - SizeOf(DWord): '4'
Array S - SizeOf()/SizeOf(DWord): ' 2.0000000000000000E+002'


Now the Application does not crash anymore.

But still the Function FillDWord() does not fill the Array correctly
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    
FillDWord(S, ArraySize, DWord(v)); 
Array S - FillDWord(): do ...
Array S - FillDWord(): Execution done.
Position 0 has the value: '30064771079'
Position 1 has the value: '30064771079'
Position 2 has the value: '30064771079'
# [...]
Position 47 has the value: '30064771079'
Position 48 has the value: '30064771079'
Position 49 has the value: '30064771079'
Position 50 has the value: '0'
Position 51 has the value: '0'
Position 52 has the value: '0'
# [...]
Position 94 has the value: '0'
Position 95 has the value: '0'
Position 96 has the value: '0'
Position 97 has the value: '0'
Position 98 has the value: '0'
Position 99 has the value: '0'
Array S - FillDWord(): Check done.
Demo App done.
Heap dump by heaptrc unit of
/usr15/proyectos/pascal/strings/demofilldword
6 memory blocks allocated : 719/728
6 memory blocks freed     : 719/728
0 unfreed memory blocks : 0
True heap size : 327680
True free heap : 327680


It is only filled half the way and not with the correct value.

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marcov · 2020-11-09 12:52

your array is int64 (8-byte) while your fill values are 32-bit.
(4-byte)

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domibay_hugo · 2020-11-09 13:06

Yes, you are right!

The actually Value in this Use Case is low and always positive, so I
expected it to cover the positive range of Int64

When I changed the Array " S " to "Array of Cardinal" FillDWord()
filled in the correct value.
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  S :
Array [0..ArraySize - 1] of Cardinal; 
But in my Use Case I actually need Int64 values.
So, I found that I must use the FillQWord() Function which works also
with 8 Byte values.
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    
FillQWord(S, ArraySize, v); 
Thank you very much for your constructive hints.