Bug in typecasting integer to array of characters
lazarus / 62-General

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Avinash · 2023-11-17 10:24

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 type 
LongRec = record    case byte of      0: (Lo, Hi: Word);      1:
(Chars4: array [0..3] of Char);  end; var I: LongInt;    S:
String;begin   S := 'test';   I := $5A5A5A5A;  {'ZZZZ'} 
WriteLn(LongRec(I).Chars4 + S);   I := $5A5A5A;    {'ZZZ'#0} 
WriteLn(LongRec(I).Chars4 + S);   I := $5A5A;      {'ZZ'#0#0} 
WriteLn(LongRec(I).Chars4 + S);   I := $5A;        {'Z'#0#0#0} 
WriteLn(LongRec(I).Chars4 + S);   I := 0;          {#0#0#0#0} 
WriteLn(LongRec(I).Chars4 + S);end.
Output by Borland TP (correct):
QuoteZZZZtest
ZZZ test
ZZ  test
Z   test
    test
Output by FPC 3.2.2 Win32 (incorrect):
QuoteZZZZtest
ZZZtest
ZZtest 
Ztest  
test

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Fibonacci · 2023-11-17 10:34

I dont think its a bug, I would expect the output to look like it is
looking. #0 is char array terminator, casted to string, then "s"
appended, looks ok.

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marcov · 2023-11-17 11:05

It might not even be Free Pascal, but merely the change of OS output
functionality. TP might even differ between using Crt and not.

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Avinash · 2023-11-17 11:18

The output has nothing to do with it, it was originally discovered in
a binary context.

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 type 
LongRec = record    case byte of      0: (Lo, Hi: Word);      1:
(Chars4: array [0..3] of Char);  end; var I: LongInt;    S:
String;begin  I := $5A5A5A5A; {'ZZZZ'}  S := LongRec(I).Chars4; 
WriteLn(Length(S):2,' ',S);   I := $5A00005A; {'Z'#0#0'Z'}  S :=
LongRec(I).Chars4;  WriteLn(Length(S):2,' ',S);end.
FPC 3.2.2 Win32:
Quote
 4 ZZZZ
 1 Z

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Fibonacci · 2023-11-17 11:25

So whats wrong? $5A00005A as PChar casted to String will have length
of 1

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marcov · 2023-11-17 11:52

I think his beef is that pchar automated conversions are afaik a
Delphi thing.

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Avinash · 2023-11-17 12:18

Discovered thing:

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 Q0:
array [0..3] of Char;    Q1: array [1..4] of Char;begin  Q0 :=
'Z'#0#0'Z';  Q1 := 'Z'#0#0'Z';  WriteLn(Q0);  WriteLn(Q1);end.
Output:
QuoteZ  
Z  Z

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Fibonacci · 2023-11-17 12:40

Is it a bug if its done on purpose?

zerobased = as PAnsiChar (to first nil)
otherwise = print full array

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
fpc_Write_Text_Pchar_as_Array(Len : Longint;var f : Text;const s :
array of ansichar; zerobased: boolean = true); iocheck;
compilerproc;var  ArrayLen : longint;  p : pansichar;Begin  If
(InOutRes<>0) then   exit;  case TextRec(f).mode of    fmOutput {
fmAppend gets changed to fmOutPut in do_open (JM) }:      begin      
p:=pansichar(@s);        if zerobased then          begin            {
can't use StrLen, since that one could try to read past the end }    
      { of the heap (JM)                                             
 }            ArrayLen:=IndexByte(p^,high(s)+1,0);            {
IndexByte returns -1 if not found (JM) }            if ArrayLen = -1
then              ArrayLen := high(s)+1;          end        else    
    ArrayLen := high(s)+1;        If Len>ArrayLen Then         
fpc_WriteBlanks(f,Len-ArrayLen);       
fpc_WriteBuffer(f,p^,ArrayLen);      end;    fmInput: InOutRes:=105  
else InOutRes:=103;  end;End;
PS. Thats something different than your first post

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Warfley · 2023-11-17 13:34

This is at most a bug in the implementation of the write intrinsic,
and even then I would not be so sure about that.

Static char Arrays are often used to interface with C apis such as
Berkeley sockets. C strings are #0 terminated, therefore treating
static char Arrays as C Strings actually makes quite some sense.

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nanobit · 2023-11-17 14:49

Quote from: Fibonacci on November 17, 2023, 01:40:19 pmzerobased = as
PAnsiChar (to first nil)
otherwise = print full array

This is by design. Only zero-based array is compatible with PChar.
Then string( array) is like string( pchar). This is also Delphi
compatible.