NewStr/AssignStr dont conform to Delphi standard
lazarus / 64-FPC development

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dje · 2022-12-10 08:46

Delphi's NewStr returns a valid pointer for empty strings. FreePascal
returns nil.
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 S :=
NewStr('')
Delphi's AssignStr accepts nil and allocates new memory. FreePascal
throws an "Access violation" on nil.
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
AssignStr(S, 'New String');
Effectively it means NewStr('')'s must be replaced with at lease a
space, or the following code.
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     P :=
NewStr(' ');    AssignStr(P, ''); 
Since empty strings return a nil, all code which de-references empty
strings must check for nil before doing so.

Tested against Delphi 11 and FPC 3.2.2

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dje · 2022-12-10 08:52

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 {$mode
delphi} program StrTest; uses  SysUtils; var  P: PString; begin  try 
 P := NewStr(''); // Replace with space to make it work   
AssignStr(P, 'This will crash');    Writeln('Before: P = "', P^,
'"');    AssignStr(P, 'A Second ansistring');    Writeln('After : P =
"', P^, '"');    DisposeStr(P);  except    on E: Exception do
WriteLn(E.Message);  end;  ReadLn;end.

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dje · 2022-12-10 09:49

Just an update: It would appear GetStr/DisposeStr originate from TP7
via the "Objects" unit.
TP7 also returns nil for GetStr('') but doesn't contain a AssignStr
function.

I checked the FreePascals Object unit, and the documentation is clear:
QuoteIf the string is empty then Nil is returned.
https://www.freepascal.org/docs-html/rtl/objects/newstr.html

But, it then contains the procedure SetStr which does accept a nil, so
this code doesn't crash:
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 {$mode
delphi}program StrTest;uses SysUtils, Objects;var P: PString;begin 
try    P := NewStr('');    SetStr(P, 'This now wont crash');   
Writeln('Before: P = "', P^, '"');    SetStr(P, 'A Second
ansistring');    Writeln('After : P = "', P^, '"');    DisposeStr(P); 
except    on E: Exception do WriteLn(E.Message);  end;  ReadLn;end. 
I guess the 3 GetStr/AssignStr/DisposeStr functions in SysUtils could
be removed if they arn't going to be updated.
The Objects unit is enough for legacy code.

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nanobit · 2022-12-10 20:38

If functions need more nil checks, you could report them in the bug
tracker.

NewStr('') returning nil (but in Delphi: global NullStr) could be seen
as preferable:
Spreading the NullStr (PString) via pointer variables is risky because
they should be used only for read access, whereas write access (p^:=
s;) would change the global NullStr.

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marcov · 2022-12-10 21:02

The sysutils ones are probably for backwards compatibility with 16-bit
Delphi 1, and should be treated separate from unit objects.

Please file a bug with a reproduction case.