[fpc-devel] Found two Delphi incompatibilities

[Sergei Gorelkin]

Thursday, August 31, 2006, 12:35:33 PM, Michael wrote:


MVC> On Thu, 31 Aug 2006, Sergei Gorelkin wrote:

>> Hello,
>>
>> While porting some Delphi code, I found the following two
>> incomatibility issues. Should I report them as bugs?
>>
>> Sample 1: It compiles both in Delphi and FPC, but FPC executable fails
>> at runtime. Delphi inserts temporary string variable and conversion
>> (array of char -> string), but FPC treats the pointer literally.

MVC> Did you check the '@ returns typed pointer' setting in FPC ?

Oops... That was a false alarm - FPC compiler behaves the same way as
Delphi here. They both generate conversion code in {$T+} and do not
generate it in {$T-}.
So my sample appears to be legal only in {$T+}, which isn't a default.

What makes a difference is actually TStrings.SetTextStr implementation.
Delphi accesses the argument as null-terminated string, so it works perfectly
well even without conversion. FPC implementation treats argument as
AnsiString, accesses its Length field and eventually segfaults...

>> -----
>> const
>>  TestData: array[0..7] of Char = 'abc'#10'def'#0;
>>
>> procedure Test1;
>> var
>>  sl: TStringList;
>> begin
>>  sl := TStringList.Create;
>>  sl.Text := string(@TestData[0]);  // <- fails here
>>  sl.Free;
>> end;
>> -----
>>
>> Sample 2: This one compiles with Delphi (again, it inserts necessary
>> conversion Wide -> Ansi), but does not compile with FPC, neither in
>> objfpc nor in Delphi mode.
>>
>> -----
>> function Test2: string;
>> var
>>  buf: array[0..255] of char;
>>  len: Integer;
>> begin
>> { (skipped) I read USB string descriptor into buf. After that,
>>  buf[0] contains length of descriptor in bytes, buf[1] = 3,
>>  and rest is filled with Unicode chars. Now I want to convert it into
>>  ASCII...
>> }
>>  len := Integer(buf[0]) shr 1;
>>  SetString(result, PWideChar(@buf[2]), len-1);
>> end;

MVC> What is the error ?

It is 'Incompatible type for arg no.2: Got "PWideChar", expected
"PChar"'.

MVC> Michael.
MVC> _______________________________________________
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-- 
Best regards,
 Sergei                            mailto:sergei_gorelkin at mail.ru