Subj : Re: "Delphi" DLL
To   : Andy Gorman
From : Alexander Grotewohl
Date : Sat May 08 2021 20:02:46

On 08 May 2021, Andy Gorman said the following...
 
 AG> I'm creating a DLL that is going to get a value passed into it (path to a
 AG> file).  I am able to pass in a value, but it's getting truncted in the
 AG> DLL.
 AG> 
 AG> Here's the code:
 AG> 
 AG> library SimpleLib;
 AG> 
 AG> Uses
 AG>         SysUtils;
 AG> 
 AG> Var
 AG>   Path : String;
 AG>   o : Text;
 AG> 
 AG> function MySucc(AVal : PWideString;  pLen : Integer) : PWideString;
 AG> stdcall; begin
 AG>   SetLength(String(AVal), pLen);
 AG>   Assign(o, 'c:\temp\simp.txt');
 AG>   Rewrite(o);
 AG>   Write(o,String(AVal));
 AG>   Close(o);
 AG>   Result := AVal;
 AG> end;
 AG> 
 AG> exports
 AG>   MySucc;
 AG> End.

are you calling this from C# then? it would seem SetLength() is doing something to a pointer that is not technically a 'PWideString'. (a PWideString has a known length, see https://wiki.freepascal.org/Character_and_string_types)

String() on the other hand (depending on compiler settings) would force AVal to a old pascal style string where string[0] is a byte for the length. so if the length is stored in memory at str[-1] and str[0] (this is not something you'd normally interact with.. these are managed types and the compiler handles this), you're only getting one byte of the length.

is there a reason you're using PWideString? SysUtils has a StrPas function that makes it fairly easy to do things like UnicodeStringVar:=StrPas(AVal); where AVal is a PWideChar .. which is usually enough for me to interact with C/C++ from pascal. 

sorry for the lack of code.. but i'd be unable to test it if it's called from other than pascal ;) but hopefully it's enough

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