R – AnsiString return values from a Delphi 2007 DLL in a Delphi 2009 application

ansistringdelphidelphi-2009dllstring

I have a DLL compiled with D2007 that has functions that return AnsiStrings.

My application is compiled in D2009. When it calls the AnsiString functions, it gets back garbage.

I created a little test app/dll to experiment and discovered that if both app and dll are compiled with the same version of Delphi (either 2007 or 2009), there is no problem. But when one is compiled in 2009 and the other 2007, I get garbage.

I've tried including the latest version of FastMM in both projects, but even then the 2009 app cannot read AnsiStrings from the 2007 dll.

Any ideas of what is going wrong here? Is there a way to work around this?

Best Answer

The internal structure of AnsiStrings changed between Delphi 2007 and Delphi 2009. (Don't get upset; that possibility has been present since day 1.) A Delphi 2009 string maintains a number indicating what code page its data is in.

I recommend you do what every other DLL on Earth does and pass character buffers that the function can fill. The caller should pass a buffer pointer and a number indicating the size of the buffer. (Make sure you're clear about whether you're measuring the size in bytes or characters.) The DLL function fills the buffer, writing no more than the given size, counting the terminating null character.

If the caller doesn't know how many bytes the buffer should be, then you have two options:

  • Make the DLL behave specially when the input buffer pointer is null. In that case, have it return the required size so that the caller can allocate that much space and call the function a second time.

  • Have the DLL allocate space for itself, with a predetermined method available for the caller to free the buffer later. The DLL can either export a function for freeing buffers that it has allocated, or you can specify some mutually available API function for the caller to use, such as GlobalFree. Your DLL must use the corresponding allocation API, such as GlobalAlloc. (Don't use Delphi's built-in memory-allocation functions like GetMem or New; there's no guarantee that the caller's memory manager will know how to call Free or Dispose, even if it's written in the same language, even if it's written with the same Delphi version.)

Besides, it's selfish to write a DLL that can only be used by a single language. Write your DLLs in the same style as the Windows API, and you can't go wrong.

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