[fpc-pascal] Delphi ShareMem -DLL-pchar

afpTeam afpteam at sbcglobal.net
Wed Sep 6 05:27:17 CEST 2006

----- Original Message ----- 
From: "Matt Emson" <memsom at interalpha.co.uk>
To: "afpTeam" <urbansound at sbcglobal.net>; "FPC-Pascal users discussions" 
<fpc-pascal at lists.freepascal.org>
Sent: Tuesday, September 05, 2006 7:45 PM
Subject: Re: [fpc-pascal] Delphi ShareMem -DLL-pchar

>> Can anyone comment on the Delphi unit "ShareMem" and if FPC ever did any 
>> work to incorporate what this Delphi unit is doing for DLL shared memory 
>> managment?  I'm looking so far, at the docs on creating a memory manager, 
>> and at the strings unit, which has some pchar utilities in it.
> I don't use it any more. I use FastMM or BucketMM.
> you only really need SHareMem if you are passing String types 
> (string/AnsiString etc) to a DLL. It doesn't sound like you're doing that.
> Beware, Sharemem is slow and buggy in my experience. Also, you would need 
> sharemem in FPC as Delphi assumes you are sending your Delphi types 
> (strings, classes etc), that dictate you need Sharemem, to another Delphi 
> process.
Good.  Especially that I don't have access to the Delphi program itself, 
only that it provides a DLL stub interface, but that's part of the 
problem... the authors chose to use variant types as the parameters being 
passed, so as I write a DLL to interface with it, I'm being force- fed large 
allocation variant types, having to use largestring models and but only a 
single shortstring that comes along for the ride with the variants. 
Fortunately, as this is the client side, I can do a socket interface to that 
half of the two apps and still survive, even remote the application if I 
choose to.

The other app, which is serving data to my DLL and hense to Delphi through 
my DLL, is a more modern platform and has a much more germane approach to 
Win32 memory management.  It has been interfaced with VB and several other 
newer compiler based apps, quite succesfully.  It is the one however, which 
is dictating AsciiZ, but then showing examples of "String" type to be fed to 
it, rather than pchars.  I was successful in exchanging data to it with FPC, 
even typecasting pchars to it, via an EXE, and hopefully that may fare well 
in a DLL also, so long as I don't have to convert that back to Delphi 
variants, but just send to Delphi, via socket.   Damn, is that ugly, or 
what?  LOL

>> This article, http://delphi.about.com/od/objectpascalide/l/aa103003a.htm 
>> explains much about the Delphi related, Win32 DLL pointer and shared 
>> memory problems.  I'm working on creating a DLL between two Win32 
>> applications, but having a lot of exception and pointer related problems 
>> to do with pchar and asciiz data transfer.
> the only way to avoid issues is to make sure the process that allocates 
> the buffer deallocates it. That fixes most issues. C gets round it because 
> you generally use the same compiler and Malloc etc. With Delphi and FPC, 
> you are going to have issues because they are two completely different 
> compilers.


> I'd do something like:
> function getFpcStrBuffer(buffer: pchar; const size: integer): boolean;
> and
> procedure freeFpcStrBuffer(buffer: pchar); //maybe size too?
> then
> var
>  b: pchar;
> begin
>  if getFpcStrBuffer(b, 21) then
> try
>     callAFpcDllRoutine(b, 21);
>  finally
>    freeFpcStrBuffer(b);
>  end;
> Do that in the DLL yhou want to pass data TO. So FPC would implement it, 
> for example. Get the Delphi code to then allocate and free buffers using 
> that routine. Something ike that.
> Avoid using STRING/ANSISTRING/WIDESTRING when passing to a DLL like the 
> plague.
> Sorry lack of sleep.. must go to bed. Hopefully that makes sense.
> M
Thanks for the suggestions.  Ditto sleep.  Will try some tomorrow but mostly 
you're suggesting the same thing I think I've been seeing.

Thank you.


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