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From: Jason Moxham <J DOT L DOT Moxham AT maths DOT soton DOT ac DOT uk>
Message-Id: <200108200319.EAA03624@malus.maths.soton.ac.uk>
Subject: Re: dll-helpers
To: cygwin AT cygwin DOT com
Date: Mon, 20 Aug 2001 04:19:32 +0100 (BST)
In-Reply-To: <3B7FF33A.8000104@ece.gatech.edu> from "Charles Wilson" at Aug 19, 2001 01:11:22 PM
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> 
> This is really a "how to build dll's" question and belongs on the cygwin 
> list, not private mail.  For your convenience, I've copied this message 
> to the list and set the Reply-To; appropriately.

Sorry , I didn't send it to cygwin list as I wrongly assumed that it
wasnt a cygwin specific question , just a gcc+windows question and as
your site has this type of thing...

anyway , thanks

> 
> Jason Moxham wrote:
> 
> > Using the latest cygwin I can compile and run the "c" version of
> > dllhelpers-0.2.7 , but what I'm having trouble  with is , if my dll
> > contains referances  to varibles/fns  in the main exe or another dll
> > then I get undefined referances when I try to compile my dll
> 
> 
> Right.  you can't do that on windows.  DLLs (unlike unix shared 
> libraries) are not allowed to have any unresolved symbols.  Thus, you 
> can't have forward references from the DLL to a client executable.

ARGHHHHH!!!!

I should of checked before I started programming , windows has so many
arbitary limitations , I dread the day when I have to change to a
multi-threaded version of my program

> There are two ways to proceed:
> 
> 1) put the "common" data items in a second DLL that can be built 
> independently of the first DLL and the executable.  Then, when building 
> the original DLL, link against this second one.  When building the 
> executable, link against both DLLs.
> 
> or
> 
> 2) Create hook functions in your DLL to set a DLL-internal pointer to 
> point to your exe's variable.  Within the DLL, only use the pointer in 
> your operations.
>

Thanks , I can use either of these methods , although the first seems
easier

Jason Moxham
j DOT l DOT moxham AT maths DOT soton DOT ac DOT uk

 
> e.g.
> 
> DLL:
> int * ext_data;
> void set_ext_data(int * c) {ext_data = c};
> In dll, use *ext_data.
> 
> client exe:
> int data;
> set_ext_data(&data);
> in client, use data.
> 
> --Chuck
> 
> 
> > 
> > for example
> > 
> > add to usedll.c  (not in main fn)
> > 
> > int jayglob;
> > 
> > 
> > 
> > add to cdll.c 
> > 
> > int jay(void){return jayglob;}
> > 
> > 
> > add to cdll.h
> > 
> > int	jay(void);
> > extern int jayglob;
> > 
> > 
> > then when I try to compile cygcdll.dll I get undefined referances to
> > jayglob
> > 
> > 
> > 
> > Of course under linux , no problems
> 
> 
> 
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