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Subject:	building a cygwin DLL to run on non-cygwin app
Message-Id:	<20020815235332.19EDF3DEB@cavall.jtang.org>
Date:	Thu, 15 Aug 2002 19:53:32 -0400 (EDT)
From:	tang AT jtang DOT org (J. Tang)

I have scoured the web and the mailing lists to no avail.  How do I
build a DLL to dynamically run with a non-cygwin application?

Background:

A vendor has supplied a program that links to a DLL at runtime to do
stuff.  I wish to write a DLL that this program may use.

Here is some psuedocode from my DLL:

long foo (<some params>) {
  /* do some bit manipulations */
}

I can build this fine using the ld/dlltool/ld/dlltool/ld trick.  This
also works if I just use the -shared flag to ld.

The vendor's program, which I'll just call 'V', happily accepts it.  At
runtime, V has a parameter that specifies the DLL (i.e., my DLL must be
relocatable).  Everything works as expected.

Now, suppose that I want to add some debugging info to my DLL, like so:

#include <stdio.h>
long foo (<some params>) {
  printf ("Entering foo.\n");
  /* do some stuff */
}

Compiling with ld fails because of the unresolved symbol printf().  I
can make this error go away by adding a '-lc' flag to the end of ld.
The DLL now compiles and builds.

When I now run it under V, I get the application error:

  The instruction at "0x77f6ce4c" referenced memory at "0x00000010".
  The memory could not be "written".

This seems to imply that linking to libc alters the DLL fundamentally.
I have tried the following:

  - Try compiling with -mno-cygwin.  I find that I still need the '-lc'
    flag, and the application error still occurs.
  - Remove the printf() call by leave '-lc'.  Although V works, I really
    do want the debugging data.
  - Try changing the entry point with '-e
    __cygwin_noncygwin_dll_entry AT 12'.  Apparantly this tricked worked
    with an older release of cygwin, but not anymore.

The closest reference I have found is Mumit Khan's comment "if you're
loading these from non-Cygwin application, eg., from an MSVC
application..." at
http://www.xraylith.wisc.edu/~khan/software/gnu-win32/dllhelpers.html.
I tried this (bullet #3 above) but it no longer works.

This is all with the latest stable release of the cygwin tools.

Hopefully somebody here has insight on how to do this deceptively easy
task.

-- 
Jason Tang  /  tang AT jtang DOT org  /  http://www.jtang.org/~tang

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