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Date: Fri, 1 Apr 2005 14:55:06 -0500 (EST)
From: Igor Pechtchanski <pechtcha AT cs DOT nyu DOT edu>
Reply-To: cygwin AT cygwin DOT com
To: Douglas Merrell <doug AT merrell DOT org>
cc: cygwin AT cygwin DOT com
Subject: Re: Cygwin dll initialization from win32 application
In-Reply-To: <424D9F14.7000209@merrell.org>
Message-ID: <Pine.GSO.4.61.0504011444550.14073@slinky.cs.nyu.edu>
References: <424D9F14 DOT 7000209 AT merrell DOT org>
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On Fri, 1 Apr 2005, Douglas Merrell wrote:

> On Tue, 11 Mar 2003 15:34:07 -0500, Christopher Faylor wrote:
> > On Tue, Mar 11, 2003 at 12:48:15PM -0500, Rich LeGrand wrote:
> > > Hi all,
> > > This has been discussed before, but I have been unable to find any
> > > solutions to this problem.
> > >
> > > Basically I'm trying to call cygwin functions in the cygwin1.dll
> > > from a Windows (msvc) application by calling LoadLibrary() and
> > > GetProcAddress().
> > >
> > > I can see that the cygwin authors have made provision for this
> > > (or what looks like a provision) with dll_dllcrt0 (a non-cygwin
> > > initialization function.)  But when I call it, it crashes.
> >
> > This has been broken for some time.  So far no one has stepped
> > forward to fix the problem in cygwin, so it has remained broken.
>
> ----------------------------------------------------------------
> I ran into this same problem and investigated a bit. Windows
> appears to be loading cygwin1.dll differently depending on whether
> it is:
>   1) linked into the application and loaded automatically by
>       the Windows loader at runtime (i.e.- compiled with gcc)
> or
>   2) loaded dynamically by an application call to LoadLibrary
>      (i.e.- compiled with MSVC++)
>
> The only difference in the two loaded modules is one double
> word at 0x610d2f4c in the .data section (0x610d2f4c memory
> address, or 0x000d194c file offset in cygwin1.dll).
>
> In case 1) the value at the location in memory is exactly as
> it appears in the raw cygwin1.dll file: 0x00000000.
>
> In case 2) the system appears to have changed the value at
> this location during the LoadLibrary call to 0x00000001.
>
> dll_dllcrt0 checks this hardcoded value (see below) as does
> the lower level function dll_crt0_FP11per_process. If the
> value is 0x00000000, everything works just fine. If the value
> is anything other than 0x00000000 (i.e.- in the case where
> the library is dynamically loaded using LoadLibrary), the
> application crashes with an AccessViolation.
>
>  ? What is this hardcoded value?
>
>  ? Why is the Windows LoadLibrary function changing the value
>    at this location?

Instead of staring at the assembly code, why not build a debug version of
cygwin1.dll and look at the actual source?  Or even download one -- IIRC
all of the snapshots are still built with debugging info intact.

FWIW, the source for dll_dllcrt0 is in the (appropriately named)
dll_init.cc in the src/winsup/cygwin directory.  The hard-coded location
is probably one of the static variables.

Another point to note is that dll_dllcrt0 isn't called directly -- it's
called from cygwin_attach_dll, which is called from _cygwin_dll_entry
(which is basically DllMain).  This should be enough to get you started in
debugging and fixing this.

And remember: with Cygwin, <http://cygwin.com/acronyms/#PTC>.  Information
on how to submit patches is at <http://cygwin.com/contrib.html>.
HTH,
	Igor
-- 
				http://cs.nyu.edu/~pechtcha/
      |\      _,,,---,,_		pechtcha AT cs DOT nyu DOT edu
ZZZzz /,`.-'`'    -.  ;-;;,_		igor AT watson DOT ibm DOT com
     |,4-  ) )-,_. ,\ (  `'-'		Igor Pechtchanski, Ph.D.
    '---''(_/--'  `-'\_) fL	a.k.a JaguaR-R-R-r-r-r-.-.-.  Meow!

"The Sun will pass between the Earth and the Moon tonight for a total
Lunar eclipse..." -- WCBS Radio Newsbrief, Oct 27 2004, 12:01 pm EDT

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