mail/cygwin32

From ian@cygnus.com Tue Nov  3 23:23 EDT 1998
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Date: Tue, 3 Nov 1998 20:25:28 -0500
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From: Ian Lance Taylor <ian@cygnus.com>
To: gvaughan@oranda.demon.co.uk
CC: tanner@gmx.de, oliva@dcc.unicamp.br, gord@trick.fig.org,
        bug-libtool@gnu.org
In-reply-to: <363F3F85.2B31574@oranda.demon.co.uk>
	(gvaughan@oranda.demon.co.uk)
Subject: Re: Inter-library dependencies in libtool
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X-Gnus-Article-Number: 2   Wed Nov  4 01:39:12 1998

   Date: Tue, 03 Nov 1998 17:38:13 +0000
   From: "Gary V. Vaughan" <gvaughan@oranda.demon.co.uk>

   It would seem that the dll code has bitrotted =(O|  Pity.  Ian, do you
   have time/want to fix this, or do you want to pass the torch on?

I no longer have access to a Windows machine, nor do I have all that
much interest in the problem, so I'd say that somebody else had better
pick up the torch.

Incidentally, I believe that DJ Delorie is working on adding DLL
support directly to ld, which will mean that dlltool is no longer
required, and should make it possible to greatly simplify the win32
hacks in dlltool, perhaps even simply using the standard GNU ld code.

   Shouldn't libtool notice that it is running on cygwin32 and pass the
   -no-undefined option by itself?  It seems to go against the raison
   d'etre for libtool to force the Makefile developer to figure this out...

This kind of goes to the heart of libtool.  libtool wants to present a
particular interface for using shared libraries.  In order to do this,
it assumes that the system supports certain capabilities.  One of
those is that the system can support undefined symbols in shared
libraries.

That means that on systems which do not permit shared libraries to
have undefined symbols--AIX and Windows--libtool doesn't really work.

The --no-undefined option is a hack which tells libtool that the
shared library has special characteristics which permit libtool to
create a shared library on AIX or Windows, or any other supported
platform.

I think the general idea is that you should use the --no-undefined
option whenever possible.  If you do, you will be able to create
shared libraries on AIX and Windows.  If you do not or can not, you
will not be able to create them.

libtool should not add a --no-undefined option itself.  If it used
that option inappropriately, then building the shared library would
fail.  Instead, libtool users should always use --no-undefined if they
can.

Of course, there are problems.  For example, in the GNU binutils, I
can arrange matters such that --no-undefined will work on Windows, but
to do so I have to link various libraries together and I have to link
against special Windows system libraries.  So I do that, which means
that I have to change the options I pass to libtool based on the
system.

In other words, the interface which libtool presents is deficient.  It
does not successfully hide the system on which it is running, and it
forces the code which calls libtool to make adjustments.

I doubt there is any wholly acceptable solution here.  The only
workable one I can see would be to effectively enhance Windows and AIX
shared libraries such that they support creating shared libraries with
undefined symbols.  On Windows, this could be done by doing the link
once, checking for undefined symbols, creating little stub routines
for those symbols which track down the symbols in some other open DLL,
compiling those stubs, and linking them into the DLL.  Perhaps
something similar is possible on AIX.

Of course even that will not make Windows DLLs identical to ELF shared
libraries.  ELF shared libraries permit the main program to override a
symbol in the shared library, and Windows DLLs do not.

Ian

When libtool links DLLs, it strips some command line switches. It's probably
the case that this is ok for most situations. However, this potentially breaks
Mingw32 support in the Cygwin environment.

In order to get Mingw32 support in Cygwin, the compiler must be invoked with
the -mno-cygwin switch. If libtool strips this switch out during the link
process, the resulting binary will get linked with the wrong import libraries.

The following is a small example. I've edited the output a bit for
readability. When I send this email, long lines will get broken up, so it
still might be a bit difficult to read.

-------------------------

$ libtool --version
ltmain.sh (GNU libtool) 1.3.3 (1.385.2.181 1999/07/02 15:49:11)


$ cat xx.c

extern __declspec(dllexport) int func(void);

int func()
{
}


$ libtool --mode=compile gcc -mno-cygwin -c xx.c
mkdir .libs
gcc -mno-cygwin -c  -DPIC xx.c -o .libs/xx.lo
mv -f .libs/xx.lo xx.o
ln -s xx.o xx.lo

$ libtool --mode=link gcc -mno-cygwin -o libxx.la xx.lo \
	-version-info 0:0:0 -no-undefined -rpath /usr/local/lib

rm -fr .libs/libxx.la .libs/libxx.* .libs/libxx.*

generating symbol list for `libxx.la'

test -f .libs/libxx-0-0-0.dll-ltdll.c || sed -e "/^# \/\* ltdll\.c starts here
\*\//,/^# \/\* ltdll.c ends here \*\// { s/^# //; p; }" -e d <
/usr/local/bin/libtool > .libs/libxx-0-0-0.dll-ltdll.c

 test -f .libs/libxx-0-0-0.dll-ltdll.o || (cd .libs && gcc -c
libxx-0-0-0.dll-ltdll.c)

 dlltool --export-all --exclude-symbols
DllMain@12,_cygwin_dll_entry@12,_cygwin_noncygwin_dll_entry@12 --output-def
.libs/libxx-0-0-0.dll-def .libs/libxx-0-0-0.dll-ltdll.o  xx.o

 sed -e "1,/EXPORTS/d" -e "s/ @ [0-9]* ; *//" < .libs/libxx-0-0-0.dll-def >
.libs/libxx.exp

echo EXPORTS > .libs/libxx-0-0-0.dll-def

 _lt_hint=1; for symbol in `cat .libs/libxx.exp`; do echo " $symbol @
$_lt_hint; " >> .libs/libxx-0-0-0.dll-def; _lt_hint=`expr 1 + $_lt_hint`; done

 test -f .libs/libxx-0-0-0.dll-ltdll.c || sed -e "/^# \/\* ltdll\.c starts
here\*\//,/^# \/\* ltdll.c ends here \*\// { s/^# //; p; }" -e d <
/usr/local/bin/libtool > .libs/libxx-0-0-0.dll-ltdll.c

 test -f .libs/libxx-0-0-0.dll-ltdll.o || (cd .libs && gcc -c
libxx-0-0-0.dll-ltdll.c)


gcc -Wl,--base-file,.libs/libxx-0-0-0.dll-base -Wl,--dll -nostartfiles -Wl,-e,
__cygwin_dll_entry@12 -o .libs/libxx-0-0-0.dll .libs/libxx-0-0-0.dll-ltdll.o
xx.o

 dlltool --as=as --dllname libxx-0-0-0.dll --exclude-symbols
DllMain@12,_cygwin_dll_entry@12,_cygwin_noncygwin_dll_entry@12 --def
.libs/libxx-0-0-0.dll-def --base-file .libs/libxx-0-0-0.dll-base --output-exp
.libs/libxx-0-0-0.dll-exp

 gcc -Wl,--base-file,.libs/libxx-0-0-0.dll-base
.libs/libxx-0-0-0.dll-exp -Wl,--dll -nostartfiles -Wl,-e,__cygwin_dll_entry@12
 -o .libs/libxx-0-0-0.dll .libs/libxx-0-0-0.dll-ltdll.o  xx.o

 dlltool --as=as --dllname libxx-0-0-0.dll --exclude-symbols
DllMain@12,_cygwin_dll_entry@12,_cygwin_noncygwin_dll_entry@12 --def
.libs/libxx-0-0-0.dll-def --base-file .libs/libxx-0-0-0.dll-base --output-exp
.libs/libxx-0-0-0.dll-exp

 gcc
.libs/libxx-0-0-0.dll-exp -Wl,--dll -nostartfiles -Wl,-e,__cygwin_dll_entry@12
 -o .libs/libxx-0-0-0.dll .libs/libxx-0-0-0.dll-ltdll.o  xx.o

(cd .libs && rm -f libxx.a && ln -s libxx-0-0-0.dll libxx.a)

dlltool --as=as --dllname libxx-0-0-0.dll --def
.libs/libxx-0-0-0.dll-def --output-lib .libs/libxx.a

creating libxx.la

(cd .libs && rm -f libxx.la && ln -s ../libxx.la libxx.la)

---------------

Notice how the 'gcc' lines do not contain the -mno-cygwin switch. This switch
should not get stripped.

Jon Leichter
jon@symas.com