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Re: Windows DLLs from Unix with minimum effort

From: Jason Curl
Subject: Re: Windows DLLs from Unix with minimum effort
Date: Thu, 01 Nov 2007 22:58:13 +0100
User-agent: Thunderbird (Windows/20070728)

Roumen Petrov wrote:
Jason Curl wrote:
Brian Dessent wrote:
Jason Curl wrote:

./configure CC="gcc -mno-cygwin" LDFLAGS="-Wl,--kill-at" --target=i586-pc-mingw32 --disable-static
on Cygwin to generate the Windows DLL

Target is not the right thing to use here.  Target only has meaning in
the context of building tools that themselves generate code, such as
compilers, linkers, assemblers, etc.  If you want to simply indicate
that you are cross-compiling a library for a host different than the
current one then you use --host. Also, if you're going to use Cygwin as
a "Fake mingw" then you probably also should use --build=mingw as well.
Thanks - all of my initial problems. My test code now compiles and links with the DLL. However, when I move the test program out into another folder and copy the DLL into the Windows System path, it doesn't execute. I haven't figured out how to run it without the "libtool" wrapper. When I observe the dependencies using Microsoft tools, I see there are none other than my library and MS libraries (e.g. Cygwin is not there).

The library works correctly when I use say, Visual Basic 6.

Please, could you prepare sample test case.
I cannot reproduce described issue.
Well, I think I've figured it out today (albeit I'm testing on a different machine, similar software though) and there are two executables. One in the build directory and one in .libs. e.g.

 libtest.exe      <-- Doesn't seem to work? No idea
                      what this is...
   libtest.exe    <-- Will work when "libmofo-1.dll"
                      is in the path, e.g. copied to
                      this dir.

Can anybody explain what libtool is doing?? It appears to do a lot of nifty stuff, but I don't see any dependencies on "libmofo" from "libtest.exe" in either case. I'll attach a minimal example when I'm back at work tomorrow.

And the directory it runs from (.libs) indicates it is actually the source "lt-libtest.c" that relies on a shell, so as soon as I move the executable to a "virgin" computer without Cygwin, the program "libtest.exe" won't work.
I have also another subdirectory that tests the library but I can't link to it using mingw. First it doesn't recognise that the name was simplified (it looks for address@hidden for example instead of _my_func/my_func).

You need to understand that the --kill-at is a linker option. It cannot change behavior of the compiler, and when gcc is told to use the stdcall
calling convention for a function, it includes the normal stdcall
decorations on the symbol. There is no way to change this AFAIK, unless
you use __attribute__((alias)) or something.
Ooh sounds horrible.
So if you want to use these symbols without the stdcall decoration, then you have to get the linker to jump throuh hoops. --kill-at removes them
from names that are exported, but that still doesn't change the fact
that the compiler emits calls to the decorated names. For that you will
have to use --enable-stdcall-fixup, which will link calls to address@hidden to
_foo if there is no address@hidden found anywhere.  Or, you could create an
import library and link with that.

But instead of all of that I think I would just simply use
--add-stdcall-alias.  This should result in both a decorated and
undecorated alias of each symbol being exported, so you don't have to
try to pretend that gcc isn't emitting calls to decorated names when it
really is.
I can probably live with that solution, I might look how to use export/import library definitions as it would also mean defining precisely the ordinals in the Windows DLLs. I'll take a deeper look at Erik's suggestion, it looks simple enough except I'm not entirely happy with the DEF file generation he uses.

You could pass "def"  file to the linker as example:


Cool, sounds like something that Erik mentioned earlier in "libsndfile.dll" that I'll look into.

I'd also like to generate .lib files (what is the .a file that is generated anyway? Is that the .lib import library?)


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