Cross compile a shared Library under Linux for windows

Question

I want to get a windows-DLL, but I want to compile it under Ubuntu-Linux.

Building an Executable was simple: env GOOS=windows GOARCH=386 go build wrapper.go generates a wrapper.exe, that behaves as expected.

but building a DLL with env GOOS=windows GOARCH=386 go build -buildmode=c-shared wrapper.go results in error:

running gcc failed: exit status 1
gcc: error: unrecognized command line option ‘-mconsole’; did you mean ‘--compile’?

I would prefer not to install and run go under windows, because my complete tool chain is running under Ubuntu

go version go1.15.6 linux/amd64

This seems very compiler implementation specific,so what's your go version and go env? — leaf bebop
– leaf bebop, Commented Jan 22, 2021 at 10:33
Also what gcc libraries did you install for compiling to windows? — TehSphinX
– TehSphinX, Commented Jan 22, 2021 at 10:36
I'm currently not using it to build c-shared libraries (and it could use a Go version update), but I did successfully build c-shared libraries with it in the past: github.com/tehsphinx/golang_cgo_windows/blob/master/Dockerfile. If you want to quickly test it, its also on docker hub: hub.docker.com/r/tehsphinx/golang_cgo_windows/… — TehSphinX
– TehSphinX, Commented Jan 22, 2021 at 10:42
what makes me wonder is, that building a .exe is working, but not building a .dll — Peter Miehle
– Peter Miehle, Commented Jan 22, 2021 at 10:42

kostix · Accepted Answer · 2021-01-22 11:21:55Z

If you would pass -x to the call to go build -buildmode=c-shared ..., you'd notice that in that mode the linker from the Go toolchain calls out to the external C linker; for instance, here on GNU/Linux with Go 1.15.x, I have:

mkdir -p $WORK/b001/exe/
cd $WORK/b001/exe/
/home/username/devel/golang-1.15.6/pkg/tool/linux_amd64/link -o cshared.dll -importcfg $WORK/b001/importcfg.link -buildmode=c-shared -buildid=OJVN3iT0GI_DEAMVbLDu/o9eT_YGfUiRe07beNQAA/-xRRfDcM8nVc03rltdqz/OJVN3iT0GI_DEAMVbLDu -extld=gcc $WORK/b001/_pkg_.a
# command-line-arguments
loadinternal: cannot find runtime/cgo
/home/username/devel/golang-1.15.6/pkg/tool/linux_amd64/link: running gcc failed: exit status 1
gcc: error: unrecognized command line option ‘-mconsole’; did you mean ‘--compile’?

Note that pkg/tool/linux_amd64/link is called with -extld=gcc, and from go doc cmd/link, we gather that

-extld linker
Set the external linker (default "clang" or "gcc").

My guess is that in order to produce a C-compatible dynamic library, the Go toolchain relies on an external C linker, and that is carried out by cgo machinery — at which there's actually a hint in the documentation of -buildmode=c-shared:

-buildmode=c-shared
Build the listed main package, plus all packages it imports,
into a C shared library. The only callable symbols will
be those functions exported using a cgo //export comment.
Requires exactly one main package to be listed.

Hence my guess is that in order to do what you want you have to:

Install a cross-compiler supporting Windows/i386 — you can start with this.
Set up the environment before calling go build as explained in the cgo docs so that the Go toolchain calls the Windows-specific linker.
Verify it works by running go build with the -x command-line option.

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