How to vary cgo lib path by os?

[David Karr]

I'm aware of the "build constraints" mechanism, which lets individual files be only compiled on specific oses or architectures.

I am constructing an app that has a single cgo module, which is the "meat" of the application (longest module). In my poc, the cgo header specifies a lib path with "linux" in the path, to point to the linux libraries. I haven't constructed a Makefile for this yet, but this works well with vscode and simple testing.

I now have to consider how to build this on Windows, although the deployment target for this will be Linux. I have to allow for testing of it on Windows, if possible. I thought perhaps that I could change the lib path in the cgo header to reference "${GOOS}" instead, but it doesn't seem to recognize that.

It's not reasonable to use build constraints for this, because it seems like I would have to duplicate the entire module in order to vary the lib path.  That would be absurd.

What are reasonable strategies for this, including facilitating testing of it in vscode on both Linux and Windows?

[Ian Lance Taylor]

If I'm reading this correctly, you can address this by using
GOOS/GOARCH build tags in your #cgo comments. See, for example,
https://go.dev/src/runtime/cgo/cgo.go?#L14. This is documented at
https://pkg.go.dev/cmd/cgo#hdr-Using_cgo_with_the_go_command.

Ian

[David Karr]

So you're saying I could do this:

    // #cgo linux LDFLAGS: -L${SRCDIR}/../lib/linux -llib1 -llib2 -llib3 -llib4

    // #cgo windows LDFLAGS: -L${SRCDIR}/../lib/windows -llib1 -llib2 -llib3 -llib4

If so, that would be much better than duplicating the entire file. It looks a little odd to duplicate the library list, but if that's the best we can do, I guess that's it.

I'll test this later.

[David Karr]

Hm, I'm going to guess you're going to tell me that I can't quite do that, as I just verified that, unless I'm simply doing it wrong.  I wrote almost exactly what I showed there.  I'm having some trouble figuring out how to set up the C compiler on Windows, but I have verified with other applications that I can cross-compile a build on Linux for Windows, so I simply ran this:

    GOOS=windows go build -o target/dist/windows-amd64

This fails with:

    imports voltagems/voltagefuncs: build constraints exclude all Go files in /home/dk068x/git/voltagego/voltagefuncs

Any way to fix this?

[Ian Lance Taylor]

Here you are cross-compiling from Linux to Windows. With Go that is
easy but when using cgo it requires a Linux -> Windows C
cross-compiler in order to compile the parts of the program that are
written in C. If you have such a cross-compiler installed, you can
make this work by writing

GOOS=windows CC=linux-to-windows-cc CGO_ENABLED=1 go build ...

Ian