Go package linkage with a C library

Question

I hope this is a basic question. I am trying to build a Go package which includes functions from a library written in C. The structure is basically as follows:

package too

/*
#cgo LDFLAGS: -L/usr/local/lib include -lbar
#include mybar.h
*/
import "C"

func MyGoWrapper () {
  C.orig_func()
}

Running go build foo.go fails with an "undefined reference" for orig_func. Note that the header is mybar.h; I created a prototype for orig_func that was not included in the original library. Do I need to recompile the library first, including this header file, before it will link with the Go build? Or am I misunderstanding something else entirely?

Answer 1

When linking against an external library, you do need to separately compile it for your target architecture. cgo can't replace the configure/make (or whatever) to compile the library; it only knows how to build a few .c files in your package directory, and a library's build process might be more complex.

---

I'm less sure of how to accomplish the larger task of linking in an external library when cross-compiling (and I'm not sure what you've already done). The (closed) Go bug on cross-compilation with cgo looks useful here. You may want to build the Go toolchain with some environment variables set that are described in godoc cmd/cgo:

To enable cgo during cross compiling builds, set the CGO_ENABLED environment variable to 1 when building the Go tools with make.bash. Also, set CC_FOR_TARGET to the C cross compiler for the target. CC will be used for compiling for the host.

After the Go tools are built, when running the go command, CC_FOR_TARGET is ignored. The value of CC_FOR_TARGET when running make.bash is the default compiler. However, you can set the environment variable CC, not CC_FOR_TARGET, to control the compiler when running the go tool.

CXX_FOR_TARGET works in a similar way for C++ code.

The bug also mentions someone who uses -ldflags="-extld=$(CC)" (where $(CC) is the name of the cross-compiler they want to use).

In your example code there's an explicit -L/usr/local/lib and I don't think that'll work: I think when you build libraries for the target, you're going to want to put them in a directory distinct from the lib for your host arch. For example, this ARM cross-compilation HOWTO uses a /usr/local/arm-linux prefix or install_root in some places.