cgo: multiple definition of cgo functions error when linking go-compiled c-archive library

[reusee]

What version of Go are you using (go version)?

go version devel +c99483feb8 Sat Jun 3 17:04:56 2017 +0000 linux/amd64

What operating system and processor architecture are you using (go env)?

GOARCH="amd64"
GOBIN=""
GOEXE=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOOS="linux"
GOPATH="/home/reus"
GORACE=""
GOROOT="/home/reus/go"
GOTOOLDIR="/home/reus/go/pkg/tool/linux_amd64"
GCCGO="gccgo"
CC="gcc"
GOGCCFLAGS="-fPIC -m64 -pthread -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build884024617=/tmp/go-build -gno-record-gcc-switches"
CXX="g++"
CGO_ENABLED="1"
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"

What did you do?

lib.go

package main

import (
	"C"
	"fmt"
)

func main() {
}

//export Foo
func Foo() {
	fmt.Printf("Foo!\n")
}

lib.c

extern void Foo();

void foo() {
  Foo();
}

build static library

 go build -buildmode=c-archive -o foo.a 

call.go

package main

/*
extern void foo();

#cgo LDFLAGS: ./foo.a
*/
import "C"

func main() {
	C.foo()
}

build call.go

go build call.go

What did you expect to see?

no build error

What did you see instead?

# command-line-arguments
./foo.a(go.o): In function `_cgo_topofstack':
/home/reus/go/src/runtime/asm_amd64.s:2327: multiple definition of `_cgo_topofstack'
/tmp/go-build074487992/command-line-arguments/_obj/_cgo_main.o:/tmp/go-build/command-line-arguments/_obj/_cgo_main.c:5: first defined here
./foo.a(go.o): In function `_cgo_panic':
/home/reus/go/src/runtime/cgo/callbacks.go:45: multiple definition of `_cgo_panic'
/tmp/go-build074487992/command-line-arguments/_obj/_cgo_main.o:/tmp/go-build/command-line-arguments/_obj/_cgo_main.c:6: first defined here
./foo.a(go.o): In function `crosscall2':
/home/reus/go/src/runtime/cgo/asm_amd64.s:12: multiple definition of `crosscall2'
/tmp/go-build074487992/command-line-arguments/_obj/_cgo_main.o:/tmp/go-build/command-line-arguments/_obj/_cgo_main.c:2: first defined here
./foo.a(000001.o): In function `_cgo_wait_runtime_init_done':
/home/reus/go/src/runtime/cgo/gcc_libinit.c:35: multiple definition of `_cgo_wait_runtime_init_done'
/tmp/go-build074487992/command-line-arguments/_obj/_cgo_main.o:/tmp/go-build/command-line-arguments/_obj/_cgo_main.c:3: first defined here
./foo.a(000001.o): In function `_cgo_release_context':
/home/reus/go/src/runtime/cgo/gcc_context.c:11: multiple definition of `_cgo_release_context'
/tmp/go-build074487992/command-line-arguments/_obj/_cgo_main.o:/tmp/go-build/command-line-arguments/_obj/_cgo_main.c:4: first defined here
collect2: error: ld returned 1 exit status

[ianlancetaylor]

-buildmode=c-archive is designed for linking Go code into a C program. You are trying to use it to link Go code into a Go program. That can't work. That's not what it's for.

What are you really trying to do?

[KDr2]

@iangudger but the below simple example doesn't work either:

$ cat mde.go
package main

/*
int Add(int i) {
        return i + 1;
}
*/
import "C"
import "fmt"

func main() {
}

//export Foo
func Foo() {
        fmt.Printf("Foo!\n")
        C.Add(1)
}

$ go build -buildmode=c-archive mde.go
# command-line-arguments
/tmp/go-build123475493/command-line-arguments/_obj/mde.cgo2.o: In function `Add':
./mde.go:5: multiple definition of `Add'
/tmp/go-build123475493/command-line-arguments/_obj/_cgo_export.o:/data/kdr2/.../mde.go:5: first defined here
collect2: error: ld returned 1 exit status

[ianlancetaylor]

The simple example is an unfortunate limitation, but a documented one. Quoting https://golang.org/cmd/cgo/#hdr-C_references_to_Go:

Using //export in a file places a restriction on the preamble: since it is copied into two different C output files, it must not contain any definitions, only declarations. If a file contains both definitions and declarations, then the two output files will produce duplicate symbols and the linker will fail. To avoid this, definitions must be placed in preambles in other files, or in C source files.

[KDr2]

Got it, thanks.