cmd/cgo: c code that was compiled with go, cannot also be used by go

[adam-hanna]

Please answer these questions before submitting your issue. Thanks!

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

$ go version
go version go1.10.3 linux/amd64

Does this issue reproduce with the latest release?

I have not tried with any go1.11.x releases

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

$ go env
GOARCH="amd64"
GOBIN="/home/adam/go/bin"
GOCACHE="/home/adam/.cache/go-build"
GOEXE=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOOS="linux"
GOPATH="/home/adam/go"
GORACE=""
GOROOT="/home/adam/.gvm/gos/go1.10.3"
GOTMPDIR=""
GOTOOLDIR="/home/adam/.gvm/gos/go1.10.3/pkg/tool/linux_amd64"
GCCGO="gccgo"
CC="gcc"
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"
GOGCCFLAGS="-fPIC -m64 -pthread -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build321160569=/tmp/go-build -gno-record-gcc-switches"

What did you do?

You can find a demo, here.

// lib/lib.go
package main

import "C"
import "log"

//export myPrint
func myPrint(str *C.char) {
    gStr := C.GoString(str)
    log.Println(gStr)
}

func main() {}

//main.go
package main

//#include <stdlib.h>
//#include "./lib/lib.h"
import "C"
import "unsafe"

func main() {
    cs := C.CString("hello, world!")
    defer C.free(unsafe.Pointer(cs))
    C.myPrint(cs)
}

$ go build -o ./lib/lib.so -buildmode=c-shared ./lib/lib.go
$ go run main.go
# command-line-arguments
In file included from ./main.go:3:0:
cgo-builtin-prolog:7:48: error: conflicting types for '_GoString_'
cgo-builtin-prolog:6:44: note: previous declaration of '_GoString_' was here
In file included from ./main.go:3:0:
cgo-gcc-export-header-prolog:27:20: error: 'GoString' redeclared as different kind of symbol
cgo-builtin-prolog:8:12: note: previous declaration of 'GoString' was here

What did you expect to see?

$ go build -o ./lib/lib.so -buildmode=c-shared ./lib/lib.go
$ go run main.go
hello, world!

What did you see instead?

$ go build -o ./lib/lib.so -buildmode=c-shared ./lib/lib.go
$ go run main.go
# command-line-arguments
In file included from ./main.go:3:0:
cgo-builtin-prolog:7:48: error: conflicting types for '_GoString_'
cgo-builtin-prolog:6:44: note: previous declaration of '_GoString_' was here
In file included from ./main.go:3:0:
cgo-gcc-export-header-prolog:27:20: error: 'GoString' redeclared as different kind of symbol
cgo-builtin-prolog:8:12: note: previous declaration of 'GoString' was here

[AlexRouSg]

Possibly related to #22192

[adam-hanna]

@AlexRouSg I think this is a little different. This err is happening because _GoString_ is being defined twice.

It's defined in cmd/cgo/out.go L#1722:

#ifndef GO_CGO_PROLOGUE_H
#define GO_CGO_PROLOGUE_H
...
typedef _GoString_ GoString;
...
#endif

But also in the header file ./lib/lib.h L#53:

#ifndef GO_CGO_PROLOGUE_H
#define GO_CGO_PROLOGUE_H
...
typedef _GoString_ GoString;
...
#endif

I don't know why the #ifndef GO_CGO_PROLOGUE_H isn't doing the trick. It seems to me like that should prevent this error from happening.

[AlexRouSg]

@adam-hanna

That issue is generally about c-shared not working (or is not expected to work) with go programs. This is about the generated header defining types twice, so that's why I thought it's related.

[adam-hanna]

@AlexRouSg ahh, my apologies. I didn't understand. Thanks!

[gopherbot]

Change https://golang.org/cl/152079 mentions this issue: cmd/cgo: use preprocessor macros to avoid prolog redefinitions