cmd/cgo: cgo + static const int results in "undefined reference" on linux

[p8a]

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

$ go version
go version go1.14.7 linux/amd64

Does this issue reproduce with the latest release?

Have not tried

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

go env Output

$ go env
GO111MODULE=""
GOARCH="amd64"
GOBIN=""
GOCACHE="/root/.cache/go-build"
GOENV="/root/.config/go/env"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOINSECURE=""
GONOPROXY=""
GONOSUMDB=""
GOOS="linux"
GOPATH="/go"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/usr/local/go"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/usr/local/go/pkg/tool/linux_amd64"
GCCGO="gccgo"
AR="ar"
CC="gcc"
CXX="g++"
CGO_ENABLED="1"
GOMOD=""
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-build954734301=/tmp/go-build -gno-record-gcc-switches"
root@06ba4e8d3414:/go/src/bitbucket.org/threattrack/vipre-mac/foo#

What did you do?

Linking of a program referring to static const int fails with undefined reference:

foo.h

static const int BAR = 1;

foo.go

package main

/*
#include "foo.h"
*/
import "C"
import "fmt"

const (
  BAR = int(C.BAR)
)

func main() {
  fmt.Println(BAR)
}

What did you expect to see?

Compilation succeeds on linux as it does on darwin

What did you see instead?

go build
/usr/bin/ld: $WORK/b001/_cgo_main.o:/tmp/go-build/cgo-generated-wrappers:2: undefined reference to `BAR'
collect2: error: ld returned 1 exit status

[AlexRouSg]

This seemed to be working at 1.10.8 and stopped working since 1.11

cc @ianlancetaylor

[cherrymui]

static in C means it is local to the its compilation unit. Arguably C and Go are different compilation units (and we do compile them separately).

[p8a]

@cherrymui in this case what would be the recommended way to keep the Go consts in sync with their C counterparts - outside of redefining them which will be a maintenance nightmare ?

This code also compiles and works on darwin with no issues.

[ianlancetaylor]

I think this is also mentioned in #39136.

One difference is that GCC emits a DW_AT_location for BAR where clang emits a DW_AT_const_value. Another difference is that when compiling this:

static const int BAR = 1;
enum { V = (BAR) * 1 };

clang compiles the code without error but GCC reports

foo.c:2:8: error: enumerator value for ‘V’ is not an integer constant

I'm not sure why clang doesn't give an error in that case. clang's behavior seems correct for C++ but not for C.

Note that using #define should work for both compilers.

[AlexRouSg]

I found the commit that broke it, da76981