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cmd/cgo: generated header for buildmode=c-archive lacks exported symbols for imported library #27150

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jpap opened this issue Aug 22, 2018 · 1 comment

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@jpap
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commented Aug 22, 2018

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

go version go1.10.3 darwin/amd64

Does this issue reproduce with the latest release?

Unfortunately, yes.

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

GOARCH="amd64"
GOBIN=""
GOCACHE="/Users/jpap/Library/Caches/go-build"
GOEXE=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GOOS="darwin"
GOPATH="/Users/jpap/go"
GORACE=""
GOROOT="/usr/local/Cellar/go/1.10.3/libexec"
GOTMPDIR=""
GOTOOLDIR="/usr/local/Cellar/go/1.10.3/libexec/pkg/tool/darwin_amd64"
GCCGO="gccgo"
CC="clang"
CXX="clang++"
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 -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/folders/f8/2x23x3n92nb_mjyj10v_ssph0000gn/T/go-build284257526=/tmp/go-build -gno-record-gcc-switches -fno-common"

What did you do?

  1. Create the following files in $GOPATH/src/github.com/exporttest,
lib/lib.go
lib+main.go

having contents:

// lib/lib.go
package lib

import (
	"C"
	"fmt"
)

//export ExportedFromLib
func ExportedFromLib() {
	fmt.Println("ExportedFromLib")
}

func init() {
	fmt.Println("lib.init")
}
// lib+main.go
package main

import (
	"C"
	"fmt"

	_ "github.com/exporttest/lib"
)

//export ExportedFromMain
func ExportedFromMain() {
	fmt.Println("ExportedFromMain")
}

func main() {
	// Empty, but required for buildmode=c-shared
}
  1. Run go build -buildmode=c-archive -o libtest.a lib+main.go

  2. Observe that in the header file libtest.h, ExportedFromMain is included, but ExportedFromLib is not.

What did you expect to see?

  • Both exported functions ExportedFromLib and ExportedFromMain should be exported.

What did you see instead?

  • ExportedFromLib was not exported in the header file, but it is exported in the generated static library, as the following sample C code shows:
// test.c
#include "libtest.h"
#include <stdio.h>

// Declared here as it does not exist in generated libtest.h
extern void ExportedFromLib();

int main() {
	ExportedFromLib();
	return 0;
}
  • C call to static library executes the exported function ExportedFromLib that does not appear in the header file.
$ gcc -o test test.c -L. -ltest
$ ./test
lib.init
ExportedFromLib
$ 

@ianlancetaylor ianlancetaylor changed the title Generated header for buildmode=c-archive lacks exported symbols for imported library cmd/cgo: generated header for buildmode=c-archive lacks exported symbols for imported library Aug 22, 2018

@ianlancetaylor

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commented Aug 22, 2018

Yes, that is how it works. The expectation is that your c-archive will only export functions from the main package, just as is done with a normal Go package.

The fact that a symbol marked with //export in some other package winds up being visible in the c-archive is more or less a bug.

Closing as working as expected.

@golang golang locked and limited conversation to collaborators Aug 22, 2019

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