cgo linking issue with C++ dynamic libraries

[DineshReddyK]

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

go version go1.10.1 linux/amd64

Does this issue reproduce with the latest release?

Yes.

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

go env Output

$ go env
cp:~/containerfolder/API_Gateway_5G_cOAM_Productized$ go env
GOARCH="amd64"
GOBIN=""
GOCACHE="/root/.cache/go-build"
GOEXE=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOOS="linux"
GOPATH="/root/go"
GORACE=""
GOROOT="/usr/lib/golang"
GOTMPDIR=""
GOTOOLDIR="/usr/lib/golang/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-build634932465=/tmp/go-build -gno-record-gcc-switches"

What did you do?

I have a C++ shared library in /opt/xx/lib64. Headers are in C. In my go code, I am including proper header and linker options using CPPFLAGS/CFLAGS and LDFLAGS. Still while building go I am getting 'undefined reference` error.

import (
    //#cgo LDFLAGS: -L/opt/xxx/lib64/ -lsessionmgmt
    //#cgo CPPFLAGS: -I/opt/xxx/include/
    //#include <stdlib.h>
    //#include <sessionMgmt/sessionMgmt.h>
    "C"
    "unsafe"
)

What did you expect to see?

successful build

What did you see instead?

I am getting this linking errors:

/tmp/go-build609413262/b163/_x002.o: In function `_cgo_e38947506e9d_Cfunc_um_get_lastlogin_info':
/tmp/go-build/cgo-gcc-prolog:52: undefined reference to `um_get_lastlogin_info'
collect2: error: ld returned 1 exit status
make[3]: *** [Makefile:48: vet] Error 2

Library is exist in the right path and do contain the required method. Not sure why this error.

Binary file /opt/xxx/lib64/libsessionmgmt.so matches

Also tried using the pkg-config with cgo. Still the same linking error appears.

[beoran]

Is the function um_get_lastlogin_info compiled as extern "C" in the C++ library ? Otherwise this might be due to the name mangling C++ applies.

[DineshReddyK]

Thanks a lot :). Yes, extern "C" was missing. After generating the .so with the changes, linking worked fine.

But from the cgo documentation, I see that there is already support for c++ right. Any reason, why is it mandatory to keep extern "C".

[AlexRouSg]

cgo is integrating C and Go, not C++ and Go. You can compile and use C++ sources/libs but to use those inside Go sources you need to make it into something you can call from C.

[DineshReddyK]

Thanks for the info. I will close this.